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

Convective Discussion

Garden variety thundershowers possible for New Brunswick, and significant wind gusts and rainfalls for Northeastern Newfoundland.

Technical Discussion

The main risk is for Newfoundland, especially the northwest. Low level moisture is moving in with the west/southwesterlies, there will be good insolation as the cells move east, and there’s some PVA ahead of a trough approaching from the Gulf of St. Lawrence.  Also, dewpoints  at 12Z were in the 16-17 C range, especially  for Cantral and eastern areas. Precipitable water is in the 30-40 range from the 12Z soundings, and shear is about 40 kts. CAPE could be up to 500 J/kg, and possibly larger if daytime heating is higher than forecast. With dry midlayers and a fairly classic  daytime heating tephi profile, healthy downdrafts are likely, followed by good local downpours. With freezing levels between 110-130 hft, hail is unlikely, especially combined with the relatively low CAPE. Onset should be early to mid afternoon.

For New Brunswick: For this case precipitable water is about 25, the freezing levels are too high for significant hail, and while low levels will become unstable there is an inversion near 14,000 feet that probably will inhibit significant convection. Guidance hints at the inversion gradually eroding away and allowing  some convection to occur. If so, they will be classic popcorn, with some downdrafts and locally heavy showers. Preipitable water is near 24 mm, and shear is near 30 kts.  For Nova Scotia there is a bare chance of an isolated thundershower or two.

Regional Impacts

Labrador: none.

Newfoundland: Thunderstorms over much of the interior, with the northeast having a risk for strong and gusty winds, followed by local heavy downpours. Significant hail is unlikely.

New Brunswick: isolated shallow based cells capped near midlevels.

Nova Scotia: Isolated cells east.

Prince Edward Island: none.

Doug Mercer

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

Convective Discussion

Convective weather is possible for parts of every province, but possible severe weather should be restricted to New Brunswick and offshore waters, mainly for the Southern Scotian Shelf. For New Brunswick the main threats are gusty winds and local downpours. For the Scotian Shelf heavier rainfall is likely and there is a chance if significant gusts for southernmost waters.

Technical discussion

For New Brunswick there is a trough approaching from the northwest with  good insolation near it. While moisture aloft is low, and precipitable water is only about 25 mm, there is fairly good surface moisture. There is moderate straight line shear near 35 kts. If and when cells initiate, this should give showers that might reach warning criteria, and downdrafts, especially as the trough should be an organising feature. Conditions should improve towards evening as the insolation dies away. For Nova Scotia expect continued locally heavy showers. Downdrafts will be inhibited by a near surface inversion.

For the Scotian shelf There is a possibility of downdrafts over southern waters, which may become significant. There’s also precipitable water amounts near 40 mm, which may give significant downpours.

For Newfoundland the profiles are too stable for surface initiation. There may be isolated elevated cells ahead of the trough approaching from the west, combined with some uplift along the south coast.

For Labrador the main likelihood is for the west. Insolation looks good, and there is a trough moving through to enhance convergence. I tried some higher temperatures for La Grande 4 and Goose Bay, and they only give CAPEs to 100-200 J/kg.  If cells appear, weak dry microburst are possible.

Regional Impacts

New Brunswick: Local heavy downpours and gusty winds, gradually diminishing by evening.

Nova Scotia: Thundershowers, and occasional gusty winds

Prince Edward Island: Thundershowers.

Newfoundland: Chance of embedded elevated convection.

Labrador: Thundershowers with gusty winds possible, especially in western Labrador.

Doug Mercer

Day 2 Convective Outlook for Atlantic Canada Valid for June 29th, 2018

 

Day Two Convective Outlook

 

An increase in the low level moisture as well as a cold trough at 500 mb will act to destabilize the airmass tomorrow for central Labrador and much of the Maritimes. New Brunswick will see the highest potential for severe weather with surface CAPES over 1500 J/kg and Precipitable Water values > 30mm. It is likely that heavy downpours could become an issue  in thunderstorms and to a lesser degree there is some potential for squall line development through the afternoon which could bring strong winds as well. Areas that receive large amounts of rain tonight will be under the greatest threat should thunderstorms develop tomorrow.

