Convective Discussion
Upper troughs moving through the region do not seem to be appropriately timed with advecting fuel sources, this combined with upper level smoke over parts of Atlantic Canada should serve to suppress severe convection over all regions except over parts of the southern marine districts of Newfoundland and southeastern districts of the Maritimes, where thunderstorms will likely persist throughout the period.
Technical Discussion
A well developed upper trough will continue to move eastward through the region over the period. The most unstable regions are over NB in the wake of this upper trough where at 500 cold trough will move through today – this presents an interesting case that is discussed more below. The jets ahead of this primary upper trough (not the 500 hPa cold trough) do not line up well for any additional lift although CYYT may be in the right entrance region later this afternoon. Low to mid-level lapse rates ahead of this upper trough are too stable for the generation of severe convection, and the most unstable regions, are too try without sufficient mid-level moisture advection present.
Focusing in on New Brunswick, the 12Z Caribou sounding demonstrates that insitu air has unstable lapse rates with possibly sufficient moisture to generate some convection, albeit unlikely to be severe, the problem is a trigger source. The primary deeper-layer moisture axis is expected to remain north, with little change expected in the dewpoint with the air advecting into the region. As previously mentioned the primary upper trough has moved well east, so only a weakening surface cold front –may- be able to help CU/TCU growth this afternoon. The larger inhibiter of severe convection is widespread smoke aloft over the province which will decrease insolation, and thus the amount of available energy to use for severe convective development. In the absence of a strong dynamical trigger, the auto-convect temperature as calculated off the 12Z Caribou sounding is 29 degrees, and with the smoke aloft affecting maximum insolation and thus the maximum daytime temperature, severe convection is not expected.
The well established trough over the southern Maritime, and eventually Newfoundland marine, districts will likely remain active today. Primary threats with these potential thunderstorms would be wind gusts capable of tapping into the 850 and 700 40 kt jet that was observed at CYYT (little change in magnitude is expected until well south of the marine districts), so potentially between 30 and 40 knots in the strongest thunderstorms over the warmest water. Cloud top cooling may allow thunderstorm activity to continue over the Southeastern Grand Banks through the latter hours of this outlook.
Photo of the smoke aloft – many thousands of feet up - (Purple-Blue Haze well established over New Brunswick, parts of NS and PEI, and near the front moving through Labrador).
Regional Impacts
Nova Scotia, New Brunswick, Newfoundland, Labrador, & Labrador Marine: No severe convection is expected.
Nova Scotia and Newfoundland Marine: Isolated to occasional thunderstorms likely over the warmest waters of Laurentian Fan, with a risk extending to the Southern Grand banks as the trough moves east. Primary threat in thunderstorms are gusts to 30 to 40 knots.
Marshall Hawkins
Operational Meteorologist – Atlantic Storm Prediction Centre, Meteorological Service of Canada
Environment and Climate Change Canada / Government of Canada
Marshall.Hawkins@canada.ca / Tel: 902-426-7779
Météorologue opérationnel – Centre de prévision des intempéries - région Atlantique, Service météorologique du Canada
Environnement et Changement climatique Canada / Gouvernement du Canada
Marshall.Hawkins@canada.ca / Tél. : 902-426-7779