Day 1 Convective Outlook for Atlantic Canada Valid for July 31, 2018

Convective discussion:

 

Isolated to scattered thunderstorms will develop today across portions of Labrador associated with a weak cold front crossing the area..

 

Technical Discussion:

 

A weak cold front will trigger ISOL/SCT TS today as it swings across Labrador. 12Z YZV tephi shows a strong capping inversion at 600 mb, and given the weak dynamics and modest surface heating, that cap will be difficult to erode. No severe weather is XPCTD.

 

KCAR tephi looks like a “loaded gun” profile, but there is no trigger to develop any TS. SCT ACC/TCU is XPCTD across NB.

 

Regional Impacts:

 

Labrador: Isolated/scattered non-severe thunderstorms.

 

Newfoundland: nil

 

Nova Scotia: nil.

 

Prince Edward Island: nil.

 

New Brunswick: nil.

 

Jeremy

 

 

 

 

 

 

 

Day 1 Convective Outlook for Atlantic Canada Valid for July 30, 2018

 

Convective discussion:

 

Scattered thunderstorms will persist today over central and eastern LAB under the influence of an upper level trough of low pressure. Scattered thunderstorms will develop early this afternoon over central and east/northeast NL in association with a surface trough and 500 mb short wave.

 

Technical Discussion:

 

Conditions look pretty good for near severe thunderstorms across portions of NL this afternoon. From 15-21 UTC there is a strong 250 jet mb with divergence over SE NL, placing the risk area above in the left exit region (which will promote ageostrophic motions). 12Z tephi from YJT shows good shear at close to 45 knots, good drying in the mid levels which would lead to possible strong wind gusts at the surface. Looking at SAT PIX there appears to be an area of PVA moving into WRN NL (associated with a somewhat decent short wave trough at 500 mb) which will move eastward and add to the potential for thunderstorms once the convective temperature is reached. 12Z surface analysis shows a surface trough near WRN NL (which should act as a trigger). Adjusting the YJT surface temperature to 27 degrees produces close to 1000 CAPE, with fairly good moisture near the surface. In addition instabilities are pretty good (LI’s of minus 4 to minus 6). All these ingredients point to the potential for severe thunderstorms, likely from the Buchans/Millertown area to regions north and east. For now near severe cells are expected, but this area will have to be monitored this afternoon and into early this evening.

 

Over LAB scattered TS will continue today east of the upper trough. Looking at the 12Z YYR sounding, freezing levels are pretty low, but the shear is really weak so no severe weather is expected. Small hail is almost a given under some of these cells.

 

Regional Impacts:

 

Newfoundland: Scattered afternoon/evening thunderstorms from central and east/northeast. Slight risk of severe.

 

Labrador: Scattered non-severe thunderstorms. Small hail likely.

 

Nova Scotia: nil.

 

Prince Edward Island: nil.

 

New Brunswick: nil.

 

Jeremy

 

 

 

 

 

 

 

Day 1 Convective Outlook for Atlantic Canada Valid for July 29, 2018

 

Convective discussion:

 

Scattered thunderstorms associated with an upper level short over the Maritimes will move over WRN NL and SE LAB this afternoon and evening. Isolated thundershowers over parts of CNTRL/WRN LAB in the vicinity of an upper low will become more widespread this afternoon. Scattered thunderstorms will develop over ERN NB, ERN and NRN NS, PEI and Cape Breton this afternoon.

 

Technical Discussion:

 

A short wave trough and associated PVA triggered some early morning thunderstorms across CNTRL/NERN NB, and this area will move into WRN NL and SE LAB this afternoon. Upper air analysis shows a strong 250 mb jet axis (around 100 knots) and also a strong 500 mb jet (around 55 knots) with decent divergence. This area will not necessarily need lift from the surface, so cloud cover shouldn’t inhibit convection. There is an upper trough at 500/700 mb progressing slowly eastward, so falling heights and clearing skies should trigger some cells over portions of the MRTMS this afternoon. In addition a surface trough/cold front (analyzed at 12Z to be crossing NB) should aid in thunderstorm development. There is ample moisture near the surface, but the question is how quickly the surface dewpoints drop. 12Z tephi analysis shows good shear across the region (35-45 knots) but the instability is not as impressive as yesterday. In addition, adjusting the profiles for a Tmax of 26-27 produces about 500-700 CAPE, so the Buoyancy will not be that great either. Some storms could produce gusts to 40 knots with torrential downpours (precipitable water is 30-40 mm). Hail will not be a concern given the low buoyancy and associated weaker updrafts in cells. That being said small hail is possible within the stronger storms. Severe weather is not anticipated, however a very isolated severe cell in ERN NB, ERN/NRN NS and PEI cannot be ruled out.

 

Regional Impacts:

 

New Brunswick: Scattered non-severe thunderstorms across the east this afternoon. Slight risk further to the west.

 

Prince Edward Island: Scattered non-severe thunderstorms this afternoon.

 

Newfoundland: Scattered afternoon/evening thunderstorms in the west. Slight risk in the east.

 

Labrador: Scattered non-severe thunderstorms.

 

Nova Scotia: Scattered non-severe thunderstorms this afternoon in the north and east (as well as Cape Breton).

 

Jeremy

 

 

 

 

 

 

 

Day 1 Convective Outlook for Atlantic Canada Valid for July 28, 2018

 

Convective discussion:

 

Scattered thunderstorms should develop early this afternoon over ME and advect into western NB. Scattered storms will move into central and western Labrador as well. There is a risk of thundershowers/storms along the warm front crossing NL today.