 

Mel Lemmon

Day 1 Convective Outlook for Atlantic Canada Valid for June 18th, 2018

 

Convective Discussion

 

There are a couple areas of interest today one over Western Labrador the other over the southwestern portion of the Maritimes. Over Western Labrador the dynamics for severe weather are favourable this morning but becoming less of an issue later in the day.  Non the less there is still some potential for thunderstorms with Gusts in the 60-70 km/hr range mainly do due to the combination of developing downdrafts and mixing of stronger winds aloft. Just west of Labrador there is an indication that dry microbursts could develop but this airmass is not likely to move over Labrador until tonight. Convective rainfall is not expected to be an issue.

 

Over Southwestern NS and NB embedded TS are possible beginning later this evening and into tomorrow morning. This is likely cause some heavier rainfall amounts in thunderstorms and possibly localized flooding.

 

Regional Impacts

 

For Labrador

Gusty winds possible for western Labrador should TS develop this afternoon and early this evening.

 

NS and NB

Embedded TS causing heavier downpours in rain that will continue through tonight.

 

For PEI

No impacts expected.

 

For Newfoundland

Very low probability of Convective Showers becoming TS early today just north of the island.

 

Mel Lemmon

Day 1 Convective Outlook for Atlantic Canada Valid for June 27th, 2018

Convective Discussion

Most of the region will see stable conditions today with very little potential for convective weather to develop. There is however some favourable dynamics coming together over central Newfoundland for this afternoon and early this evening. The main two being the upper level Jet and the slow moving surface trough slowly crossing over the island later today. In general low level moisture is not overly favourable but afternoon dew points could reach 12 degrees which given sufficient solar insolation could generate some weak thunderstorms. The overall balance between shear and updrafts will likely not be sufficient for any organized convection however given the very low freezing levels (< 9000 ft AGL) small hail is likely. Thunderstorms should they develop should be concentrated just ahead and in the surface trough where convergence will be a contributing factor.

Regional Impacts

For New Brunswick.. Nova Scotia.. PEI and Labrador.

No Impacts Expected.

For Newfound

Should thunderstorms develop this afternoon no significant impacts are expected however local rainfall ~ 10mm with small hail is possible.

Mel Lemmon

Convective Outlook for Atlantic Canada Valid for June 25th, 2018

Convective Discussion

Technical Discussion

Two areas will have a chance of convection, for northwestern New Brunswick for this afternoon ending this evening, and for the Atlantic coasts of Newfoundland and Nova Scotia.

For New Brunswick there is marginal moisture and low CAPEs from the 12Z Caribou sounding. But there is an upper level  thermal  trough approaching from the northwest giving some cooling aloft, combined with some good insolation and a weak trough  as a trigger.  Showers should not be heavy, but the cells are moving slowly, so there is a chance of training.. Given high shear near 60 knots and low CAPEs, warning level  downdrafts are unlikely, but there may be some good gusts .

For Atlantic  waters there is a developing low moving across the Scotian Shelf to lies near the Southwestern Grand Banks. Isolated thundershowers are occurring near and a little west of the low center, and are mainly elevated convection. Precipitable water is good, between 35-40 mm. Heavy showers are likely offshore, but gusty winds due to convection are unlikely. Leave that for the main low.

Regional Impacts

New Brunswick: a few thundershowers with a chance for gusty winds northwest.

Nova Scotia: slight chance of thundershowers along the Atlantic Coast this afternoon into the evening.

Newfoundland: slight chance of thundershowers for southern Burin Peninsula and southern Avalon Peninsula tonight into Tuesday morning..

Doug Mercer

Day 1 Convective Outlook for Atlantic Canada Valid for June 23rd, 2018

 

Convective Discussion

 

A ridge of high pressure will continue to maintain fair conditions over the region today. Moisture from a frontal system currently south of New England is starting push northward. The leading edge of the high cloud is already over the Maritimes and it should move over southern Newfoundland this afternoon. The stream of moisture, along with some instability, will reach Nova Scotia tonight bringing an increase in the likelihood of thundershowers. Isolated thundershowers are expected over westernmost regions later this evening, and after midnight for the rest of southwestern shore.

