Day 1 Convective Outlook for Atlantic Canada Valid for September 3rd, 2018

 

Convective Discussion

·         A frontal disturbance over Quebec may bring a  few potentially severe thunderstorms to northwesternmost areas of New Brunswick late in the day-evening and night

·         The same disturbance may trigger isolated thundershowers over parts of Labrador during the day

·         Elsewhere, a quasi-stationary ridge will prevent any convection.

 

Technical Discussion

 

High pressure system continues to prevail over southern Maritimes and Newfoundland. A frontal disturbance west of the region is triggering elevated thunderstorms over southern Quebec and central Labrador this morning. The  thermal ridge associated with this feature will slowly advance over NB today with un-modified SWEAT indices suggesting non-severe thunderstorms. However moisture content will continue to increase in response to a 25-30kt southwesterly low-level jet so that by the end of the day modified indices are indicating a potential for severe thunderstorms especially during the evening and night period. Precipitable water estimates from this morning sounding are already in the 40-50mm range in parts of the risk area (25-30mm over Labrador) resulting in a potential for bouts of heavy showers with amounts approaching warning thresholds.

 

Regional Impacts

 

Northwestern New Brunswick:  Isolated thunderstorms possible during the evening and night.  Heavy downpours are possible (25+mm).

 

Labrador:  Isolated thunderstorms possible during the afternoon and early evening. Brief heavier showers and wind gusts to 70km/h are possible.

 

 

Jean-Marc

 

Day 1 Convective Outlook for Atlantic Canada Valid for September 2nd, 2018

Convective Discussion

·         A frontal disturbance over Quebec/New England may bring isolated/non-severe thunderstorms to northernmost areas of the Maritimes later today

·         A cold front over Labrador may trigger thundershowers over southeastern portions this afternoon

·         Scattered thundershowers are continuing over the Gulf Stream mainly south of the Scotian Slope waters

·         Elsewhere, a quasi-stationary ridge will inhibit significant convection.

 

Technical Discussion

 

High pressure system prevails over southern Maritimes and Newfoundland. A frontal disturbance west of the region is triggering elevated thunderstorms (eastern Ontario and Western Quebec this morning). The  thermal ridge associated with this feature will slowly advance towards northern NB later today with K indices suggesting isolated thunderstorms becoming likely especially during the tonight period. Precipitable water estimates from this morning sounding are already in the 30-40mm range in the risk area, and 40-50mm near the thermal ridge resulting in a potential for bouts of heavy showers, especially tonight.

 

The cold front over Labrador is generating some instability with towering cumulus tops extending well above the freezing level, but with limited penetration into the glaciation levels. However, the combination of mediumsurface heating - extra lift provided by the mountainous areas - and the active front could generate a small cumulonimbus capable of giving brief heavier showers and wind gusts to 70km/h. Behind the cold front, westerly gusts of around 80km/h are likely to persist.

 

For slope waters, clusters of convection mainly south of forecast waters will continue. Scattered thundershowers are expected.

 

Regional Impacts

 

Northwestern New Brunswick:  Isolated thunderstorms possible during the late afternoon and evening. Brief heavier showers are possible.

 

Labrador:  Slight risk of a thunderstorms possible during the afternoon and early evening. Brief heavier showers and wind gusts to 70km/h are possible. Very windy in the westerlies behind the cold front with gusts to 80 km/h.

 

Slope waters of the Scotian Shelf and Southern Grand Banks: Scattered thundershowers with no severe impacts expected.

 

 

Jean-Marc

 

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

 

 

Convective Discussion

A cold front approaching Labrador from the west may give a few thundershowers to westernmost regions this evening. There may be some gusts and small hail. Over most other areas a QS ridge will inhibit significant convection. For slope waters, there will be scattered thundershowers for much of the day and tonight.