 

Technical Discussion:

 

There is decent support from the surface to 250 mb for the potential of severe thunderstorms across the western half of NB this afternoon. A fairly strong jet streak will move across the region this afternoon placing most areas within the yellow threat zone above in the right entrance of that jet. A fairly strong 500 mb jet of 45 knots was analyzed at 12Z and an associated upper trough over the eastern GRTLKS/WRN New England. There is quite a bit of moisture from 850 mb to the surface, and instabilities are good as well. The 12Z KCAR tephi shows upwards of 1000 CAPE if modifying for a surface temperature of 27 degrees C (CAPE is somewhat fat as well with decent low-mid level lapse rates). Precipitable water is close to 40 mm, and there is about 30-35 knots of shear from 0-6 km. 12Z surface analysis shows a slowly approaching surface front, which will act as the trigger, since atmospheric heights don’t really drop at all through the course of the day. If cells develop, local torrential downpours and strong wind gusts appear as the main threats. FZLVL are around 12,000 feet so small hail is also possible. Over western/central LAB non-severe cells are expected…however local torrential downpours and modest wind gusts are also a possibility with some storms. And isolated thundershowers embedded along a warm front will continue across gulf waters and extreme NW NL today.

 

Regional Impacts:

 

New Brunswick: Possible hail, downpours and gusty winds over western sections this afternoon/evening.  Risk of thundershowers for areas adjacent to and east of the severe threat zone.

 

Prince Edward Island: Slight risk for Prince County this evening.

 

Newfoundland: Slight risk in the far NW.

 

Labrador: Scattered thunderstorms across western/central regions.

 

Nova Scotia: nil

 

Jeremy

 

 

 

 

 

 

 

Day 1 Convective Outlook for Atlantic Canada Valid for July 27, 2018

Convective discussion:

 

Risk of thundershowers over northern New Brunswick this afternoon and evening. They could become severe over northwestern areas. Risk of non-severe thundershowers over southwestern Labrador today and tonight.

 

Technical Discussion:

 

There is a supportive upper level jet approaching northwestern N.B. this afternoon. It has a diffluent flow and an embedded approaching vorticity centre. The Caribou sounding is likely the most representative and it suggests CAPE values in the 1000 J/kg range, shear of 30 kts and precipital water near 30 mm. There are no clear trigger mechanisms other than afternoon surface heating in the relatively cloud free skies currently over that region and a potential subtle surface trough. Local downpours and hail are possible. The flow is generally uni-directional and the higher winds are quite elevated. Gusty winds are still a possibility with water loading. The mid-levels are fairly moist so evaporative cooling will not aid significantly in the wind generation.  

 

Regional Impacts:

 

New Brunswick: Possible hail, downpours and gusty winds over northwestern sections this afternoon/evening.  Risk of thundershowers northeast.

 

Nova Scotia and Prince Edward Island: Nil

 

Newfoundland: Nil.

 

Labrador: Chance of thundershowers southwest.

 

 

 

 

 

 

 

Day 1 Convective Outlook for Atlantic Canada Valid for July 26, 2018

 

Convective discussion:

 

There is the risk of heavy rain and embedded thundershowers over parts of the Maritimes today and tonight with the approach of an upper level trough tracking eastward. 

 

Technical Discussion:

 

The upper trough that has been persistent over the eastern United States and southern Quebec is beginning to slowly push eastward towards the Maritimes and weaken as it tracks across New Brunswick this evening and overnight.  There is the risk of isolated thundershowers along and ahead of the trough this afternoon and evening for New Brunswick and western parts of Nova Scotia. With increasing moderate wind shear of 40-45kt, CAPE values near 500 J/kg  as well as high precipitable water of 40-50mm, heavy downpours and brief wind gusts are possible. At this time thundershowers do not look to be severe but the concern will be heavy rainfall where total rainfall values could reach over 50mm in some areas especially over western New Brunswick and southern New Brunswick.

 

Regional Impacts:

 

 

New Brunswick and Nova Scotia: Risk of thundershowers this afternoon and evening with locally heavy downpours possible.  

 

Prince Edward Island: Locally heavy showers in TCu possible.

 

Southwestern marine waters: Thundershowers this afternoon.

 

Labrador: Locally heavy showers over central and eastern section in TCu.

 

Newfoundland: Nil

 

 

Roberta McArthur

 

 

Day 1 Convective Outlook for Atlantic Canada Valid for July 25, 2018

 

Convective discussion:

 

There is the risk for non-severe thundershowers over southern Labrador, in the Annapolis valley area of Nova Scotia and over the southwestern marine waters this afternoon and evening.

 

Technical Discussion:

 

A warm front currently over southern Labrador is lying near quasi-stationary today. There is the slight risk of embedded thundershowers along and north of the front today and into the  overnight hours where cloud top cooling may help to destabilize the air mass further. Precipitable water values are ranging between 40-50mm so locally heavy downpours are possible. The area from Goose Bay toward Cartwright is already under a rainfall warning.

 

There is a slight risk of a thundershower over Nova Scotia this afternoon, especially in the Annapolis Valley where clouds are expected to clear, allowing convective temperatures to be reached. There is no real trigger over the area other than ample moisture and moderate instability so anything that develops will not be long lived but given the high precipitable water levels, a locally heavy downpour is possible.

 

On the northwestern edge of the ridge, some thundershowers are occurring over West Scotian Slope and approaching Georges Bank and will likely continue to slowly push northward today.

 

Regional Impacts:

 

Nova Scotia: Slight chance of a thundershower.  

