Convective Discussion
A low pressure system over the southern Maritimes will continue to produce rain over western NF&Lab today. The inversion along the frontal boundary will generate severe wind gusts to the lee of the Cape Breton Highlands and the mountain range on the west coast of Newfoundland.
Further west, a surface cold front extending southward from a low over western Quebec will reach the lower St. Lawrence/Maine/New Brunswick later today. A 30 kt southwesterly low-level jet ahead of this feature is pushing enough moisture over northern NB to sustain some instability this afternoon once the low cloud/fog dissipates. This will take a few more hours since the lowest 5 000 feet is saturated (CAR sounding is showing this). However once the cloud cover breaks, instability will increase and a number of ingredients will be coming together to support the development of strong pulse Storms. The deep layer shears calculated from this mornings upper wind soundings are probably too strong to support long-lived bowing lines, however the presentation of the CAR hodograph suggests veering winds with heights, and if this pattern was maintained after convective initiation this afternoon formation of a tornado might not be out of the question. Having said this, as the air mass becomes depleted of the higher water content in the mid-levels - downdraft CAPE values between 400 to 600 J/kg will evolve, at that point the Storms could potentially produce damaging wind gusts in excess of 90 km/h. This appear to be the main threat for today.
Regional Impacts
New Brunswick: Over Northern regions this afternoon, strong pulse type thunderstorms capable of producing brief downpours, small hail and damaging winds. Storm motion will be eastward at 50+ km/h. Hourly rainfall rates of 15-25mm/h are also likely during brief downpours.
Isolated thunderstorms from central Maine may propagate over the mid and lower Saint John River Valley this evening.
Elsewhere across the rest of the Atlantic provinces, no thunderstorms are expected.
Jean-Marc