 

Technical Discussion

 

The cold front will start becoming unstable in western Labrador in the evening (with a slight possibility for late afternoon). PVA will be strongest near 00Z, and some weak lift from the left entrance of the 250 mb jet will help. An associated thermal trough from 700 mb up will give some slight cooling to midlevels as well, but not enough to give significant CAPE. Shear will be on the high side, at 40+ knots, especially relative to low CAPE values expected. However, if cooling aloft is stronger than predicted, the CAPEs may be high enough for a few hundred Joules/kg, giving some local and brief heavy showers of 5-15 mm, and some gusts to 70 kph.  Rainfall estmates are based on a fast moving cell and 30 mm of precipitable water. Freezing levels will be about 9500 AGL (11500 ASL), giving a possibility of small hail.

 

For slope waters, there’s an area of weak embedded convection mainly south of forecast waters, but occasionally creeping into our region. There is low to moderate shear, and about 20-30 mm of precipitable water. The main trigger is weak troughing south of the ridge. Isolated to scattered thundershowers are expected, but, no hail, and wind gusts are inhibited by a warm nose near 850 mb.

Regional Impacts

 

Labrador:  Isolated thundershowers possible mainly this evening west of Churchill Falls and from near Wabush to the north. Possible wind gusts to 70 kph, and a small chance of small hail.

 

Slope waters of the Scotian Shelf and Southern Grand Banks: Isolated to scattered thundershowers. No severe impacts expected.

 

Elsewhere: no thundershowers expected.

 

Doug Mercer

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

 

 

Convective discussion:

 

No convective weather is expected today .

 

Technical Discussion:

 

Drier and stable air in the wake of a cold front will suppress any significant convection today.

 

Regional Impacts:

 

New Brunswick: Nil.

 

Nova Scotia: Nil.

 

Prince Edward Island: Nil

 

Newfoundland: Nil.

 

Labrador: Nil.

 

 

 

 

 

 

 

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

 

Convective discussion:

 

Risk of non-severe thundershowers over Atlantic coastal regions of Nova Scotia and central/Newfoundland this afternoon and evening.

 

Technical Discussion:

 

A cold front with an associated low pressure system is sweeping across Atlantic Canada today. Very moist tropical air (dew points 20 degrees plus) ahead of the front get replaced by much drier and stable air in its wake. Morning soundings are not showing much CAPE ahead of the front so just a risk of a thundershower is expected ahead of this fast moving feature. The thundershower risk is surface based for Nova Scotia and embedded for Newfoundland. Lots of clouds already in Newfoundland ahead of the approaching low pressure system.

 

Regional Impacts:

 

New Brunswick: Nil.

 

Nova Scotia: Risk of non-severe thundershowers.

 

Prince Edward Island: Nil

 

Newfoundland: Risk of non-severe thundershowers.

 

Labrador: Nil.

 

 

 

 

 

 

 

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

 

Convective Discussion

 

Showers and possible thundershowers to push into western New Brunswick late this afternoon and evening as a shortwave trough moves into the area from the west.

 

Technical Discussion

 

This morning a low pressure system is producing some isolated showers over Newfoundland into the Labrador Sea. There remains a slight risk of a thundershower over Belle Isle Bank as the low tracks continues northeastward today. Most of the Maritimes are under fair weather this afternoon under a weak ridge of high pressure however the next shortwave trough will approach western New Brunswick by early this evening. A surface low pressure currently over central Quebec is expected to track along the St. Lawrence River eastward through northern New Brunswick overnight.

 

The environment within the warm sector and along the cold front will be conducive for the development of thunderstorms as low level moisture, surface heating and strong westerly shear increase this afternoon. Since the cold front is not expected to push through New Brunswick until the overnight hours, the main concern will be what may develop ahead of the cold front in the warm sector in the afternoon.