 

Labrador: Chance of thundershowers with local downpours.

 

Southwestern marine waters: Thundershowers this afternoon.

 

New Brunswick, Prince Edward Island and Newfoundland: Nil

 

 

 

Roberta McArthur

 

Day 1 Convective Outlook for Atlantic Canada Valid for July 24, 2018

Convective discussion:

 

Risk of non-severe thundershowers over southern Labrador today and tonight.

 

Technical Discussion:

 

A warm front will move into southern Labrador today. Abundant cloud is associated with the warm front so just a chance of embedded thundershowers near and north of the front are expected. Precipital water is 30-50 mm so local downpours in thundershowers are possible.

 

Regional Impacts:

 

New Brunswick: Nil.  

 

Nova Scotia and Prince Edward Island: Nil

 

Newfoundland: Nil.

 

Labrador: Chance of thundershowers with local downpours.

 

 

 

 

 

 

 

Day 1 Convective Outlook for Atlantic Canada Valid for July 23, 2018

 

Convective discussion:

 

Convective activity will be generally low in Atlantic Canada today due to lack of sunshine throughout much of the Maritimes and Newfoundland. However, isolated t-shwrs are likely in NS where sunshine and high dew points initiate convection. Although Cape Breton is not in the minor threat area on the map, any t-shwr activity in Antigonish County could move into Inverness County.

 

Unlike yesterday, Labrador shouldn’t see any significant convection due the passage of a cold front followed by a ridge of high pressure.

 

Technical Discussion:

 

If convection occurs in the Maritimes and Newfoundland, CAPE will be low due to warm air aloft. Shear is low in NS so t-shwrs that occur would be non-severe. Shear is moderate in NB, PE, and NF so longevity and separation of updrafts and downdrafts could occur if t-shwrs formed.

 

T-shwr activity is unlikely in NB, PE, and NF due to cloud and showers and lack of a trigger other than sunshine which generally is not expected or not expected to persist. Sunshine could be a trigger in NS.

 

Regional Impacts:

 

New Brunswick: very unlikely

 

Nova Scotia: Isolated non-severe thundershowers likely Valley and northern mainland.

 

Newfoundland: Unlikely.

Labrador: Very unlikely

 

 

 

 

 

 

 

 

Day 1 Convective Outlook for Atlantic Canada Valid for July 22, 2018

Convective discussion:

High probability of thunderstorms in western and central Labrador. Heavy rain, gusty winds and possible hail would likely accompany these thunderstorms. There is a slight possibility of severe conditions.

Thundershowers will likely persist south of NS and NF with the southern Avalon remaining in the risk of non-severe thundershowers.

Technical Discussion:

CAPE values could exceed 1000J/kg in western Labrador. In addition, 0-6 km shear would be in the 40-50 knot range. Supercells and squall lines are possible.

Regional Impacts:

New Brunswick: Nil

Nova Scotia and Prince Edward Island: Nil, except a very slight risk northern mainland NS if a period of prolonged sunshine occurs this afternoon.

Newfoundland: Chance of a thundershower on the Avalon.

Labrador: Likely.

Day 1 Convective Outlook for Atlantic Canada Valid for July 21st, 2018

Convective Discussion

Isolated thundershowers are occurring near Wabush/Lab City this morning along a frontal boundary that’s sitting across the region. As the boundary slowly progresses east-northeastward today, this activity should continue to progress along that zone. Later today (just south of the front) convection is likely to become more widespread once surface heating is maxed. A weak upper low and associated surface low south of NS will gradually move northeastward today and tonight giving isolated thundershowers to portions of NS and NL.

Technical Discussion

There is a slight risk of severe thunderstorms for extreme western Labrador this afternoon and early evening. Upper air analysis shows a fairly strong jet streak, placing WRN LAB in the right entrance region (though there is no diffluence aloft) There is a weak upper 500 mb trough north of the region but that really doesn’t advect into the region as the day progresses. There is a decent tongue of moisture at 850 analyzed from the Great Lakes into south-central Quebec. The question is how far northeastward will this stretch through the course of the day. The instability is there and 12Z tephis shows about 30-40 knots of shear, in addition the surface boundary being the trigger. Moisture will be the key ingredient to generate strong storms this afternoon. YZV shows good lapse rates in the low-mid levels with drying conditions about 700mb. Adjusting the surface T and Td can generate CAPEs of close to 1000 J/Kg and associated DCAPEs of around 600-800. If cells develop, strong wind gusts (close to 80 km/h), small hail and heavy downpours will be the concern.

The upper level low and associated PVA south of NS will give some isolated thundershowers as it progresses to the northeast (although it will be weakening over time). Precipitable water is quite high so the only concern would be torrential downpours. Even in the presence of embedded TCU, heavy rain can be expected.

Regional Impacts

For Labrador

Possible strong winds, small hail, frequent lightning and heavy downpours for the YWK area. OTWZ isolated thundershowers within the grey zone.

For Nova Scotia and Newfoundland:
Isolated thundershowers giving torrential downpours are possible.

For New Brunswick and PEI:

Nil.

Jeremy

Day 1 Convective Outlook for Atlantic Canada Valid for July 20th, 2018r

 

Convective Discussion

 

Isolated thundershowers are occurring west of Labrador (halfway to James Bay) again this morning. This activity should slowly move eastward during the day. Cumulus clouds are becoming more widespread over central and eastern Labrador. Isolated thunderstorms will likely develop especially over higher terrain.

