Convective discussion:
Possible severe thunderstorms for this afternoon into the evening, with a slight chance for supercells, for New Brunswick.
Some isolated to scattered thunderstorms possible for western Labrador. Isolated thundershowers over the Southern Grand Banks.
Technical Discussion:
A low pressure system near James Bay will move to lie over Ungava Bay tonight.
For New Brunswick there will be increasing low level moisture and warm advection in the warm sector, along with increasing shear as the cold front from the low approaches. Current model tephis show an almost classic shotgun setup, with the inversion breaking between 15-21Z. Currently CAPES are forecast in excess of 1000 J/kg, and precipitable water about 30-40 mm. Surface and low level dewpoints are also good, with 20 C likely this afternoon at the surface, and a couple of degrees lower at 850 mb. It gets dry near and above 700 mb, which will support downdrafts. The shear starts a bit light at 25 kts, but increases into the evening to 35 kts. Finally Storm Relative Helicity is in the 150-250 m^2/s^2.
If a supercell does develop later this evening, strong downdrafts are likely with DCAPES in the 1200 J/kg range based on KBUF, KGYX, and WMW upstream. There will likely be heavy downpours, and nickel sized hail is also possible.
On the other hand the steering flow winds will be west to southwesterly, so it could skim by to the north, and the trailing front will also be mainly in the north. Also, the Bulk Richardson Number indicates CAPE dominating shear, so it may be more of a multicell scenario than supercells, and in fact the guidance is hinting at a prefrontal line. In that case, the downdrafts and rain is still likely, but freezing levels of 15,000 ft, make hail unlikely.
For Labrador the warm front will pass through western Labrador early this afternoon, quickly followed by the cold front. In the west, if the system passes a little farther north, there may be some convection with heavy rainfall.
For the Southern Grand Banks, a low slowly moving away will five isolated thundershowers until the system passes east overnight tonight.
Regional Impacts:
New Brunswick: heavy downpours, gusty winds, maybe small hail, and maybe a supercell. Wind will be an issue if a prefrontal squall line forms.
Labrador: Possibly heavy downpours.
Southern Grand Banks: Wind gusts and local heavy showers ending overnight.
Nova Scotia/Prince Edward Island/Newfoundland/Iles de la Madeleine: nothing significant.
Doug Mercer
Lead Meteorologist, Meteorological Service of Canada
Environment Canada / Government of Canada
doug.mercer@ec.gc.ca / Tel:902-426-9200
Météorologue principal, Service météorologique du Canada
Environnement Canada / Gouvernement du Canada
doug.mercer@ec.gc.ca / Tél: 902-426-9200