 

Regional Impacts

 

Localised downpours giving 20+mm are expected over western Nova Scotia.

 

 

 

Jean-Marc

 

Day 1 Convective Outlook for Atlantic Canada Valid for June 22nd, 2018

 

Convective Discussion

 

Very quiet day across the region with no thunderstorms expected as a ridge of high pressure builds in today through the Maritimes. A trough of low pressure will bring gusty winds and showers to western Labrador however thunderstorms are not expected.

 

Regional Impacts

 

No impacts do to convective weather expected.

 

Mel Lemmon

 

 

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

 

 

Convective Discussion

Alas the first day of summer is here, however this is not reflected in the weather and this is true for convective weather as well. A cool somewhat dry airmass will move across the region today, there is a very low chance that TCU’s could become CB’s over western NFLD early this afternoon as the cold pool moves westward today. By late this afternoon surface moisture will begin to deplete and deep convection will become less likely.

 

 

Regional Impacts

 

Labrador… NB… PEI… NS… No Impacts expected

 

NFLD… There is a greater than zero probability that low topped thunderstorms  could pop up. Brief showers with nil sig weather expected should they happen.

 

Mel Lemmon

Day 1 Convective Outlook for Atlantic Canada Valid for June 19th, 2018

 

Convective Discussion

In general the potential for thunderstorm development is quite low across the region today. There is a slight chance of TCU’s developing just behind the through while low level moisture is still present. There is a slight chance that TCU’s over western Newfoundland could develop into weak thunderstorms. There will likely be some cooling aloft which will aid in destabilizing the atmosphere, this along with some solar insulation and convergence in the general trough area could generate some weaker updrafts. The risk may continue into the evening along a secondary surface trough.  

 

Regional Impacts

 

NB PEI and NS

No impacts are expected.

 

For Newfoundland

Isolated CB possible this afternoon and early this evening for western and central parts of the island. No hazardous weather expected.

 

Mel Lemmon

Day 2 Convective Outlook for Atlantic Canada Valid for June 18, 2018

Convective Discussion

Two “waves” of thundershowers or thunderstorms are possible on Monday for the Maritimes. No significant convection expected for Newfoundland or Labrador.

Technical Discussion

A two event scenario is looking more likely as the 12Z guidance from both the RDPS and the GFS are keying in on a warm-sector trough moving through the region prior to the passage of the primary cold front.  

<  (1)     Morning warm sector low-to-mid level trough…

Confidence in the location of convection with the passage of this warm-sector trough is low. The new foreign and domestic NWP timing of the warm-sector trough that is calculated to arrive near 12Z over western New Brunswick is consistent with the previous 06Z adjustment  (see previous convective outlook); however, the parameterized thunderstorm activity is not in agreement. The 12Z GFS maintains a solution with convection traveling eastward through northern New Brunswick while the 12Z regional, valid at the same time, has its “thunderstorms” over Fredericton and Southern York County. 12Z 1.5 PVU analysis for the upstream conditions around the upper trough associated with the surface cold front is acceptable; however, no extra confidence was gained on trying to pint-point the leading warm-sector trough, so confidence remains low on that feature. Should thunderstorms form with this feature there is a risk of severe convection associated with this trough. A low level jet near 50 knots at 850 hPa combined with NWP dewpoints near 12 degrees over western and northwestern New Brunswick.  Main threats with this feature – should one form – would be significant rainfall eclipsing 25 mm/h rainfall rates, and peak wind gusts near 50 knots. Givin the increasing shear scenario, the chance of a supercell forming along this trough line is increasing but remains minimal as modified NWP soundings are generating only 500 J/kg with NWP 0 to 6 km shear near 45 knots.  To reitorate, confidence with the dynamics of this feature is low.