 

Technical Discussion

 

The cold pool associated with the upper low northeast of Labrador will continue to support convective clouds today. The main focus will be over southeastern regions where surface temperatures will reach 25. Convection will be enhanced by higher terrain especially south of Lake Melville. Moisture in this airmass is limited so the main threat would be brief high winds (gusts around 70km/h or even a little more are possible).

To the west, a low over northern Ontario with a warm front extending eastward continues to support some elevated convection with a help a weak (80kt) anticyclonic upper jet. Some of this dynamic might approach western Lab late in the day but the prime area for surface-based convection will likely remain south of the frontal feature (and out of Lab) where higher wind gusts appear to be the main threat.

 

Regional Impacts

 

New Brunswick, Prince Edward Island, Nova Scotia and Newfoundland

Nil.

 

For Labrador

Lightning and possible gusts for eastern Labrador.

 

 

 

Jean-Marc Couturier

 

Day 1 Convective Outlook for Atlantic Canada Valid for Month Day, Year

 

Convective Discussion

 

Isolated thundershowers are occurring over northern Quebec this morning. Smaller cumulus clouds are widespread across Labrador. Also, Radar is currently detecting an area of rain over southeastern Newfoundland.

 

Technical Discussion

 

A surface low and its associated frontal wave east of Hudson Bay will advance over northern Quebec today. A 20 knot southwesterly low-level jet is supplying ample moisture and instability to support thunderstorms southeast of the low. Temperatures in the 20-24 degree range and dewpoints approaching 15 should be enough to fuel isolated thundershowers with CAPE values in excess of 500 J/kg west of Labrador this afternoon. Some of these storms could propagate over Western Lab with some help from a 40-50 knot westerly upper jet.

 

A frontal band continues to maintain a very moist air mass over southeastern Newfoundland. The sounding from St John’s West indicates a moist profile from the surface up to 850mb and again from 500mb and up, with precipitable water of 47mm! A series of vorticity centres will support isolated thundershowers along the frontal band throughout the day.

 

Regional Impacts

 

New Brunswick, Prince Edward Island and Nova Scotia

Nil.

 

For Newfoundland

Heavy downpours for the southeast.

 

For Labrador

Lightning and possible gusts for western Labrador.

 

 

Jean-Marc Couturier

Day 1 Convective Outlook for Atlantic Canada Valid for July 18, 2018

 

Convective Discussion

 

Convection is occurring ahead of a cold frontal trough stretching from central Labrador south through eastern Nova Scotia toward the southern marine district. The trough will continue to push slowly eastward today to lie over Newfoundland by early tomorrow morning.  Rainfall rates have been the primary concern with this front as it brought close to 50 mm over central New Brunswick yesterday and this morning a heavy band of rain and embedded thundershowers moved over mainland Nova Scotia giving total rainfall amounts greater than 40 mm. Once the band moves off of Nova Scotia early this afternoon, it is expected that the heaviest rainfall rates  will now remain over the southern marine districts but may reach portions of southern Newfoundland.

 

Technical Discussion

 

Over central Labrador and Newfoundland: The dynamics near the low and along the cold front have weakened overnight. There is a strong 250mb jet but is not well aligned to help enhance any convection.  However with cooling heights aloft there is still the risk for non severe thundershowers this afternoon as the front continues to approach Newfoundland. The highest rainfall is forecasted to remain over the marine district but there may still be some high rainfall rates that make it into southern Newfoundland today and overnight. Precipitable water values near 40 mm have already streamed northward toward YJT.

 

For eastern Nova Scotia and the southern marine districts: Heavy showers with embedded thundershowers will continue to push into Cape Breton in the next hour. The heaviest showers are aided by a 30KT LLJ over eastern NS extending northward towards Port Aux Basques. There is a weak low pressure area developing in the trough southwest of Georges Bank and is forecast to move northeastward into the southern slope waters late this evening. There is already frequent lightning near and ahead of the low as it lies still within the warmer gulf stream waters. That activity may continue into the southern slope waters this afternoon and overnight where SSTs continue to be warm. Gusts to 35 KTS are possible over the southern slope waters of Georges Bank and West Scotian Slope  The convection will likely become more isolated as it approaches our cooler waters further north an low level stability increases, however cloud top cooling may provide enough upper level energy to keep convection going overnight. Over the southeastern Grand Banks toward East Coast, there is a bit of upper level instability this morning firing up a few thundershowers but they are not expected to be severe and should diminish during the day.

 

Regional Impacts

 

For eastern Nova Scotia and the southern marine waters

Lightning, heavy downpours and gusts are possible.

 

For Newfoundland

Lightning and heavy downpours for the south coast.

 

For Labrador

Lightning and brief gusts for central and eastern Labrador.

 

 

Roberta McArthur

 

 

Day 1 Convective Outlook for Atlantic Canada Valid for July 17, 2018

 

Convective Discussion

 

An intensifying low pressure system over central Quebec will track northeastward into western Labrador by this evening. Showers and thundershowers have already developed ahead of the cold front this morning over Maine and northwestern NB. Showers with the risk of severe thunderstorms are possible beginning near noon and continuing into the afternoon and early evening as the front progresses eastward. Besides frequent lightning, heavy downpours and strong gusts will be a threat along and ahead of the cold front this afternoon and tonight. Some thundershower activity is likely over western Labrador this afternoon and evening in the vicinity of the low. For our marine area, there is the risk of thundershowers over the Grand Banks today and tonight.