 

Looking at KCAR 12Z sounding, temperatures from 850mb upward are quite warm. However upper heights and colder air aloft will be pushing into Maine and New Brunswick will be falling though out the day, increasing the instability aloft. There is also not much cloud cover over New Brunswick currently so areas may be able to achieve their max daytime heating.  A westerly 40KT 850mb LLJ will be approaching western New Brunswick by the afternoon and with 0-6km shear near 50KTS, strong wind gusts (potentially near 40KTS) are possible especially where isolated cells develop into a squall lines. There is sufficient deep level shear so the possibility of supercells cannot be ruled out. Increasing precipitable water values of 40mm will also mean heavy rainfall rates are possible although cells should be moving quite quickly. At this time it appears the strongest threat for severe thunderstorms will be over southern Quebec and Maine where upwards of 2000J/kg may be reached. Areas of western New Brunswick mainly from Grand Falls and Victoria County to Fredericton and York County will be closely watched as cells could maintain long enough to advect into those areas before convection weakens in the evening.

 

Regional Impacts

 

New Brunswick: Showers and possible thundershowers with lightning strong wind gusts and locally heavy downpours this evening over western NB. Showers with a slight risk of a thundershower may extend into eastern NB overnight.

 

PEI and Nova Scotia: An isolated heavy showers is possible overnight as the cold front moves through.

 

Newfoundland and Labrador: Scattered TCU and associated showers as the trough moves out of Newfoundland this afternoon and the approaching low pressure system moves into southwestern Labrador.

 

 

 

Roberta McArthur

 

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

 

Convective Discussion

 

Showers are expected ahead and along an upper trough extending from northern Labrador towards central Quebec that will push eastward today. There is the risk for thundershowers to develop over southern Quebec and Maine this afternoon that may continue into northwestern New Brunswick this evening. There is also the slight risk of a thundershower this afternoon and evening over central and southern Labrador.

 

Technical Discussion

 

A upper trough stretches from northern Labrador southwest toward the central United States with several short wave troughs lying within it.

 

One wave lies south of Hudson Bay and will track eastward towards southern Labrador by midnight.  Strong convection should remain west of our region today as the associated cold front and strong LLJ lie over southern Ontario. Looking at the KCAR sounding, there is currently a good capping inversion at 500mb but with the thermal ridge moving eastward today, there may be enough cooling aloft along with surface heating to breach that cap. There is however a 40KT 850mb LLJ currently over the St. Lawrence River which will help to provide uplift as it pushes into western New Brunswick early this evening. With increasing shear along with relatively high precipitable water values reaching 40mm, any showers or thundershowers that develop could give locally heavy rainfall rates and strong wind gusts. Given the timing of the trough passage being late evening and overnight, it is expected that much of the convection will dissipate as it tracks across northern New Brunswick overnight.  

 

For central and southern Labrador, they will be in a favourable left exit region of a 250mb 110KT jet as another short wave trough moves through northern Labrador with a cold front extending through central and southern Labrador. Deep convection is however limited by both 500mb and 700mb thermal ridges moving through the area so unless they can get some additional surface heating, they may not be able to break the cap and will only see TCU and showers with gusts potentially reaching 30 KTS along the cold front.

 

Regional Impacts

 

New Brunswick: Showers and possible thundershowers with locally heavy downpours and strong wind gusts late this afternoon or evening over northwestern NB. Elsewhere TCU and possible showers extending toward southern NB this evening.

 

Labrador: Showers this afternoon with a slight risk of a thundershower this afternoon.

 

PEI, NS, Newfoundland and Marine waters: No significant weather expected.

 

 

Roberta McArthur

 

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

 

Convective Discussion

 

Slight chance of thundershowers across parts of NB tonight associated with a short wave trough. A slight chance of thundershowers near or along the Atlantic Coast of NS associated with a weak surface trough.

 

Technical Discussion

 

The dynamics and thermodynamics around ATL Can are relatively weak today so very little in the way of convection is expected. There is a 500/700 mb trough upstream over the GRTLKS region, but that is not expected to advect into the region later today. A shortwave trough at 500 mb will cross NB tonight which may trigger an isolated thunderstorm or 2. Also, a weak surface trough will cross over NS tonight giving a slight RSK of TSRA. The 12UTC sounding from YJT shows decent LLV moisture and 35 knots or so shear, but also 2 fairly strong capping inversions. Once daytime heating is maxed SCT TCU are likely with just the slightest RSK of a TSRA across CNTRL/ERN NL.