   (2)  Late afternoon / evening primary cold front passage…

The primary cold front is expected to begin its descent over the region late Monday afternoon or early Monday evening – there remains some discrepancy with the timing across foreign and domestic solutions. Ahead of this feature projected afternoon dewpoints for the majority of New Brunswick are near 16 degrees, and similar for the Cobequids for NS and Prince County, PEI. Dew points in excess of 20 degrees are calculated by some NWP for western New Brunswick. Lifted indecies for Monday afternoon over New Brunswick range from 0 to -2 over western New Brunswick, with the odd -4 over Maine. CAPE values relative to the 12Z RDPS run are suggesting sub 500 J/kg for New Brunswick by 18Z Monday afternoon, with between 500 to 1000 J/kg of CAPE for central and western Maine. The largest uncertainty with the formation of late afternoon convection ahead of / associated with this descending cold front is the cloud coverage after the passage of the morning warm-sector trough and the timing of the cold front. Naturally,should an increased cloud coverage scenario be incorrect and more insolation is available then the thunderstorms will be more likely to form, CAPEs will be larger (slight modifications to some NWP soundings extend the 500-1000J/Kg swath further eastward into New Brunswick and produce near 1200-2000 J/kg westward), and the probability of more significant severe convective weather will increase. Zero to 6 km shear remains near 50 knots…therefore their remains a risk of supercell formation ahead of/associated with this feature. Furthermore, should the leading edge of the cold front be faster, as the 12Z GFS is currently suggesting, a more significant afternoon scenario could emerge for New Brunswick, but again, it all depends on the cloud coverage in the wake of the initial trough.

Final notes: Freezing levels are expected to remaind near 13,000 feet for the majority of NB so the threat of hail is not as signficiant as torrential downpours. NWP generated helicity ahead of this cold front fluctuates from sever to non-severe. Confidence is low in the RDPS NWP helicity; that being said, super cell composite parameters from the 00Z RDPS run are near 30 for northwestern New Brunswick Monday afternoon. If the cloud cover is sufficiently low, and all other energetics remain the scame, there will like be an active cold front scenario producing longer lived, multicelled line of thunderstorms with strong wind gusts, and torrential down pours. If the cloud cover remains too high, conditionally-stable to stable low levels would inhibit surface based convection ahead of the cold front. Along the cold front, heavy showers and scattered thundershowers are still likely, but the more severe threat would like remain over Maine, where on average, cloud cover is expected to be lower, and convective indecies are more significant.

The graphical outlook above is assuming a cloud cover scenario that is more condusive to thunderstorm development.

Regional Impacts

For Newfoundland: Nil significant convective weather.

For Labrador: Slight risk around the upper sfc/upr low Monday aft/evening over south western labrador (primarily south of Labrador city and Churchill falls). Severe convection is not expected.

For PEI and Nova Scotia: Possible thundershowers (perhaps a thunderstorm) associated with the passage of that mid level trough. Pimary impacts should convection materialise would be high rainfall rates, potentially exceeding 25 mm/h, and gusty winds potentially exceeding 40 knots. Thundershowers and Thunderstorms possible Monday evening into Tuesday morning with the passage of the cold front. Primary threats with these features would be high rainfall rates, and gusty winds.

For New Brunswick: Thundershowers or thunderstorms possible Monday morning depending on the vigor of the warm-sector trough. Primary threats with this feature would be strong wind gusts near 50 knots , and downpours generating  rainfall rates in excess of 25 mm/h.  Similar conditions for the passage of the primary cold front, should the cloud cover regime be conclusive to thunderstorm formation.

Marine Impacts: Risk of thunderstorms/thundershowers associated with the warm-sector trough, as well as the passage of the cold front. Primarily for western and southern gulf waters.

Marshall Hawkins

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

 

Convective Discussion

 

No significant convective weather is expected for Atlantic Canada over the next 24 hours; however, there is a risk of thundershower activity over New Brunswick overnight tonight and into Monday morning as conditions favouring the development of thundershowers move into the region.  In the short term over the marine districts, no signficiant convective weather is expected but there is a risk of thundershower activity along the wave moving through the southwestern grand banks.

 

Technical Discussion

 

A relatively dry air mass combined with surface and upper lever ridging over the region will inhibit the deveopment of severe convective weather today for Atlantic Canada.  However, the ridge begins to degrade the fastest is over northwestern New Brunswick as a slow moving warm front will begin to track into the region later today… and will usher in larger dewpoints along with increasing 0 to 6 km shear. Recent changes in numerical weather guidance suggest that a weak trough will approach the western edge of the province prior to 12Z. This adjustment is met with some skepticism as there are currently thunderstorms active along its leading edge which may affect the NWP’s ability to project the true timing and subsequent arrival.  Should a severe (or near-severe) thunderstorm form near 12Z over northwestern New Brunswick, the primary threats would be high rainfall rates and strong wind gusts near or exceeding 35 knots.