 

Technical Discussion

 

For New Brunswick: The cold front is currently over the St. Lawrence river valley and should reach northwestern NB by this evening. KCAR sounding this morning indicates a capping inversion in the low levels however this is expected to break by the early afternoon with some insolation. There remains quite a bit of cloud cover already pushing into central NB so max heating will be a critical factor of whether severe thunderstorms could develop ahead and along the cold frontal passage. Cooling aloft with the approaching cold front will help to destabilize and continue convection over NB this afternoon ahead of the front. With dew points in the upper teens over northern NB, there will be ample low level moisture for thunderstorms to draw from. Sufficient CAPE values of above 1000 J/kg and moderate shear values of 25KT increasing to 35KT by this afternoon will also help maintain thunderstorms should they continue to develop. With upstream precipital water values of 30-50mm, torrential downpours are possible and some cells could reach severe criteria of 25mm/hr or total rainfall exceeding 50mm. With freezing levels expected to be maintained above 14,000ft, the risk for hail >2cm seems minimal.

 

For western Labrador: There is the slight risk for thundershowers near the vicinity of the low this afternoon but the system will begin to weaken this evening as it moves into central Labrador. With a 30Kt LLJ moving into the area, there is the possibility of gusts reaching 70 km/hr.

 

For the southern marine district: There is a risk for convection over the southern Grand Banks today and tonight. Given the relativity cool SSTs, gusts above 35KTS are not likely to penetrate the stable marine layer.  With the approaching cold front, thunderstorms are possible beginning tonight and overnight for the southeastern marine waters.

 

Regional Impacts

 

For NB

Lightning, locally heavy downpours and brief gusts are possible. There is also the very slight risk of small hail.

 

For Labrador

Lightning, gusty winds and local downpours possible for western Labrador.

 

For the southern marine waters

Lightning, locally heavy downpours and brief gusts are possible.

 

Roberta McArthur

 

 

 

 

Roberta McArthur

 

Operational Meteorologist / Météorologue Opérationnel

Atlantic Storm Prediction Centre / Centre de prévision des intempéries de la Région de l'Atlantique

Environment and Climate Change Canada / Environnement et Changement climatique Canada

3rd Floor, 45 Alderney Drive / 3ième étage, 45 Alderney Dr.

Dartmouth NS B2Y 2N6

roberta.mcarthur@canada.ca

Telephone | 902-426-9200

 

Day 2 Convective Outlook for Atlantic Canada Valid for July 17, 2018

 

 

Convective Discussion

 

Upper level trough over central Quebec and southern Ontario today will be approaching the Maritimes tomorrow. An intensifying low pressure system currently near James Bay is expected to track northeastward into western Labrador by Tuesday evening. Some thundershower activity is likely over western Labrador again tomorrow along the warm from in the vicinity of the low. Warm and humid conditions will continue for the Maritimes ahead of the cold front which is expected to approach northwestern New Brunswick by  Tuesday evening and slowly progress eastward. Showers and thundershowers are expected to develop early afternoon over northwestern NB ahead of the front. Uncertainly lies with the amount of cloud cover ahead of the cold front which could inhibit max heating especially over extreme northwestern NB. Besides frequent lightning, heavy downpours and strong gusts will be a threat along and ahead of the cold front. With upstream precipital water values of 30-50mm, torrential downpours are possible and some cells could reach severe criteria.

 

Roberta McArthur

 

Day 1 Convective Outlook for Atlantic Canada Valid for July 15, 2018

 

Convective discussion:

 

Risk of non-severe thundershowers over southeastern Nfld. and much of New Brunswick.

 

Technical Discussion:

 

In New Brunswick cape values of up to 750 J/kg however very low shear values (ie. No high or low level  jets in the proximity). High freezing level of 12K feet will reduce any hail size.

A convective temperature of 24 degrees is required however there is currently considerable low level cloud in NW New Brunswick  limiting solar heating. No real trigger to initiate convection either. If/when convection is realized the main threat is localized rainfall due to the light flow. Precipital water is 30-40 mm.

 

Regional Impacts:

 

New Brunswick: Possible thundershowers. Possible high local rainfall amounts in heavier showers.  

 

Nova Scotia and Prince Edward Island: Nil

 

Newfoundland: Chance of a thundershower.

 

Labrador: Nil.

 

 

 

 

 

 

 

Day 2 Convective Outlook for Atlantic Canada Valid for July 14, 2018

Convective Discussion

Slight risk for convection later in the day Saturday over northwestern Labrador as a trough moves in from the west. Meanwhile a slow moving trough south of Nova Scotia may generate conditions favourable for thunderstorms over most southern waters of the maritime marine district and possibly into the Newfoundland’s southwestern Grand Banks.

Technical Discussion

An upper trough moving toward Labrador has a history of producing thunderstorms in its wake over northern Quebec. The timing for the passage of this feature over western Newfoundland is not that great and the risk for severe convection during the outlook period for western Labrador is low.  Over the southern Maritime Marine district similar instability remains and upper level triggered thunderstorms are possible with primary threats being wind gusts of 35 knots or slightly more if they form. Of course the mixing of these winds would be subject to the in-situ stability over the marine areas.

Regional Impacts

Newfoundland, Labrador, New Brunswick, PEI, Nova Scotia, and Labrador Marine: No severe convection expected.

Maritime and Newfoundland Marine: Slight risk of thunderstorms around the through over southern waters. Primary thunderstorm threats should they form would be wind gusts near gale force.

Marshall Hawkins

Day 1 Convective Outlook for Atlantic Canada Valid for July 13, 2018

Convective Discussion

No severe convection is expected across Atlantic Canada today with the exception of and increased risk for severe convection over  the southern Maritime  and Newfoundland marine districts.

