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