Convective Discussion
Non-severe thundershowers possible for much of Atlantic Canada, with possibly near severe impacts for northern and western Labrador this evening, for central New Brunswick for this afternoon possibly into this evening, and for northwestern Newfoundland.
All of this is related to an occluded low south of Ungava Bay, and a frontal system extending southwest from Labrador southwest into Maine. This is giving some dynamic support, mainly for New Brunswick for Today and Labrador for tonight. There’s some convection over Labrador ahead of the occluded low, and some insolation over the west. Dewpoints are a bit on the low side (10-15 C), but PWAT is in the 30 mm range, which isn’t bad. The upper jet approaching has some PVA, and when it sinks southeast there will be some enhanced lift from the Left Exit to the northwest. Shear is on the high side at 50+ kts, which isn’t likely to drop with the approaching jet, and will reduce local rainfall amounts, and may be too high relative to the CAPE to produce supercells.. The basic idea is that convection will persist into tonight.
For New Brunswick. Ingredients are healthy for convection, which may near severe status. Currently there is scattered convection in the clearer air ahead of the front, which may argue for more moderate cells. Moisture is good as the surface with 20C dewpoints, and at 850 mb near 14C dewpoints. PWAT is near 50 mm. Instability is okay with CAPEs currently near 3-400 J/kg, and possibly a bit higher this afternoon. Later this afternoon CAPEs may near 500-1000 J/kg, which may be enough to near severe. The shear is currently near 30 kts, which is marginal for severe conditions, but will increase a little this afternoon.
For Newfoundland the approaching front will act a bit like the conditions for New Brunswick, with marginal shear (30-40 kts) , probably lower CAPEs, slightly lower dewponts (near 16 C), and PWAT near 35 mm. Because of the timing and orientation of the trough, things should max out later in the afternoon roughly near the Deer Lake region.
Regional Impacts
Labrador: Non-severe thundershowers for most areas developing this afternoon with high intensity beginning this evening, with stronger cells north of the Labrador Highway and west of the mid Labrador coast. Impacts will be mainly local heavy downpours and some gusty winds. There may also be some hail further north.
Newfoundland: Non-severe thundershowers beginning early in the afternoon. Impacts will be local heavy downpours, followed by gusty winds. The Deer Lake region and points north on the Great Northern Peninsula should have more significant impacts.
Maritimes: Non-severe thundershowers today for New Brunswick and a chance for Prince Edward Island, with the most significant impacts for the southern half of New Brunswick except for the Fundy Coast. For this evening a risk of thundershowers Nova Scotia. Impacts will be local heavy downpours and gusty winds.
Marine waters: Embedded convection possible for all Maritimes waters today and tonight, with stronger cells over southernmost waters. Also a slight chance near the Labrador Coast, and the Northeast Coast of Newfoundland.
Doug Mercer