 

Regional Impacts

 

New Brunswick and Nova Scotia: slight risk tonight.

 

Newfoundland: very slight risk this afternoon and evening.

 

Labrador: nil.

 

Jeremy

 

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

 

Convective Discussion

 

Upper troughs moving through the region do not seem to be appropriately timed with advecting fuel sources, this combined with upper level smoke over parts of Atlantic Canada should serve to suppress severe convection over all regions except over parts of the southern marine districts of Newfoundland and southeastern districts of the Maritimes, where thunderstorms will likely persist throughout the period.

 

 

 

Technical Discussion

 

A well developed upper trough will continue to move eastward through the region over the period. The most unstable regions are over NB in the wake of this upper trough where at 500 cold trough will move through today – this presents an interesting case that is discussed more below. The jets ahead of this primary upper trough (not the 500 hPa cold trough) do not line up well for any additional lift although CYYT may be in the right entrance region later this afternoon. Low to mid-level lapse rates ahead of this upper trough are too stable for the generation of severe convection, and the most unstable regions, are too try without sufficient mid-level moisture advection present.

 

Focusing in on New Brunswick, the 12Z Caribou sounding demonstrates that insitu air has unstable lapse rates with possibly sufficient moisture to generate some convection, albeit unlikely to be severe, the problem is a trigger source. The primary deeper-layer moisture axis is expected to remain north, with little change expected in the dewpoint with the air advecting into the region. As previously mentioned the primary upper trough has moved well east, so only a weakening surface cold front –may- be able to help CU/TCU growth this afternoon. The larger inhibiter of severe convection is widespread smoke aloft over the province which will decrease insolation, and thus the amount of available energy to use for severe convective development. In the absence of a strong dynamical trigger, the auto-convect temperature as calculated off the 12Z Caribou sounding is 29 degrees, and with the smoke aloft affecting maximum insolation and thus the maximum daytime temperature, severe convection is not expected.

 

The well established trough over the southern Maritime, and eventually Newfoundland marine, districts will likely remain active today. Primary threats with these potential thunderstorms would be wind gusts capable of tapping into the 850 and 700 40 kt jet that was observed at CYYT (little change in magnitude is expected until well south of the marine districts), so potentially between 30 and 40 knots in the strongest thunderstorms over the warmest water. Cloud top cooling may allow thunderstorm activity to continue over the Southeastern Grand Banks through the latter hours of this outlook.

 

Photo of the smoke aloft – many thousands of feet up - (Purple-Blue Haze well established over New Brunswick, parts of NS and PEI, and near the front moving through Labrador).

 

Regional Impacts

 

Nova Scotia, New Brunswick, Newfoundland, Labrador, & Labrador Marine: No severe convection is expected.

 

Nova Scotia and Newfoundland Marine: Isolated to occasional thunderstorms likely over the warmest waters of Laurentian Fan, with a risk extending  to the Southern Grand banks as the trough moves east. Primary threat in thunderstorms are gusts to 30 to 40 knots.

 

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 August 23, 2018

 

Convective Discussion

 

The Maritimes are expected to remain quiet on the severe convection front, while over Labrador in the wake of a frontal wave, conditions may produce thunderstorms this afternoon.

 

Technical Discussion

 

Nil sig convection is expected over the Maritimes, post frontal, capped soundings suggest that although there is a well stacked upper trough moving through, dew points in the low teens, and jets aloft, that there will be insufficient thermodynamics/dynamics to create significant convection during the valid period of this product.

 

Over Newfoundland, the fuel is present ahead of the cold front, and the upper level jets are further to the west than the surface cold front – only a 35 knots jet extends ahead of this feature. CYJT and CYYT profiles exhibit some potential instability; contrasting between CYJT, CYYT, and CYZV profiles for warm sector to post-cold frontal, mid level lapse rates do not favour deep convection.