Technical Discussion

Multiple capping inversions across the region resulting from an upper ridge to the west and a slow moving upper trough over the region will not provide conditions favourable for the development of severe convection over the majority of Atlantic Canada. For the southern Maritime/Newfoundland marine districts, persisting upper level divergence associated ahead of the upper trough will assist in the continued development of convection throughout the period. Aside from lightning,  the primary risks with these cells would be wind gusts near 35-40 knots. The threat of wind is the largest where water temperatures are warmest in the shaded area above.

Regional Impacts

New Brunswick, Nova Scotia, PEI, Newfoundland, Labrador: No severe convection expected.

Labrador Marine: No severe convection expected.

Maritime Marine/Newfoundland Marine: Thunderstorm risk is largest over the southern waters. Primary threat – wind gusts near or exceeding 35 knots (stability dependent w.r.t sea surface temperatures), otherwise, no severe convection expected.

Marshall Hawkins

Day 1 Convective Outlook for Atlantic Canada Valid for July 12, 2018

Convective Discussion

Other than the embedded convection associated with  Post-Tropical Storm Chris, no severe convection is expected across Atlantic Canada today. For details regarding the impacts of Chris, please refer to official forecasts from the Canadian Hurricane Centre, Newfoundland and Labrador Weather Office, and the Atlantic Storm Prediction Centre.

Technical Discussion

Primary threat for severe convection is in the developing frontal regions of Post-Tropical Storm Chris encompassed by the grey area above. The effects generated by these thunderstorms would likely not be a severe departure from the already predicted synoptic conditions. As the warm front develops toward the end of the period, cloud top cooling may begin to play a roll over the Northern Grand banks for the generation of nocturnal thunderstorms.  Otherwise, no significant weather is expected over the remaining regions in Atlantic Canada.

Regional Impacts

Newfoundland:  Heavy embedded showers and isolated thunderstorms associated with the rain bands of Chris.  The impacts driven from synoptic scale processes are more likely to be an issue than an individual embedded thunderstorm cell. Primary threats with these thunderstorms would be wind gusts and downpours, both factors are not likely to be a significant departure from the prevailing weather conditions produced by Chris.

Newfoundland Marine: Heavy embedded showers and isolated thunderstorms expected with the passage of Chris. During the latter hours of this outlook, once the warm front is more developed, the risk of thunderstorms extends northward to the Northern Grand Banks. Primary threat with these thunderstorms would be wind gusts and these gusts are not likely to be a significant departure from the weather synoptic conditions produced by Chris.

Maritime Marine: Heavy embedded showers and isolated CBs over eastern regions associated with the passage of the developing fronts. Primary threat with these thunderstorms would be wind gusts and these gusts are not likely to be a significant departure from the prevailing weather conditions produced by Chris.

Nova Scotia: Moderate to at times heavy embedded showers expected over eastern Cape Breton associated with the passage of Chris otherwise no significant convection expected.

Labrador: Isolated TCU possible over southeastern regions this afternoon. No significant convection expected.

New Brunswick, PEI: No significant convection is expected.

Marshall Hawkins

Day 1 Convective Outlook for Atlantic Canada Valid for July 11, 2018

Convective Discussion

A trough swings through the region and then begins to interact at the mid levels with Hurricane Chris towards the end of the period. Shower or thundershower activity is possible for parts of the Nova Scotia Spine and possibly further up to Cape Breton. Less of a threat of thundershowers moving into central Newfoundland. The most active region, or potential threat, for severe convection is over the southern Nova Scotian Marine district which will be more impacted by the northeastward track currently forecast by the Canadian Hurricane Center.

Technical Discussion

A well structured upper trough moving over the region will pass over Nova Scotia and western Newfoundland during the outlook period. CAPE Values remain low across all the region and can only be modified to produce CAPE values less than 500 J/kg through significant  mechanical lift (via upper level trigger) or unphysical autoconvection temperature for the regions in the slight risk zones. Freezing levels remain above 10k feet for the maritimes with a decrease expected post trough passage, significant hail is not expected and shear values are marginal ahead of the trough.

There is uncertainty as to how this trough will interact with Hurricane Chris toward the end of the outlook period. It will likely begin to undergo deformation during the later half of the period which increases the uncertainty involved with the northward transport of moisture content. This primarily effects the likelihood of stronger convection over the southern Nova Scotia marine district.

Regional Impacts

Newfoundland: Slight chance of a thundershower ahead of the trough later this afternoon mainly confined to the interior. Increased mid level moisture transport ahead of the trough may supply sufficient fuel; however it depends on how much drying occurs as the air mass with more moisture crosses the land. Mid level lapse rates (NWP) are only favourable should moisture increase prior to the trough passage. No severe convection is expected.

Labrador: Southeast of the trough occasional TCU activing with dry gusts possible. Risk of a thundershower is low, isolated CB possible. No severe convection is expected. Northwest of the trough although vorticity remains near the upper low, only TCU generation is expected( and maybe an isolated CB) as there is insufficient moisture for severe convection. Slight risk of a thundershower here. No severe convection is expected.

Nova Scotia: Showers and/or thundershowers possible from the interior of southwestern Nova Scotia northeastward along the spine to and including the highlands of Cape Breton. The chance of severe convection is low; however, thundershowers are possible which would include moderate to heavy downpours. Rates and downpour accumulations are currently expected to remain below warning thresholds. Freezing levels are near 12,000 ft and are expected to drop in the wake of the feature, significant hail is not expected.

New Brunswick & PEI: No significant severe weather is expected. Isolated TCU activity over eastern PEI possible later this afternoon.