 

There is an increasing probability of conditions favouring severe convection moving into New Brunswick Monday afternoon…a day 2 outlook will be issued later today.

 

Regional Impacts

 

Newfoundland and Labrador:  Nil significant convective weather.

 

Nova Scotia and PEI: Nil significant convective weather.

 

New Brunswick: There is a risk for non-severe convection over much of the province overnight tonight and into Monday morning as conditions favouring the development of thundershowers move into the region from the west. Over extreme northwestern New Brunswick there is a very slight risk of a severe thunderstorm by 12Z on Monday morning. At this time a severe thunderstorm watch for Monday morning is not anticipated due to lack of evidence supporting this more rapid eastward movement of the afformentioned trough; however should subsequent observations indicate that this trough is moving faster, adjustments to the forecast may be made. Should a faster scenario arise, primary threats for this region include downpours with rates near 25 mm/h and wind gusts near 35 knots.  There is an increasing risk for severe convection Monday afternoon, please refer to the day 2 outlook (to be published later today).

 

Atlantic Marine Areas:

Chaleur-Miscou: Nil significant convective weather expected, chance of thundershowers tomorrow morning over the Baie de Chaleur, with a larger risk in the afternoon (refer to day 2 outlook when available).

Southwestern Grand Banks: No significant convective weather expected, chance of thundershowers along the wave as it continues to move eastward today.

                    

Marshall Hawkins

 

Operational Meteorologist – Atlantic Storm Prediction Centre, Meteorological Service of Canada

Environment and Climate Change Canada / Government of Canada

Marshall.Hawkins@canada.ca  / Tel: 902-426-7779

 

Météorologue opérationnel – Centre de prévision des intempéries - région Atlantique, Service météorologique du Canada

Environnement et Changement climatique Canada / Gouvernement du Canada

Marshall.Hawkins@canada.ca / Tél. : 902-426-7779

 

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

Convective Discussion

Isolated thundershowers continue just south of the Grand Banks today.

Technical Discussion

The upper low which was located over southern Quebec yesterday has opened-up into a large trough over NF and is currently connected to another upper low over the Labrador sea. A frontal system oriented NE-SW extends over the southeastern Grand Banks. A strong upper jet along with some vorticity continue to support a few clusters of thundershowers. This activity will continue this afternoon near/over the southern Grand Banks. The St John’s tephi shows very stable conditions in the low levels and dry air above with pecipitable water of nearly 25mm.



An area of showers over New England is supported by another jet branch descending from southern Ontario. A cold pool will advance and push this precipitation over Georges Bank this afternoon and evening.

Regional impacts

NS/NB/NL/PEI/LAB: nil sig weather.

Jean-Marc

Convective Outlook for Atlantic Canada Valid for June 14th, 2018

Convective Discussion

Isolated thundershowers continue across southern New Brunswick, northern/eastern Nova Scotia, and southern marine waters today. This activity will reach the Grand Banks tonight.

Technical Discussion

An upper low over the Eastern Townships this morning is supporting a surface low pressure system over southern Maine. A large area of rain ahead of these features extends from the Saguenay across the Maritimes to just south of Newfoundland. Based on the 12z sounding from Gray, Me, the instability in the cold pool is sufficient to supports elevated CB’s based around 10,000ft with tops above 35,000. Latest radar echo top products is a small cluster of thundershowers with these characteristics just north of the Minas Basin. Some of the soundings this morning showed precipitable water of 25-35mm. Widespread rainfall totals of 10-15mm have been reported , with peak hourly amounts of 11.5mm reported at YCX earlier this morning.

Regional impacts

NS/NB/NL/PEI/LAB: Brief localized mod-heavy rain..

Jean-Marc

Day 1 Convective Outlook for Atlantic Canada Valid for June 12th, 2018

Convective Discussion

No convective weather expected across Atlantic Canada today.

