Day 1 Convective Outlook for Atlantic Canada Valid August 11, 2019

 

Convective Discussion

There will be much less convection in Atlantic Canada today than yesterday overall. The surface cold front and area of PVA associated with the upper trough has moved past the western Maritimes and is over the Gulf of St. Lawrence. Therefore, convective activity today should be confined to the Gulf of St. Lawrence region, western Newfoundland and parts of central and eastern Labrador. There is diffluence in upper  flow east of the Maritimes. There is also a cold trough in the 500 mb flow over the Maritimes and eastern US.

 

The 12 Z sounding from Stephenville shows significant instability above 850 mb. However, this sounding also shows a cap between 600 and 500 mb. My thinking is that falling heights associated with the approaching upper trough and the PVA should allow convection to initiate this afternoon over western Newfoundland. There should be enough sunshine to assist in initiation of convection in this area. There is significant shear in this area, 40 to 50 knots in the 0-6 km layer. CAPEs would be in the 500-1000 J/kg range. There is a possibility that storms similar to what occurred in the Maritimes yesterday could occur in the yellow region, with gusty winds, hail and heavy downpours.

 

Elsewhere in NL, There is a cap at about 650 mb limiting convection in eastern Newfoundland. Over Labrador, thundershowers have been occurring and will continue to occur today. This is associated with the upper low. These thundershowers should not be severe because the is very low shear. However heavy downpours and small hail are a possibility.

 

Regional Impacts

Maritimes…

Isolated thundershowers possible over eastern NS and PEI.

 

Newfoundland and Labrador…

Thundershowers likely  for western Newfoundland. Risk of thundershowers for central/eastern Labrador.

 

Steve

Day 1 Convective Outlook for Atlantic Canada Valid August 10, 2019

Convective Discussion

There is a chance of thundershowers over most of Atlantic Canada today. Throughout the region there is cold air aloft and freezing levels are 10,000 feet or less due to the existence of a large upper trough. In addition there is a shortwave just west of the Maritimes at 12Z this morning. There is also a cold trough at 500 mb just west of the Maritimes as indicated by 12Z soundings. There is plenty of sunshine throughout the region for the convective temperature to be reached this afternoon. The southern Maritimes is also in the left exit region of a 250 mb Jet.

Over NS and southeastern NB, shear values are forecast to be unidirectional from the southwest and near 30 knots in the 0-6km layer. CAPE values are forecast to be near 1000 J/kg over these areas. These are the areas highlighted in yellow, where there is a chance of severe thunderstorms today with strong wind gusts and possible hail. The lapse rates are steep between 850 and 700 mb over these areas contributing to strong buoyancy and also of potential for significant DCAPE giving strong wind gusts. The low freezing levels will contribute to the risk of small to medium sized hail.

Elsewhere, CAPE’s and shear are less, so thundershowers that form should not be severe.

Regional Impacts

Maritimes…Thundershowers, heavy downpours, hail and strong wind gusts are possible today. Severe thunderstorm watches may be issued for portions of the area denoted in yellow.

Newfoundland and Labrador…Risk of thundershowers for Newfoundland today or tonight. Risk of thundershowers for central/eastern Labrador.

Steve

Day 1 Convective Outlook for Atlantic Canada Valid August 9, 2019

Convective Discussion

A cold front will move across the Maritimes this afternoon and evening. A diffluent upper flow and dropping heights will favour the development of severe weather ahead and near the front. Cape values of 1000-1500 J/kg and uni-directional shear of 30-40 knots is being indicated by the 12z soundings. Skies are clearing and the fog patches lifting in the wake of last night’s rain system, so daytime heating to 25-28 degrees is very possible. CU/TCU/CB formation is expected to develop shortly after noon. This system gave numerous reports of wind damage through New England on Thursday so that is likely the biggest concern. Dcape values are indicating 90-100 km/h gusts.  Hail and torrential downpours are also possible. The precipital water is 30-35 mm. The freezing level is near 12,000 feet but there is good Cape through the hail growth zone. Oddly enough the main upper jet is well ahead of the surface cold front so not sure if the dynamics from that will play a part or not.

Regional Impacts

Maritimes…

Thundershowers, torrential downpours, hail and strong wind gusts are possible today. Severe thunderstorm watches will be issued for the area denoted in yellow.

Newfoundland and Labrador…

Risk of thundershowers for western Newfoundland tonight.

Andy Firth

Day 1 Convective Outlook for Atlantic Canada Valid August 8, 2019

Convective Discussion

A trough of low pressure and an associated upper jet will approach from the west today. Over the Maritimes embedded/non-surface based thundershowers are possible with the associated area of cloud and rain this afternoon and/or tonight.  Over western Labrador, surface based thundershowers are possible this afternoon and into the evening ahead of the approaching area of cloudiness. The energy is weak and the shear is minimal so just a risk is expected.

Regional Impacts

Maritimes…

Risk of thundershowers.

Newfoundland and Labrador…

Risk of thundershowers .

Andy Firth

Convective Outlook Valid for August 7, 2019

Convective Discussion

Today there are three things going on. For Labrador a combination of a surface low under a weak diffluent upper  jet will give non-severe thunderstorms which may persist into this evening. For New Brunswick air-mass thundershowers are possible ahead of a slowly approaching low, with a southerly flow brining in an axis of moisture up to al least 700 mb. Finally there is a low crossing the scotia shelf and giving embedded thundershowers.

For Labrador the upper trough moving in from the west is diffluent with a weak jet on the south side. Shear is around 25-30 kts, with CAPEs possibly reaching 500 J/kg, but more likely less.

Dewponts will increase slightly this afternoon, and if they reach 16 C, the 500 CAPE may occur. Downdrafts are possible, but shouldn’t be more than 60- kph.

For western New Brunswick, surface to 700 mb dewpoints are increasing and may reach 17+ C from he surface, and good insolation is expected early this afternoon with mainly clear skies in the northwest. PWATs are healthy and between 30-40 mm. Shear is low, arguing for airmass pop-ups. The only worry is the slow speeds of the potential thundershowers, which may give 25 mm/hr.

Regional Impacts

New Brunswick: mainly a chance of  non-severe thundershowers into this evening, with a chance for local downpours near or possibly exceeding 20 mm.

Labrador: a chance of non severe thundershowers, with showers up to 15-20 mm and some gusty winds.

Southern Waters: Embedded thundershowers with occasional gusty winds.

Doug Mercer