Tuesday, July 18, 2023
Regional Impacts
Today
NB: Gusty winds, small hail and downpours, rainfall rates >25mm/hr.
NS: Downpours begin this evening, rainfall rates >25mm/hr.
LAB: Isolated lightning
Tonight
NB: Downpours, rainfall rates >25mm/hr
NS: Downpours, rainfall rates >25mm/hr
PEI: Downpours, rainfall rates >25mm/hr
Magdalen Islands: Downpours, rainfall rates <25 mm/hr
LAB: Isolated lightning.
Wednesday
NS: Downpours, rainfall rates> 25mm/h
NL: Downpours, rainfall rates> 25mm/h
LAB: Scattered lightning
NB: isolated lightning.
Convective Discussion
UA’s from Caribou, Yarmouth and Maniwaki indicated only modest PWATs of 26, 32 and 20 mm respectively at 12z. Nevertheless moisture is anticipated to rebuild ahead of a weakening cold front in NW NB throughout the afternoon. Thunderstorms are expected to initiate ahead of this feature in eastern Quebec and Northern Maine and move into Madawaska and western parts of Restigouche County late this afternoon. Storms will have SBCAPEs of 1000-1500 J/kg to work with following peak insolation (assuming minimal impacts from smoke in the vicinity). Bulk shears of 35-45 kts, of primarily linear character will allow storms to sustain themselves and grow into multi-cells that could present the odd strong downdraft to bring gusty winds. In addition PWATs are expected to build to ~ 40mm by late this afternoon yielding primary hazards of rainfall rates, and gusty winds in this region of NB. Small hail is also possible, however the hail threat will be limited by less than ideal buoyancy through the HGZ, given high freezing levels, despite the favourable shear character.
A broad upper trough well to the west of the region has begun to take on a negative tilt, this will guide a new stream of tropical moisture into southern NB and western NS by the evening hours. Models suggest PWATs building up to 65mm during the overnight hours. Moist unstable profiles will be aided by a broad ascent region associated with the right entrance of an upper jet. Lightning with this stream of moisture will initially be isolated, but should become more frequent overnight with cold top cooling as this band of moist unstable air slowly moves east. Elevated MUCAPES of 500-1000 J/Kg, should yield torrential downpours, with localized flooding being possible throughout the overnight in southern NB, parts of PEI and the western half of mainland NS.
On Wednesday this same area of moisture will present a similar downpour threat for eastern NS and western NL, although lightning frequency may be lower during the daytime hours as cloud tops warm. Northern NB and much of Labrador can expect air mass thunderstorms of isolated to scattered frequency respectively due to a cooler air mass aloft moving eastward from James Bay.
Thunderstorm Outlook for Today
Thunderstorm Outlook for tonight:
Thunderstorm Outlook for Wednesday:
Allen