Technical Discussion

A low pressure system over western Labrador will track southeastward today. A trailing cold front may give thundershowers this afternoon or evening over regions near and north of the Gaspe Peninsula. CU development is expected over portions of eastern NFLD and LAB today but the airmass is dry so just a few showers will be a result. Synoptic rain and snow is expected over western LAB closer to the low.

Regional impacts

NS/NB/NL/PEI/LAB: Nil.

Andy

Day 1 Convective Outlook for Atlantic Canada Valid for June 11th, 2018

Convective Discussion

No significant weather expected across ATL CAN.

Technical Discussion

Upper level trough remains over the region today with cold, dry air in the low levels. Strong capping associated with an approaching area of high pressure is limiting the cloud development to CU and SC. There is a zero percent chance of TS activity today.

Regional impacts

NS/NB/NL/PEI/LAB: Nil.

Andy

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

Convective Discussion

This is a very low probability of non-severe thunderstorms over southern New Brunswick and Western PEI late this afternoon ahead of an approaching cold front. For Eastern PEI and Northern Nova Scotia this risk will continue into this evening. There is a very slight chance of weak embedded thunderstorms with enhanced rainfall amounts possible should they develop.

Technical Discussion

Overall the amount of low level moisture and instability is not favorable for the development of thunderstorms with most unstable CAPE values < 250 J/kg. However with the presence of the approaching cold front stretching from Labrador down into New Brunswick as a trigger and an approaching cold trough aloft there is a slight risk that thunderstorms may develop. This is risk may also be enhanced slightly over areas of higher terrain and max daytime heating later today.

Regional Impacts

New Brunswick: Showers and risk of weaker thunderstorms over southern New Brunswick beginning late this afternoon. No significant impacts expected.

Nova Scotia: There is a slight risk of weak thunderstorms late this afternoon and this evening over parts of northern Nova Scotia. No significant impacts expected.

Prince Edward Island: There is a slight risk of weak thunderstorms late this afternoon and this evening. No significant impacts expected.

Newfoundland and Labrador: There is a slight risk of weak thunderstorms late this afternoon for southern Labrador and this afternoon and  evening for western Newfoundland. No significant impacts expected.

Mel Lemmon

Day 1 Convective Outlook for Atlantic Canada Valid for June 7, 2018

Convective Discussion

 

There is a risk for non-severe thundershowers over northern New Brunswick late this afternoon into the evening and overnight with an approaching low pressure system over central Quebec tracking eastward today. Elsewhere no significant convection is expected with a ridge of high pressure stretching from Labrador to Nova Scotia keeping conditions dry.

 

 

Technical Discussion

 

The warm front in association with the low pressure system over Quebec has pushed north of New Brunswick. The ingredients necessary to initiate and maintain convection this late this afternoon over northern New Brunswick should be present. In the upper levels, there is an approaching  80KT 250mb jet and northern New Brunswick lies in the favourable left exit region. The 500mb level has increasing diffluence, slightly cooling temperatures, as well as increasing 700 and 850mb warming will help to destabilize the air mass. A trough within the warm sector is also expected to push eastward over Northern New Brunswick tonight to kick start the convection.  The biggest inhibitor are the current relatively dry low levels and moist upper levels. Upstream precipital water values are in the range of 15-20mm. There is also widespread cloud cover currently from Quebec into western New Brunswick which may delay convective initiation if convective temperatures are not reached. If they do, only CAPE values maxed near 500 J/kg will be reached. Freezing levels are low so there is the risk of small hail. 

 

Regional Impacts

 

New Brunswick: Showers and risk of thundershowers over northern New Brunswick beginning late this afternoon and may continue into the early overnight hours. Some local heavy downpours, gusts to 50 km/hr, and small hail (<2cm) are possible.

 

Nova Scotia: None

 

Prince Edward Island: None

 

Newfounland and Labrador: None

 

 

Roberta McArthur

 

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

 

Convective Discussion

 

Nil significant convective weather is expected across Atlantic Canada today. Isolated to scattered shower development is expected over central New Brunswick this afternoon and early this evening. Slight risk of convection this afternoon over southern Nova Scotian waters and late this evening/overnight over southern Newfoundland waters  as a weak low pressure system currently over southwestern Nova Scotian waters will slowly track northeastward throughout the period.