Marshall Hawkins

Day 1 Convective Outlook for Atlantic Canada Valid for July 10, 2018

Convective Discussion:

A front east of an occluded low west of Labrador will move east across the Big Land. Ove the Maritimes the same front will drop south over New Brunswick this afternoon, spreading thundershowers south across the province and into Prince Edward Island. There is a possibility of significant weather, mainly downdrafts, for New Brunswick and parts of Southern Labrador, and small hail for Southern Labrador

Technical Discussion:

Currently the cold front is over the St. Lawrence River heading south, and there is another weak trough over Central New Brunswick that has triggered a few cells along and east west line.  Most of the tephis over NB show a weak cap that should break sometime this afternoon with temperatures into the low 30’s, and enhancement with both the trough and front should organise things. Shear values near 50 knots over NB diminishing to about 35 over NS, should stay steady or increase. CAPEs should be healthy, in the 500 to 1000 J/kg range. Moisture is more marginal, with dewpoint  values near 14-17 for NB and PEI. Also precipitable water amounts are in the 20-30 mm range, so not huge.  The big risk here is for downdrafts, probably from mid-afternoon until near sunset. Midlevel cooling as the main front approaches should enhance thing sin the middle to late afternoon as well.

Labrador is similar, with CAPEs in to 500-1000 J/kg range, and shear a little lower. Over the mountains south of Lake Melville the relatively low freezing levels give a risk for hail. This may affect the Trans-Labrador Highway.

The Avalon may see isolated showers into the overnight periods

Regional Impacts:

New Brunswick: Downdrafts possibly to 90 km/h. Thundershowers.

Labrador south: Downdrafts possibly to 90 km/h. Small hail, especially over higher terrain.

Avalon Peninsula: Chance of thundershowers into the overnight period.

Elsewhere: nothing significant

Doug Mercer

Day 1 Convective Outlook for Atlantic Canada Valid for July 9, 2018

Convective discussion

A weak low/frontal feature west of Labrador will move across tonight to lie east of Labrador Tuesday morning.  This system has a cold front associated with it that will drop down over the Norther Gulf  tonight. This will give some thundershowers, and over western Labrador may give significant wind gusts.

Technical discussion

A weak low near the surface is just west of Labrador, with some support from a weak trough extending to mid-levels just west of a thermal ridge. This is moving through western Labrador and cooling aloft is occurring behind it. Moisture is currently marginal, but there are precipitable water amounts of 25-35 mm near the front and low.  Also, an axis of 18 degree surface dew points are approaching from the southwest (south of the cold front), and should start moving into Western Labrador by late this afternoon.

Shear is strong, and there are two midlevel jets approaching western Labrador, one in the extreme southwest and the other further north. Shear values are between 45 to 70 kts.  Based on the soundings and forecast afternoon temperatures, CAPEs in the 500-1000 J/kg range are expected for western Labrador, dropping to 250 over eastern Labrador.

The main threat is downdrafts, possibly reaching 90 km/h for Churchill Falls and west, followed by short intense downpours. Freezing levels of near 10,000 feet allow the possibility of small hail over the western higher terrain and the mountains, but again, moisture levels are limited.

Regional impacts

Labrador:  possibly severe downdrafts Churchill Falls and west . Elsewhere isolated to scattered thundershowers.

New Brunswick/ Northern Gulf of St. Lawrence/Western Labrador Coast/ Atlantic slope waters: isolated thundershowers.

Elsewhere: none.

Doug Mercer

Day 1 Convective Outlook for Atlantic Canada Valid for July 8th, 2018

 

Convective Discussion

In general it will be a quiet day across most of the region with a showers expected along the warm front that will push its way through Labrador, Eastern Quebec and into Western Newfoundland later today. In the short term convection will be somewhat suppressed do to the thermal ridge aloft and fairly low dew point temperatures. Later this afternoon however the dynamics and thermodynamics will become more favourable over western Labrador.  Aloft  the 250 mb jet and 500 mb jets will begin to strengthen with strong diffluence at the upper levels. As well the thermal ridge will continue to track east and we will get into some cooling aloft later today in the order of 2- 4 degrees at 500 mb, this along with the low level moisture advection will aid to destabilize the atmosphere. With the strength of the low level winds >40 kts and the impending thunderstorms, it is not unlikely that wind gusts will exceed warning criteria west of Labrador. Close attention should be paid to the speed and timing of thunderstorms that may move into western Labrador early this afternoon or evening. Should thunderstorms be very active wrt lightning watches may be required.

 

 

Regional Impacts

 

For Labrador

 

Strong winds and heavy rain possible should thunderstorms approach Western regions later today.

 

For PEI, NB, NS and Newfoundland

 

No Impacts expected.

 

Mel Lemmon

 

 

Day 1 Convective Outlook for Atlantic Canada Valid for July 6, 2018

Convective discussion:

 

A cold front will sweep across the region today. Embedded thundershowers with frequent lightning and downpours are expected ahead and near the front.

 

Technical Discussion:

 

A cold front with embedded thundershowers will track across Atlantic Canada today and tonight. Abundant cloud near and ahead of the front is limiting Cape values to below 500 J/kg. Precipital water values of 40-50 mm are producing torrential downpours. Frequent lightning is also a concern. A low level southwesterly jet of 60 knots may give gusty winds in thundershowers.  

 

Regional Impacts:

 

New Brunswick, Nova Scotia and Prince Edward Island: Heavy downpours and frequent lightning. Gusty southwest winds.

 

Newfoundland: Chance of thundershowers.