 

For Labrador, diminishing risk ahead of the trowal/warm front as it moves northward, cloud tops will warm with insolation so this threat will diminish as upper features move northward and the day goes on. In the wake of the trowal, there is an unstable region extending south of the Churchill Valley to the provincial boundary. This region will not likely be triggered by surface based convection given the current cloud cover (however cloud cover likely to continue to diminish), and is showing the hallmarks of a region able to produce convection through upper level forcing.  There is an approaching upper low and associated region of positive vorticity advection,  “decent” divergence present at the 250hPa level across all of central Labrador, and “weaker” divergence signatures as we descend through to 850 hPa.  CAPE values shown on NWP may be artificially low because of the Skinny Cape vs Virtual temperature issue, and I believe that this region may be more unstable than NWP suggests. The GFS has 1000 J/kg of CAPE over CYYR by 18Z, but overall the NWP cape envelope from the 06Z runs of the RDPS and GFS are similar (stretching from CZUM to the western portion of Eagle River). Because there are multiple Jets with left exit regions possibly reaching the area discussed above (peak Jet at 250 ~ 110kt over the middle-Gulf) , should thunderstorms form they would be able to produce strong wind gusts, potentially reaching 80 (or at worst possibly 90 km/h), or small hail as freezing levels  are near 12,000ft AGL (cross-front average).  0-6km Bulk shear this afternoon over the Labrador target region should be near 40 knots.

 

For the Maritime and Newfoundland Marine district:  Upper level triggered (via low level jet proximity) thunderstorm risk continues over the warmer waters of the Nova Scotia Marine district and that risk may extend into the west and southern regions of the Southwestern Grand Banks. Primary threat with these thunderstorms include wind gusts of 35 -40 knots associated with the passage of a similarly robust 850 jet.

 

Regional Impacts

 

Newfoundland: There is a weak risk of a thundershower with the passage of the cold front this afternoon. At this time the risk is not deemed significant enough to merit any additional action other than what is forecast currently by the NLWO. Severe convection is not expected.

 

Labrador:  Diminishing risk ahead of the trowal/warm front as it moves northward, cloud tops will warm with insolation. Thundershowers expected to develop this afternoon south of the Labrador public forecast regions, with a moderate risk for the development of severe weather mainly south of the Churchill Valley. Should thunderstorms/showers form they would be capable of producing strong wind gusts, and potentially small hail.

 

Maritime and Newfoundland marine districts: Isolated thunderstorms primarily over warm water regions. Wind gusts reaching 35-40 knots are the primary threat.

 

Labrador marine: Severe convection is not expected.

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 August 22nd, 2018

 

Technical Discussion

Low risk of non severe elevated thundershowers this afternoon and tonight over parts of NB and NS.  These thundershowers are associated with a trough that will move over the area.  If any thundershowers develop, they will be scattered.  The highest likelihood of thundershowers is over southwest NS.

 

Regional Impacts

 

Nova Scotia – Showers with embedded heavier amounts up to 10 mm in thundershowers.  Most likely over southwestern NS.

 

NB – Showers with embedded heavier amounts up to 10mm in thundershowers.

 

PEI – none.

 

Nfld & Lab – none.

 

 

Barrie MacKinnon

 

 

 

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

 

 

Technical Discussion

Low risk of non severe elevated thundershowers this afternoon and tonight over parts of NB and NS.  These thundershowers are associated with a trough that will move over the area.  If any thundershowers develop, they will be scattered.  The highest likelihood of thundershowers is over southwest NS.

 

Regional Impacts

 

Nova Scotia – Showers with embedded heavier amounts up to 10 mm in thundershowers.  Most likely over southwestern NS.

 

NB – Showers with embeddd heavier amounts up to 10mm in thundershowers.

 

PEI – none.

 

Nfld & Lab – none.

 

 

Barrie MacKinnon

 

 

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

Technical Discussion

High pressure continues to dominate most of Atlantic Canada today leading to widespread subsidence which will suppress convective activity.  Slight risk of non severe thundershowers over southwestern Labrador this afternoon and early evening due to the presence of a weak trough over the area.

