Convective Discussion
Lots of fuel remains over the Maritimes for the development of severe convection, the problem – as usual – is what will assist in the triggering of thunderstorms. For Newfoundland and Labrador in the very short term a ridge of high pressure will inhibit the development of severe convection.
Technical Discussion
An upper trough west of the region is helping support a surface low that will move through New Brunswick today. Ambient moisture remains high from the surface through to the mid levels and will continue to generate heavy shower producing TCU and embedded ACC today. Ahead of the approaching cold front relatively weak low level jets of near 30 knots or so below 500 hPa will not likely favour super cell development, but afternoon thundershowers may become severe given the amount of available moisture. Freezing levels remain similar to yesterday at near 13000-14000 feet so again, without the generation of supercells, significant hail seems unlikely. Similar again to yesterday modifying the Yarmouth sounding as well as other upstream soundings above 900 hPa does produce CAPE values between 700 to 1200 J/kg, and precipitable water is near 50 mm. The curvature of the hodograph in the warm sector would favour right moving cells but in the absence of deep vertical shear, the development of super-cells would likely be hindered. Primary threats to the Maritime Public and Marine regions include heavy embedded showers ahead of the warm front with isolated, high precipitation thunderstorms, and similar conditions in behind the warm front, in the warm sector ahead of the passing cold front where marginal winds may serve to generate peak gusts near 70 to 80 km/h. In addition, the low level jets at 850 and at 700 have left exit regions that are expected to move over the bay of Fundy this afternoon, this may serve as additional support for longer lived cells just ahead of the cold front. With the current information available, evidence to support wind gusts in excess of 90 km/h is low because the majority of wind levels below 500 remain sub 45 knots; however, all cells are expected to be able to produce torrential rain.
Over Newfoundland and Labrador a ridge of high pressure will slowly yield to the system discussed above, and as the warm front moves through in the overnight hours, cloud top cooling may assist in the development of isolated thundershowers/thunderstorms primarily over the west coast and gulf regions. Embedded ACC giving locally heavy downpours likely.
Regional Impacts
Newfoundland and Newfoundland Marine: As the warm front passes through isolated nocturnal thunderstorms or thundershowers are possible. Mainly over the west coast (extra lift) and over the Gulf regions.
Nova Scotia, New Brunswick, PEI: Heavy showers, isolated thundershowers and thunderstorms. Torrential rains in the heaviest cells with rates exceeding 25 mm / hour.
Nova Scotia Marine: Squalls possible but stability over the region may make it difficult for 35 knot winds to reach the surface. 30 knots as an upper end is a more probable gust.
Labrador and Labrador Marine: Nil significant convective weather expected.
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