 

Labrador: Nil

 

 

 

 

 

 

 

Day 1 Convective Outlook for Atlantic Canada Valid for July 5, 2018 - Resent for missing valid date

 

Convective discussion:

 

Some isolated thunderstorms are possible this afternoon and evening associated with a warm front extending from central Quebec to the Gulf of St. Lawrence pushing northeastward as well as within the warm sector over New Brunswick and Prince Edward Island.

 

Technical Discussion:

 

A low pressure system over Hudson Bay will slide eastward today. Showers with embedded thundershowers are occurring this morning over Quebec extending southeastward towards northern New Brunswick and will continue to push to the northeast In the afternoon and evening, these showers with the risk of thundershowers are possible as the front moves into southwestern Labrador stretching southeastward into the Gulf of St. Lawrence. The cold front will quickly approach southwestern Labrador by midnight tonight and will reach northwestern New Brunswick by early Friday morning. Heavy downpours are possible with this cold frontal passage but as it is moving through the area in the overnight hours, the severe potential is less.

 

The Maritimes remain in a hot and humid air mass today.  Conditions will be unstable this afternoon with interior temperatures expected to be in the low 30s. The adjusted KCAR sounding is showing CAPE values between 1500 and 2000 J/kg,  however conditions remain fairly dry in the low levels and there is a lack of sufficient low level wind shear and upward forcing for there to be any severe potential... A sea breeze is expected to set up this afternoon which will likely be the main enhancement to help start up convection over eastern New Brunswick. Precipital water values of 35mm are the area, so heavy downpours are possible should convection initiate.

 

Regional Impacts:

 

New Brunswick and Prince Edward Island PEI: brief heavy downpours and gusty winds in isolated cells.

 

Nova Scotia: Brief showers mainly along the spine.

 

Labrador: Heavy downpours and gusty winds, especially along the cold front overnight for southwestern Labrador.

 

Newfoundland: Nil.

 

 

Roberta McArthur

 

 

 

 

Day 1 Convective Outlook for Atlantic Canada Valid for Month Day, Year

 

Convective discussion:

 

Some isolated thunderstorms are possible this afternoon and evening associated with a warm front extending from central Quebec to the Gulf of St. Lawrence pushing northeastward as well as within the warm sector over New Brunswick and Prince Edward Island.

 

Technical Discussion:

 

A low pressure system over Hudson Bay will slide eastward today. Showers with embedded thundershowers are occurring this morning over Quebec extending southeastward towards northern New Brunswick and will continue to push to the northeast In the afternoon and evening, these showers with the risk of thundershowers are possible as the front moves into southwestern Labrador stretching southeastward into the Gulf of St. Lawrence. The cold front will quickly approach southwestern Labrador by midnight tonight and will reach northwestern New Brunswick by early Friday morning. Heavy downpours are possible with this cold frontal passage but as it is moving through the area in the overnight hours, the severe potential is less.

 

The Maritimes remain in a hot and humid air mass today.  Conditions will be unstable this afternoon with interior temperatures expected to be in the low 30s. The adjusted KCAR sounding is showing CAPE values between 1500 and 2000 J/kg,  however conditions remain fairly dry in the low levels and there is a lack of sufficient low level wind shear and upward forcing for there to be any severe potential... A sea breeze is expected to set up this afternoon which will likely be the main enhancement to help start up convection over eastern New Brunswick. Precipital water values of 35mm are the area, so heavy downpours are possible should convection initiate.

 

Regional Impacts:

 

New Brunswick and Prince Edward Island PEI: brief heavy downpours and gusty winds in isolated cells.

 

Nova Scotia: Brief showers mainly along the spine.

 

Labrador: Heavy downpours and gusty winds, especially along the cold front overnight for southwestern Labrador.

 

Newfoundland: Nil.

 

 

Roberta McArthur

 

Convective discussion:

 

Some isolated thunderstorms are possible this afternoon and evening associated with a warm front extending from central Quebec to the Gulf of St. Lawrence pushing northeastward as well as within the warm sector over New Brunswick and Prince Edward Island.

 

Technical Discussion:

 

A low pressure system over Hudson Bay will slide eastward today. Showers with embedded thundershowers are occurring this morning over Quebec extending southeastward towards northern New Brunswick and will continue to push to the northeast In the afternoon and evening, these showers with the risk of thundershowers are possible as the front moves into southwestern Labrador stretching southeastward into the Gulf of St. Lawrence. The cold front will quickly approach southwestern Labrador by midnight tonight and will reach northwestern New Brunswick by early Friday morning. Heavy downpours are possible with this cold frontal passage but as it is moving through the area in the overnight hours, the severe potential is less.

 

The Maritimes remain in a hot and humid air mass today.  Conditions will be unstable this afternoon with interior temperatures expected to be in the low 30s. The adjusted KCAR sounding is showing CAPE values between 1500 and 2000 J/kg,  however conditions remain fairly dry in the low levels and there is a lack of sufficient low level wind shear and upward forcing for there to be any severe potential... A sea breeze is expected to set up this afternoon which will likely be the main enhancement to help start up convection over eastern New Brunswick. Precipital water values of 35mm are the area, so heavy downpours are possible should convection initiate.

 

Regional Impacts:

 

New Brunswick and Prince Edward Island PEI: brief heavy downpours and gusty winds in isolated cells.

 

Nova Scotia: Brief showers mainly along the spine.

 

Labrador: Heavy downpours and gusty winds, especially along the cold front overnight for southwestern Labrador.

 

Newfoundland: Nil.

 

 

Roberta McArthur