 

Regional Impacts

 

None

 

 

Barrie MacKinnon

 

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

Technical Discussion

 

A large area of high pressure dominates Atlantic Canada bringing with it wide scale subsidence and very dry upper levels.  There is no convective activity expected over the region today or tonight except possibly over southern Maritime marine waters as an area of low pressure tracks over that area.  These thundershowers are not expected to be severe.

 

Regional Impacts

 

None

 

 

Barrie MacKinnon

 

Day 1 Convective Outlook for Atlantic Canada Valid for August 19, 2018

Convective Discussion

 

No thunderstorms expected over the Atlantic provinces.  

 

 

 

Jean-Marc Couturier

 

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

 

Convective Discussion

 

Rainfall warnings are continued for portions of the Maritimes. A large area of heavy precipitation with some embedded thunderstorms extends from Maine to the southwest coast of Newfoundland.

Our autostation at Sussex Corners reported an hourly rate of 30+ mm between 9 and 10 AM with the total for the area now exceeding 80 mm.

The rain will gradually end from west to east during the afternoon and evening.  

 

Technical Discussion

A low pressure system currently over southeastern Maine will travel across Nova Scotia today and exit the province to the south of Cape Breton this evening. A warm front extends from the low eastward to PEI and then to Cape Breton. Radar is detecting a band of heavy showers (and isolated thunderstorms) from the lift provided by the warm front. South of the front the air is very moist, this is highlighted by the Gray and Yarmouth 12z soundings where both are showing precipitable water in excess of 50mm.

While the lightning activity has diminished along the warm front two other areas have now become the focus, in the warm sector a 30kt southwesterly low-level jet is fueling the already moisture-rich airmass resulting in a cluster of thunderstorms over the Baie Ste-Marie area – these storms were moving eastward at about 40 km/h. There is a good chance that these storms could reach the Valley and possibly the Southwest shore early this afternoon, and a slight chance for the rest of the mainland during the rest of the afternoon and early evening. Another area to consider for thunderstorm activity is in the vicinity of the low where a few strikes were detected during the morning.

Regional Impacts.

 

Southeastern NB and Northern and Eastern NS including Cape Breton: rain heavy at times with localized amounts of 50 to 100 mm

Rest of NS: Scattered thundershowers with brief heavier showers giving amounts of 20-40 mm.

 

 

 

 

Jean-Marc Couturier

 

Day 1 Convective Outlook for Atlantic Canada Valid for August 17th 2018

Convective Discussion

 

No Thunderstorms are expected today for the Atlantic Provinces as a sharp change in the airmass with cooler temperatures and lower dewpoints across the region.

 

Regional Impacts

 

Non expected.

 

Mel Lemmon

 

Day 1 Convective Outlook for Atlantic Canada Valid for August 16th, 2018

 

Convective Discussion

 

Convective activity is expected to be confined to the southern Maritimes and southern Newfoundland today. Elsewhere the passage of a cold front mitigates convection for today. The primary threat zone is Nova Scotia from Halifax east. Heavy downpours of rain are the most likely impact. However, strong wind gusts less than 90 km/h and small hail are also possible.

 

Another area to watch is southern Newfoundland where heavy rain is a concern. There is an area of heavy rain and  t-shwrs offshore heading toward the Avalon and Burin Peninsulas that will arrive early this afternoon. Heavy showers and tshwrs will persist into the evening.

 

Technical Discussion

There is an approaching mid level trough and associated cooling aloft that will destabilize the column this afternoon and this evening over the threat area. There is 30 knots of unidirectional shear over the threat area. The 250 mb winds are around 50 knots over the threat area. CAPEs are in the 600-900J/kg range once initiation occurs.

Regional Impacts.

 

Srn NS and srn NB.

 

Hatt

 

Day 1 Convective Outlook for Atlantic Canada Valid for August 15th, 2018

 

Convective Discussion

 

Thunderstorms are expected to form ahead of a cold front approaching from Quebec. There is potential for very heavy downpours or rain, strong winds and possible hail over northern New Brunswick this afternoon and to a lesser extent over central New Brunswick early this evening. Elsewhere some thundershower activity is either occurring or possible today in the areas highlighted. Thundershowers over Labrador are giving significant lightning and possibly gusty winds. However, these thundershowers won’t likely be severe.  