 

Technical Discussion

 

Ridging moving into the region from the northwest, and multiple low pressure systems crossing the southern marine districts of both NS and NL, will generate conditions that are not condusive for the production of severe convection over the provinces today. Capping exists throughout the region and although some dynamic forcings are present, they are not expected to be strong enough to erode the capping inversions. Moisture across the region is also generally low, with the most moisture being confined well south of the Atlantic Provinces and near the Gulf Stream. There is an upper trough that is currently moving through northwestern New Brunswick and is expected to help generate TCU across central and western regions of the province this afternoon. Showers that will develop with the passage of this feature are expected to be generally light with an isolated moderate shower where day time heating and moisture availability is largest.  

 

Regional Impacts

 

Newfoundland and Labrador: Nil sig severe convective weather is expected today.

 

PEI and Nova Scotia: Nil sig severe convective weather is expected today.

 

New Brunswick: Showers associated with the passage of an upper trough are expected to develop this afternoon. No significant convective weather is expected to develop this                afternoon with the passage of the aforementioned feature.

 

Southern Nova Scotia and Southern Newfoudland marine districts: Slight chance of thunderstorms this afternoon associated with the passage of the afforementioned low. Primary regions of potential development are over the southern marine waters with the warmest sea surface temperatures, from Georges Bank to East Scotian Slope. Slight chance of nocturnal thunderstorms associated with the passage of this low over Laurentian fan late this evening and overnight for extreme southern portions of the Southwestern Grand Banks. Should thunderstorms develop, their primary threats would be wind gusts near 35 knots.

 

 

Marshall Hawkins

Day 1 Convective Outlook for Atlantic Canada Valid for June 5, 2018

Convective Discussion

 

Nil significant convective weather is expected across Atlantic Canada in the short term.  Isolated thunderstorms / thundershowers are expected to continue across parts of the southern Nova Scotia marine district today, and possibly over the eastern Nova Scotian and southern Newfoundland marine district overnight.

 

Technical Discussion

 

Multiple capping inversions remain across the region, and only thin regions of potential instability exist across the majority of actual sounding sites throughout Atlantic Canada. To overcome these various caps, significantly more moisture and daytime heating is required than is currently forecast; therefore no significant convective weather is expected across any of the provinces today.

 

The primary threat area to produce severe convective weather for the marine districts stems from the passage of a frontal wave and associated low pressure system that is forecast to continue to move northeastward, crossing from the southern NS marine district to the southern NL marine district. Current thunderstorm activity associated with this wave is producing isolated cells along the border of Sable and East Scotian Slope with cloud tops near 37,000ft (radar observed, consistent with RDPS NWP), surface wind observations through SAR/ASCAT have not been able to produce a measurement of this activity as of the time of this bulletin. Isolated cells associated with this wave are expected to continue today, and may intensify slightly overnight as cloud top cooling ahead of the warm front begins to play more of a role throughout the nocturnal hours.

 

Although there is a capping inversion across the marine district that would inhibit significant surface based convection, a strong low level jet is present below this inversion. The presence of this strong LLJ along with an unstable lower boundary layer may produce gusts near 45 knots should a cell form over the most unstable regions associated with the warmest Gulf-Stream waters for the southern Nova Scotia marine district. Given the transition to cooler water as this system moves into the Newfoundland marine area of responsibility, the persistance of thunderstorms would rely upon continual Warm Air Advection (WAA) and cloud top cooling (CTC) – the marine inversion would remain strong and likely prevent significant gusts from reaching the surface.

 

Overnight a second low pressure system may produce nocturnal thunderstorm activity over the sotuhwestern slope waters of Nova Scotia as positive voriticity advection along with WAA and CTC will be present over relatively warm waters. It is possible that thunderstorm activity that is expected to occur over eastern New England this afternoon may persist as this low moves off shore and eventually crosses into the south western NS marine district. Threats with these cells would be primarily wind gusts exceeding 35 knots.

 

Regional Impacts

 

Newfoundland and Labrador:  No significant convective weather expected.

 

New Brunswick, Pei, and Nova Scotia: No significant convective weather expected.

