Convective Discussion
Nil significant convective weather is expected across Atlantic Canada in the short term. Isolated thunderstorms / thundershowers are expected to continue across parts of the southern Nova Scotia marine district today, and possibly over the eastern Nova Scotian and southern Newfoundland marine district overnight.
Technical Discussion
Multiple capping inversions remain across the region, and only thin regions of potential instability exist across the majority of actual sounding sites throughout Atlantic Canada. To overcome these various caps, significantly more moisture and daytime heating is required than is currently forecast; therefore no significant convective weather is expected across any of the provinces today.
The primary threat area to produce severe convective weather for the marine districts stems from the passage of a frontal wave and associated low pressure system that is forecast to continue to move northeastward, crossing from the southern NS marine district to the southern NL marine district. Current thunderstorm activity associated with this wave is producing isolated cells along the border of Sable and East Scotian Slope with cloud tops near 37,000ft (radar observed, consistent with RDPS NWP), surface wind observations through SAR/ASCAT have not been able to produce a measurement of this activity as of the time of this bulletin. Isolated cells associated with this wave are expected to continue today, and may intensify slightly overnight as cloud top cooling ahead of the warm front begins to play more of a role throughout the nocturnal hours.
Although there is a capping inversion across the marine district that would inhibit significant surface based convection, a strong low level jet is present below this inversion. The presence of this strong LLJ along with an unstable lower boundary layer may produce gusts near 45 knots should a cell form over the most unstable regions associated with the warmest Gulf-Stream waters for the southern Nova Scotia marine district. Given the transition to cooler water as this system moves into the Newfoundland marine area of responsibility, the persistance of thunderstorms would rely upon continual Warm Air Advection (WAA) and cloud top cooling (CTC) – the marine inversion would remain strong and likely prevent significant gusts from reaching the surface.
Overnight a second low pressure system may produce nocturnal thunderstorm activity over the sotuhwestern slope waters of Nova Scotia as positive voriticity advection along with WAA and CTC will be present over relatively warm waters. It is possible that thunderstorm activity that is expected to occur over eastern New England this afternoon may persist as this low moves off shore and eventually crosses into the south western NS marine district. Threats with these cells would be primarily wind gusts exceeding 35 knots.
Regional Impacts
Newfoundland and Labrador: No significant convective weather expected.
New Brunswick, Pei, and Nova Scotia: No significant convective weather expected.
Southern Nova Scotia and Southern Newfoundland Marine districts: Risk of thunderstorms activity throughout the remainder of today for NS and into tonight southern NFLD Marine (21 UTC onward). Gusts today for NS/NL may peak near 45 knots – which is not entirely significant compared to the currently forecasted gale-force conditions from either issuing office. Squall watches are not expected to be required at this time.
Marshall Hawkins