 

Technical Discussion

There is a 250 mb Jet of 75 to 90 knots extending from the Gulf of Maine to northern NB to southern Labrador. There is a deep layer thermal ridge extending from Cape Hatteras to Newfoundland. The associated low level Jet is 20 to 30 knots. The primary triggers are sunshine and the cold front over southern Quebec. Cape values are over 1000 J/kg over the primary threat area. However, there is a cap at 750 – 800 mb that needs to broken before significant thunderstorms can form. Rainfall rates over northern NB could exceed 25mm in an hour due to the fact the airmass is very moist and steering flow is moderate around 20 to 25 knots. There could also be some training of cells due to unidirectional flow from the sfc to 500 mb.

Regional Impacts.

 

Primarily NB, but also Lab.

 

Hatt

 

Day 1 Convective Outlook for Atlantic Canada Valid for August 14th, 2018

 

Convective Discussion

 

Isolated thunderstorms along a frontal band which extends from James Bay to Labrador.

 

Technical Discussion

Surface ridge over the Gulf of St. Lawrence and Newfoundland will prevail today. A surface low over NY state is pushing moisture over NB and western NS, however with no trigger to speak of and limited daytime surface heating,  instability will be limited to elevated Cu,  isolated TCU and even some ACC. The Caribou and Yarmouth soundings show potential instability but a warm nose at 750mb and cloud cover prevent any surface development.

A frontal feature tied to a surface low near James Bay and very strong dynamics aloft, continue to produce organized clusters of thunderstorms again this morning. An area of high precipitable water (30+mm) stretches from the low into Labrador and is indicative of potential brief heavy showers. Storm motion is eastward under a 100 knot slightly anti-cyclonic upper jet. This core of this feature will drift eastward during the next 24 hours or so bringing a risk of thunderstorms to central and western Labrador.

Regional Impacts.

 

NB, PEI, NS and Newfoundland: none

 

Labrador:  Isolated thunderstorms producing brief heavier showers and 60km/h wind gusts over central and west portions.

 

 

Couturier

 

 

Day 1 Convective Outlook for Atlantic Canada Valid for August 13th, 2018

Convective Discussion

Isolated thunderstorms associated with a warm and humid airmass over New England will slowly propagate northward during the next day.

Technical Discussion

Surface ridge will prevail today across the region. A surface low south of New England will continue to push moisture, however with no trigger to speak of, surface-based instability will be limited to cumulus and perhaps a few towering cumulus. The Gray sounding from 12z showed some potential for CB’s but due to the incoming mid/High cloud cover coming in surface heating will not be sufficient.

A frontal feature near James Bay, supported by very strong dynamics aloft, is producing organized clusters of thunderstorms this morning. This feature will drift southeastward during the next 24 hours or so bringing a slight risk of thunderstorms to western Labrador tonight.

Regional Impacts.

Atlantic Provinces: none

Marine waters: Isolated thunderstorms for Gulf of Maine possibly reaching Lurcher late in the forecast period

Couturier

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

Convective Discussion

 

Tricky situation today over Atlantic Canada, NWP is failing on the resolution of features, and timing issues plague the obvious generation of potential convection. Isolated thundershowers are possible across the region today.

 

Technical Discussion

 

A low pressure system near southeastern Labrador will track north-northeastward today as an approaching trough begins to tilt the steering flow more “meridional”. The associated wave with this low passing through the island will continue to bring humid conditions to the island prior to the passage of the aforementioned features. Well established warm sector showers with heavy embedded showers over eastern Newfoundland will persist throughout the day as low level jets ahead of the primary front continue to pump in moisture. Saint John’s 12Z sounding demonstrated precipitable water values near 36 mm but poor lapse rates for severe thunderstorm generation. Over eastern sections of the island only showers with heavy embedded ACC showers are expected…the risk of a thundershower seems minimal given the current evidence. For western-central Newfoundland, 12Z upper air analysis suggests that there are favourable upper level dynamics for thundershower production as the left-exit of upper level jets may be able to assist in the formation of a thundershower into this afternoon. The problem here is not available fuel, but sufficient day time heating and once again poor mid level-lapse rates. Another factor mitigating the risks of severe convection is that the low level shear may not be favourable to assist TCU showers to become CB’s. A risk of a thundershower is possible this afternoon and potentially this evening associated with the passage of the troughs and the weak upper level dynamics.

