Convective Outlook Valid for July 31st, 2019

Convective Discussion

 

This could be a very active day for much of the Atlantic Provinces with the main focus of energy being over New Brunswick and possibly spreading into western PEI. In general there will be a weak surface trough from western Labrador then through New Brunswick which will help aid in the thunderstorm initiation. For most of the Maritimes there are high CAPE values between 1000-2000 J/kg and very high levels of moisture, this will lead to a high probability that heavy down pours will occur. Over Newfoundland and Labrador updrafts will be much weaker however with the high moisture content of the airmass locally heavy rain is still a possibility. Over New Brunswick the wind shear is sufficient to organize some storms, longer lived thunderstorms can be expected with a low chance of supercell and bow echo formation. Should stronger storms develop strong wind gusts 70-90 km/h and hail of 2-4 cm is possible.

 

Over Nova Scotia dynamics are not as favorable however there is a chance that back building thunderstorms could develop along the spine of nova scotia.

 

Regional Impacts

 

New Brunswick… Heavy Rain 25-50mm in heaviest thunderstorms, Hail 2-4 cm and wind gusts 70-90km/hr.

 

Nova Scotia… Heavy Rain 25 mm, locally higher amounts in back building thunderstorms.

 

PEI… Heavy Rain 25-50mm in heaviest thunderstorms, Hail 2-4 cm and wind gusts 70-90 km/hr over western areas possible.

 

Newfoundland and Labrador… Locally heavy downpours 20mm, small hail possible over western Labrador.

 

 

 

 

Convective Outlook Valid for July 30th 2019

Convective Discussion

 

An upper ridge moving over the Maritimes will suppress the convection somewhat and may inhibit convective initiation till late in the day. Overall the CAPE values are quite favorable being greater than 1000 j/kg for most of the Region. 0 – 6 km Wind shear values are of 35-40 knots over NW NB and for Newfoundland this will increase the potential for organized convection over these areas. Over Newfoundland if convection occurs it will likely begin early this afternoon while the thermodynamics still remain strong. Over New Brunswick the probability of severe weather is low but it is possible that severe thunderstorms could advect from Maine late in the day.

 

Regional Impacts

 

New Brunswick…

A general risk of thunderstorms for most areas with a heightened risk of severe storms for northwest areas, local rain of 25mm. If severe thunderstorms develop they will be very late in the day as dynamics gradually become more favorable.

Nova Scotia…

A low risk of thunderstorms that could produce local rainfall amounts near 20mm. A slight risk of that multiple thunderstorms could move over the same area causing further amounts.  

 

Prince Edward Island…

A low risk of thunderstorms that could produce local rainfall amounts near 20mm.

 

Newfoundland…

A low risk of thunderstorms that could produce local rainfall amounts near 25mm and wind gusts of 50-70 km/hr. A slight risk of that multiple thunderstorms could move over the same area causing further amounts.  

 

Labrador…

There is a chance of thunderstorms embedded in the trough that is approaching from Quebec.

 

 

Mel Lemmon

 

 

 

 

Day 1 Convective Outlook for Atlantic Canada Valid for July 29th, 2019

Convective Discussion:


Convective activity expected today over much of the Atlantic Canada with small chance of some convection becoming severe.

Lots of low level moisture and solar insolation will kick off some thundershowers across much of the Maritimes today. Shear is around the 25 – 35 kt mark and there are no substantial synoptic features to kick start and/or organize things. There is also a mid level warm nose which should cap off some but not all convective activity. So generally speaking, thundershowers over the Maritimes should be non severe but over area with shear near 35 kts, there is a chance of longer lived thundershowers that could become organized and become severe. There is also a possibility of tstorm formation over or along the Annapolis valley that could become quasi-stationary and extend out towards Antigonish county. The main threat if severe thunderstorms develop is heavy rainfall but hail and winds near criteria are also possible.

An area of low pressure over Labrador and an associated trough will kick off mid level tstorms over the area. Generally speaking, these thundershowers are expected to be non severe but a line currently over then Northeast Gulf could bring down some heavy downpours over parts of western Newfoundland.

Regional Impacts:
Maritimes: Slight chance of heavy rainfall reaching 25mm/hr.

Newfoundland: Slight chance of heavy rainfall reaching 25mm/hr over parts of western Newfoundland.

Labrador: Severe thunderstorms not expected.

Marine areas: Heavy rainfall over northeast gulf. Winds possibly reaching ~35 kts in thunderstorms.

Barrie MacKinnon

Convective Outlook Valid for July 26 2019

Convective Discussion:


Thunderstorms are unlikely over Atlantic Canada today due to very dry conditions and subsidence in the mid levels over the Maritimes and Newfoundland. The subsidence inversion in the 700 to 600 mb layer should suppress any convection exceeding that layer. These conditions are the result of a large surface high over southern areas and an approaching upper level ridge today and tonight. Showers are likely later today over southeastern and central Newfoundland. However, the dry air and subsidence aloft should prevent any TCu’s that form from obtaining sufficient height to become CB’s.


Over Labrador, relatively warm air aloft and cool air in the low levels will give a profile too stable for deep convection. In addition, mid-level cloud and moisture will suppress solar insolation.


Regional Impacts:

Maritimes: none.

Newfoundland: Scattered showers, thundershowers unlikely.

Labrador: none.

Marine areas: none. 

Convective Outlook Valid for July 25 2019r

Convective Discussion:

A slight chance of thundershowers over northern Labrador and for the southern border between Labrador and the Quebec border. Over southernmost sections of the Scotian Shelf and the southern Grand Banks occasional thundershowers.

Today a broad trough will be moving into Labrador from the west as a trigger, with some associated cooling aloft and weak destabilisation. The main inhibitor is again a lack of moisture. From 850 mb and higher there is precious little moisture, and at the surface the dew points are near 10 C or less.  PWATs over and upstream of Labrador are only near 17 mm. Now there is some low to mid level cloud moving in , which may supply the necessary moisture. Topographic forcing may help a little north and south, as previously mentioned. Impacts will be mainly some gusty winds , followed by  some brief heavy showers,  Freezing levels are again below 9000 ft, so there is a slight chance for small hail.

For the Maritimes there is little upper forcing, low shear near 20-25 kts, PWAT less than 20 mm, and low to moderate dew points. For Newfoundland the shear is higher as is the PWAT in the southeast, but upper forcing  is again limited. Finally , Stephenville and St. John’s tephis, show a moderate cap in the west and a strong cap in the east.

For offshore waters, high SSTs near the North Wall of the Gulf Stream will trigger some thunderstorms, further north isolated embedded convection is possible,

Regional Impacts:

Maritimes and Newfoundland: none.

Labrador: low chance for non-severe convection for northern Labrador and the southern border.

Marine areas: occasional non-severe thunderstorms over the southern Scotian Shelf, as well as the southern Grand Banks.

Doug Mercer