Convective Outlook Valid for August 15th, 2019

Convective Discussion

 

Atlantic Canada is mainly under the influence of a ridge of high pressure today which will suppress convection over the Maritimes. Sea breeze convergence will provide some lift and isolated showers are possible for Newfoundland this afternoon and evening. There is a weak upper level trough over North Shore Quebec moving eastward towards southern Labrador today which will provide further instability with some cooling aloft. In addition, some coastal convergence and upslope flow will help to produce some isolated thundershowers this afternoon over central and southeastern Labrador. Given only weak shear, locally heavy downpours will be the main threat. With CAPE values of around 250 J/kg and freezing levels near 8,000ft in the area, there is also a slight risk of small hail.

 

Regional Impacts

 

Labrador

South central and eastern Labrador – Risk of thunderstorms with heavy downpours and a slight risk for small hail. 

 

Maritimes and Newfoundland

None expected

 

 

 

Roberta McArthur

 

Convective Outlook Valid for August 14th, 2019

Convective Discussion

 

A quiet day across Atlantic Canada as a low pulls away northeast of Newfoundland and high pressure moves in.  No convective activity expected.

 

Regional Impacts

 

Atlantic Canada

None

 

 

Barrie MacKinnon

 

Convective Outlook Valid for August 13th, 2019

Convective Discussion

 

The focus will be over Newfoundland today where thunderstorms are expected.   A very healthy strengthening jet over the area brings 0-6 km wind shear values near 40 kts which is expected to increase to near 60 kts later this afternoon for some areas.  At the current forecasted temperatures highs, 500 J/kg can be expected with higher amounts possible if highs reach a few degrees above forecasted temps.  A trough over the Maritimes will help kick start convective activity and low freezing levels of around 8500 ft could make hail a possibility under some of these storms.   Areas that stay cloud covered throughout the day should see reduced convective activity.  It is possible that some of the cells that develop may become severe.  Heavy rainfall and hail are the main threat if  severe thunderstorms develop.  The increasing wind shear later today could actually help to limit convection as t-storms development could be sheared off.

 

Regional Impacts

 

New Brunswick

Non expected

 

Nova Scotia and PEI

None expected

 

Newfoundland

Thunderstorms of which a few severe thunderstorms are possible.  Heavy rainfall and large hail are possible. 

 

Labrador

Southernmost coastal Labrador – Risk of thunderstorms with slight chance of severe tstorms.  Heavy rainfall and large hail possible. 

 

 

 

Barrie MacKinnon

 

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