 

For western Labrador, a stacked upper centre may have sufficient vorticity to assist in lifting the odd TCU to become overzealous, thundershowers are possible simply based on the strong pre-existing angular momentum and lift present around this low…mainly south of Wabush/Lab City.

 

For Nova Scotia: Once again the Yarmouth sounding shows the potential for thunderstorm development over the province… the issue today is timing. The primary trigger should be the east-northeast ward movement of the primary, well developed, upper trough; however, linear extrapolation from 12Z to 14Z on the forward speed of this trough is was near 100km/h and although it should slow as the flow begins to tilt in a more meridional direction it may arrive too quickly. The RDPS, GFS, have some very unstable indices (lifted indices over northeastern NS and into Cape Breton are -6 to -8, GFS CAPE 1000+ J/kg ) for later this afternoon but at the current speed this primary trough may not arrive in time to fully take advantage of the extra instability created by daytime heating. So the larger threat for afternoon development then becomes the passage of the weak 850 hPa trough currently moving through New Brunswick (which is triggering thundershowers, and is unresolved by the RDPS). It is difficult to say if this 850 hPa trough will maintain its current strength as it moves east-southeastward. Although divergence ahead of the primary upper trough is present over parts of northeastern Nova Scotia at the 250hPa and 500hPa levels, nil significant divergence was identified just ahead of it, and convergence was identified in its wake. So, that being said, should this secondary feature (the 850 hPa trough) arrive mid to late afternoon, thundershowers over central then to northeastern Nova Scotia / Cape Breton are possible, and potentially a severe thunderstorm over northeastern NS and CB but they will in a tricky dynamic regime. Sea breeze fronts are not expected to develop, and convective temperatures are not expected to be reached, therefore these are not expected to be primary triggers. Should a severe thunderstorm form, strong wind gusts near 80 to 90 km/h, with small hail, and downpours may form.

 

Prince Edward Island: Queens and Prince county are not expected to be affected by the similar conditions described for northeastern Nova Scotia. Kings county is borderline, and confidence in any significant weather here is low. TCU showers are likely this afternoon, and more-so over Kings county.

 

New Brunswick: There is a weak 850 hPa trough that is moving through the region has the ability to generate TCU and Isolated CB’s. It is competing with incoming mid level stability as noted by the strong capping inversion at 700 hPa from the Caribou sounding (which  should prevent anything other then TCU generation north of this 850 hPa trough).  Isolated thundershowers are possible over the southern half of the province this afternoon. Severe thunderstorms are not expected.

 

 

Regional Impacts

 

Labrador Public:  Slight risk of thundershowers near the low south of Wabush/LabCity, TCU most likely.

 

Labrador Marine and Newfoundland Marine: No severe convection is expected.

 

Maritimes Marine: Risk of thunderstorms over the warmer southern marine regions where the gulf stream runs the hottest until passage the passage of the trough(s).

 

Newfoundland Public: Predominately showers with heavy embedded showers east. Further west, slight risk of an afternoon and evening thundershower. No severe convection expected.

 

Nova Scotia: Isolated thundershowers with a slight risk of thunderstorms primarily over northeastern Nova Scotia and Cape Breton this afternoon and early this evening. Should severe thunderstorms form, strong wind, small hail, and heavy downpours possible. Confidence at this time is low on their formation, and a severe thunderstorm watch will only likely be issued if conditions begin to align better.

 

New Brunswick: Thundershowers over the southern half of the province possible with the passage of the 850hPa trough. Thundershowers are not expected to become severe thunderstorms.

 

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