Convective Outlook Valid for Today and Tomorrow

 

Thunderstorm Outlook for Day 1

 

Regional Impacts: None for land areas of the Atlantic Provinces. A chance of embedded convection with occasional heavy showers for Nova Scotia slope waters spreading northeast towards Sable later today and approaching southeastern Newfoundland overnight.

 

Convective Discussion

There is a ridge extending from the Great Lakes ENE into  the South Labrador Sea, with a second ridge southeast of Newfoundland into the mid Atlantic. Between these two features is a northeast-southwest trough over Newfoundland.  Above the western ridge is a nearly stationary upper trough extending from Lake Erie along the St. Lawrence river towards southern Labrador. Southwest of Newfoundland the surface trough is intensifying as an inverted trough slowly moving toward the Avalon Peninsula tonight and  on Monday.

 

The basic story is that near and under both ridges things are stable under subsidence inversions around 700 to 850 mb, with very little precipitable water and near zero CAPEs, and with wind shears around 40 kts over the Maritimes, and for western Newfoundland approaching 70 kts. So little water, negligible CAPE, and high shear should kill any chance of thunderstorms.

 

For the Scotian Slope and approaching southern Newfoundland things are a bit less definitive. The aforementioned inverted trough is forecast to intensify and extend towards the Avalan. There’s deep moistire and PWs  in the 35-45 mm range, an almost tropical moist adiabatic profile. And the trough as a possible trigger. Shear is weak near and south of the trough, but just north it increases to the 30-40 kt range. My best guess is elevated weak convection with some heavy showers, extending to south of Newfoundland overnight, and maybe brushing the southern Burin or Avalon on Monday.

 

 

 

Thunderstorm Outlook for Day 2

See above.

Convective Outlook Valid for Today and Tomorrow

 

Thunderstorm Outlook for Day 1

 

Regional Impacts: no severe weather expected.

 

Convective Discussion: There is a large ridge centred north of Lake Superior and another large ridge east of the Grand Banks, with a weakening trough over the Atlantic Provinces between them. There is some dynamic support, but the ridging from the west is resulting in a fairly strong subsidence inversion capping convection for the Maritimes and western and northern Labrador.  For southeastern Labrador there is a frontal trough, but increasing cloud and low precipitable  water,  as well as a higher inversion at about 700 mb, argues for a few moderate to heavy showers at worst.

 

Over Newfoundland the story is similar with the riding inversion being weaker, but still there between 850 and 700 mb, and without the trough in Labrador. Precipitable water is a bit higher, especially in the southeast, and deep layer shear is okay, but the inversion and relatively low dewpoints for most of the island are consistent with no CAPE likely in the guidance.

 

 

Thunderstorm Outlook for Day 2: no severe weather expected.

Convective Outlook Valid for Today and Tomorrow

Thunderstorm Outlook for Day 1

 

 

 

Regional Impacts

 

Scattered showers with a slight possibility of embedded thundershowers early this afternoon over parts of the Bay of Fundy, Annapolis Valley, and northern Nova Scotia. The main threat with these will be some localised heavy rain.

 

Convective Discussion

 

A broad trough from a low pressure system over the northern Labrador Sea extends from western Newfoundland southwestward through the Gulf of St. Lawrence, PEI and the Bay of Fundy region this morning. The trough has not seen any lightning over our area this morning but some elevated convection is still possible into this afternoon. This area is in the right entrance of a 110 KT 250 mb jet with a weak low level jet over Nova Scotia. A cold pool lies south of New Jersey where the convection has been firing up this morning but the upper levels remain warmer further north, which is likely the main inhibiting factor. There is still a good amount of precipitable water available from the Yarmouth and Shearwater 12Z soundings of around 40 mm so locally heavy downpours are possible.

This trough will continue to weaken and somewhat stall over Nova Scotia and western Newfoundland into Saturday.

 

Later this afternoon, another trough will slide in from central Quebec into northwestern New Brunswick this evening. The main threat for convection will remain north of the St. Lawrence River extending north possibly reaching southwestern Labrador. Some TCUs reaching 18,000 ft in showers are expected to reach New Brunswick this evening.

 

Thunderstorm Outlook for Day 2

 

 

Regional impacts

 

None

 

Convective Discussion

 

The second trough will continue eastward over southern Labrador and northern Newfoundland with just scattered showers expected.  A sea breeze convergence over the Annapolis Valley and northern Nova Scotia may kick off some TCU in showers but a warm 700-500mb layer will likely inhibit any CB formation. The quasi-stationary trough over the southern maritime marine areas may prompt some embedded convection in the early morning hours Sunday morning.

 

 

Roberta McArthur

 

 

 

Convective Outlook Valid for Today and Tomorrow

Thunderstorm Outlook for Day 1

 

 

 

Regional Impacts

 

Scattered showers and embedded thundershowers for today and this evening over western New Brunswick. Heavy rain and brief strong wind gusts are possible.

 

Convective Discussion

 

A near stationary low pressure system resides over James Bay with a long cold frontal boundary extending south over southern Quebec and along the eastern United States this morning. Most of the dynamics remains further west over southern Quebec into New England. Ahead of the front over New Brunswick today, there is warm advection in the mid to upper levels, limiting the energy possible in embedded thundershowers. It is also expected to remain cloudy today, prohibiting any surface based convection. There is however increasing 0-6km shear, and a low level jet will help destabilize the environmental flow this afternoon. Linear lines of convection may form but will be quite fast moving. There is also ample precipitable water available of 40 to 50 mm which could prompt some localized heavy downpours.

 

Meanwhile over Labrador, a warm front will push northward into central Labrador today. There is a very slight risk of elevated convection overnight in the warm sector over southwestern parts of Labrador.

 

Thunderstorm Outlook for Day 2

 

 

The weakening trough will continue to push into Nova Scotia overnight and stall on Friday. A short wave trough will move from Quebec into northwestern New Brunswick late afternoon. At this time, TCUs are expected. With very high shear but weak CAPE, CBs are not expected at this time.

 

Roberta McArthur

 

 

 

Convective Outlook Valid for Today and Tomorrow

Thunderstorm Outlook for Day 1

 

Regional Impacts

Brief showers with risk of a thundershower for Terra Nova, Clarenville and Bonavista Peninsula.

 

Convective Discussion

Surface High pressure system continues to build over the Maritimes. High cloudiness ahead of a warm front extending ahead of extratropical cyclone Cristobal will continue to push eastward during the next day reaching Maine by Thursday. A 50kt southwesterly low-level jet will advect some low cloud and moisture, and forecast guidance is suggesting some elevated CAPE and a slight risk of thundershower early Thursday morning.

 

Over Newfoundland today, the air mass is fairly unstable but lacking moisture and diurnal heating. Towering-cumulus giving brief showers are in store for the afternoon, however with the help of a convergence line over east-central portions of the island, one or two of these TCU’s could build high enough to have glaciated tops. A slight risk of a thundershower is indicated on the western edge of a deck of strato-cumulus covering southeastern regions of the Island

 

Thunderstorm Outlook for Day 2