 

Southern Nova Scotia and Southern Newfoundland Marine districts: Risk of thunderstorms activity throughout the remainder of today for NS and into tonight southern NFLD Marine (21 UTC onward). Gusts today for NS/NL may peak near 45 knots – which is not entirely significant compared to the currently forecasted gale-force conditions from either issuing office. Squall watches are not expected to be required at this time.

 

Marshall Hawkins

 

 

Day 1 Convective Outlook for Atlantic Canada Valid June 4th, 2018

Convective Discussion

A ridge of high pressure entrenched over much of Atlantic Canada is expected to inhibit any severe convective weather for the immediate short term. Overnight tonight, a low south of Cape Cod is expected to migrate northeastward and track south of Nova Scotia – as it does so there is a slight risk for convection over southwestern Nova Scotia and across the marine regions south of the province and in the vicinity of the low centre.

Technical Discussion

An exceptionally dry air mass (throughout the majority of the column) stretching from western and central Labrador to Nova Scotia will inhibit the development of any severe convective weather through max daytime heating today and into this evening for all of Atlantic Canada. Although some weak thundershower activity has been associated with the approaching trough over New England and Southern Ontario (Ottawa River Valley) no significant convective weather is expected to move in over the Maritimes with the approach of this feature due in part to the aforementioned dry air mass. There is however a slight risk for nocturnal convective activity for Nova Scotia (and NS Marine) as  a developing low pressure system currently south of Cape Cod is expected to track northeastward throughout the period and upper level vorticity combined with a thermal ridge (and corresponding moisture axis as well as cloud top cooling) are expected to produce conditions favorable for isolated elevated nocturnal convection.

Newfoundland and coastal Labrador remain in a synoptic flow that is not conducive to the development of severe convective weather.

Regional Impacts


Newfoundland & Labrador: None via convective severe weather.


PEI: None via convective severe weather


New Brunswick: Possible moderate showers via ACC associated with the passage of the trough near 03 UTC and onward...otherwise, nil convective severe weather.


Nova Scotia & Nova Scotia Marine district: Risk of isolated nocturnal thundershowers/thunderstorms after 03 UTC. Should one develop, the primary risk (for the strongest cells) would be gale force wind gusts near 40 knots over southern waters where water temperatures are warmest.

Marshall Hawkins

Day 1 Convective Outlook for Atlantic Canada Valid for June 3, 2018

Convective Discussion

Technical Discussion

There is a low southeast of the Avalon slowly moving away to the east. Further west is an upper level jet moving in ahead of ridging, and the left exit of the jet may give some upper lift. While there is no chance of surface forcing with a strong inversion and cold water, above 850 mb stability is less than moist adiabatic.

Regional Impacts

Minimal impacts over the southern Grand Banks, mostly isolated thundershowers possible near and west of the low center.

Doug Mercer

Day 1 Convective Outlook for Atlantic Canada Valid for June 2, 2018

Convective Discussion

 

Technical Discussion

A trough of low pressure could initiate some mid level convection over the contoured area today.  The upper levels are quite warm so if convection does get initiated, CAPE values of up to 200 J/kg could be realised but higher CAPE values are not likely so severe thunderstorms are not expected.

 

Regional Impacts

None

 

Barrie MacKinnon

 

Day 1 Convective Outlook for Atlantic Canada Valid for June 1, 2018

 

Convective Discussion

 

A warm and increasingly humid air mass is approaching New Brunswick today ahead of a low pressure system over Labrador with a cold front extending into central Quebec. The atmosphere will be conducive for the development of thunderstorms in the afternoon given falling 500mb heights, increasing moisture in the low levels and high maximum temperatures. These ingredients along with the approaching cold front will help de-stabilize the air mass and initiate showers and thundershowers over northwestern New Brunswick beginning in the afternoon and spread southeastward into the evening. Due to the high precipital water content in this air mass and not much wind aloft, showers that develop will move slow and local heavy downpours are possible. The risk of thundershowers may continue into Nova Scotia overnight.

 

Regional Impacts

 

The main threat with the thundershowers will be locally heavy downpours (20-40mm possible).

 

 

Roberta McArthur