Convective Outlook Valid for August 12th, 2019

Convective Discussion

Most of the Atlantic provinces are under a dryer airmass after the passage of the cold front over the weekend, maximum dewpoints are generally around 16C which is on the low side for deep convection but still sufficient. With the lower moisture levels the overall amount of Convective available potential energy(CAPE) is generally low with the exception of southern New Brunswick where cape values could exceed 1000 j/kg. In terms of the dynamics the approaching upper level jet may provide some needed support to maintain convection later in the day, this along with falling heights and 40 to 50 knots of deep shear mean that severe thunderstorms are possible later this afternoon for New Brunswick. However, it may be a challenge to initiate thunderstorms with the lack of a strong trigger but a weak trough this afternoon may provide adequate lift to get things started. Watches may me issued for New Brunswick if thunderstorm activity begins.

 

For the remainder of the Maritimes there remains a low probability of severe thunderstorms however with improving dynamics thunderstorms could linger well into the evening.

 

For southern Labrador the dynamics, windshear and moisture shouldn’t be an issue but steepening lapse rates through the day may lead to airmass thunderstorms developing and continuing into the evening.

 

 

Regional Impacts

 

New Brunswick

Severe Thunderstorms Possible. Small hail, locally heavy downpours and potentially damaging winds. Mainly over southern New Brunswick.

 

Nova Scotia and PEI

Low probability of Severe Thunderstorms. Locally heavy downpours and strong wind gusts possible.

 

Newfoundland and Labrador

Airmass thunderstorms possible for southern Labrador.  Locally 20mm rain, small hail, some gusty winds.

 

 

 

 

Mel Lemmon

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