Convective Discussion
Tricky situation today over Atlantic Canada, NWP is failing on the resolution of features, and timing issues plague the obvious generation of potential convection. Isolated thundershowers are possible across the region today.
Technical Discussion
A low pressure system near southeastern Labrador will track north-northeastward today as an approaching trough begins to tilt the steering flow more “meridional”. The associated wave with this low passing through the island will continue to bring humid conditions to the island prior to the passage of the aforementioned features. Well established warm sector showers with heavy embedded showers over eastern Newfoundland will persist throughout the day as low level jets ahead of the primary front continue to pump in moisture. Saint John’s 12Z sounding demonstrated precipitable water values near 36 mm but poor lapse rates for severe thunderstorm generation. Over eastern sections of the island only showers with heavy embedded ACC showers are expected…the risk of a thundershower seems minimal given the current evidence. For western-central Newfoundland, 12Z upper air analysis suggests that there are favourable upper level dynamics for thundershower production as the left-exit of upper level jets may be able to assist in the formation of a thundershower into this afternoon. The problem here is not available fuel, but sufficient day time heating and once again poor mid level-lapse rates. Another factor mitigating the risks of severe convection is that the low level shear may not be favourable to assist TCU showers to become CB’s. A risk of a thundershower is possible this afternoon and potentially this evening associated with the passage of the troughs and the weak upper level dynamics.
For western Labrador, a stacked upper centre may have sufficient vorticity to assist in lifting the odd TCU to become overzealous, thundershowers are possible simply based on the strong pre-existing angular momentum and lift present around this low…mainly south of Wabush/Lab City.
For Nova Scotia: Once again the Yarmouth sounding shows the potential for thunderstorm development over the province… the issue today is timing. The primary trigger should be the east-northeast ward movement of the primary, well developed, upper trough; however, linear extrapolation from 12Z to 14Z on the forward speed of this trough is was near 100km/h and although it should slow as the flow begins to tilt in a more meridional direction it may arrive too quickly. The RDPS, GFS, have some very unstable indices (lifted indices over northeastern NS and into Cape Breton are -6 to -8, GFS CAPE 1000+ J/kg ) for later this afternoon but at the current speed this primary trough may not arrive in time to fully take advantage of the extra instability created by daytime heating. So the larger threat for afternoon development then becomes the passage of the weak 850 hPa trough currently moving through New Brunswick (which is triggering thundershowers, and is unresolved by the RDPS). It is difficult to say if this 850 hPa trough will maintain its current strength as it moves east-southeastward. Although divergence ahead of the primary upper trough is present over parts of northeastern Nova Scotia at the 250hPa and 500hPa levels, nil significant divergence was identified just ahead of it, and convergence was identified in its wake. So, that being said, should this secondary feature (the 850 hPa trough) arrive mid to late afternoon, thundershowers over central then to northeastern Nova Scotia / Cape Breton are possible, and potentially a severe thunderstorm over northeastern NS and CB but they will in a tricky dynamic regime. Sea breeze fronts are not expected to develop, and convective temperatures are not expected to be reached, therefore these are not expected to be primary triggers. Should a severe thunderstorm form, strong wind gusts near 80 to 90 km/h, with small hail, and downpours may form.
Prince Edward Island: Queens and Prince county are not expected to be affected by the similar conditions described for northeastern Nova Scotia. Kings county is borderline, and confidence in any significant weather here is low. TCU showers are likely this afternoon, and more-so over Kings county.
New Brunswick: There is a weak 850 hPa trough that is moving through the region has the ability to generate TCU and Isolated CB’s. It is competing with incoming mid level stability as noted by the strong capping inversion at 700 hPa from the Caribou sounding (which should prevent anything other then TCU generation north of this 850 hPa trough). Isolated thundershowers are possible over the southern half of the province this afternoon. Severe thunderstorms are not expected.
Regional Impacts
Labrador Public: Slight risk of thundershowers near the low south of Wabush/LabCity, TCU most likely.
Labrador Marine and Newfoundland Marine: No severe convection is expected.
Maritimes Marine: Risk of thunderstorms over the warmer southern marine regions where the gulf stream runs the hottest until passage the passage of the trough(s).
Newfoundland Public: Predominately showers with heavy embedded showers east. Further west, slight risk of an afternoon and evening thundershower. No severe convection expected.
Nova Scotia: Isolated thundershowers with a slight risk of thunderstorms primarily over northeastern Nova Scotia and Cape Breton this afternoon and early this evening. Should severe thunderstorms form, strong wind, small hail, and heavy downpours possible. Confidence at this time is low on their formation, and a severe thunderstorm watch will only likely be issued if conditions begin to align better.
New Brunswick: Thundershowers over the southern half of the province possible with the passage of the 850hPa trough. Thundershowers are not expected to become severe thunderstorms.
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
