Convective Discussion
The Atlantic Region is still under the influence of what is now a nearly vertically stacked low pressure system over Northern Newfoundland. In terms of the thunderstorm activity there is still a chance of embedded thunderstorms near the main band of heavier rain wrapping around the low. These are will be more pronounced in the trough ahead of this system, which will be near the Labrador coast today. Any thunderstorms triggered here will initiate well aloft and could generate some locally heavier downpours.
Across the Maritimes dry air and weak dynamics greatly reduce the potential for any severe weather. However, there is a very slight chance that weak thundershowers could develop over central and northern Nova Scotia as a weak trough passes through picking up a bit of moisture from the Northumberland Strait. No significant impacts are expected although there is a chance of a heavier shower likely with 10-15mm of rain with pea sized hail.
Convective Discussion
There is a low probability that thunderstorms could develop over three main areas today, which are extreme western Labrador, southwestern New Brunswick and Newfoundland.
Over Newfoundland the heaviest area of Total Precipitable Water (TPW) has swung past the Island and weakened somewhat. With TPW values in the mid 30mm range heavier rainfall in embedded convection is possible but the heaviest rainfall will likely be just north of the Island or possible over the Northern Peninsula.
For extreme western Labrador a band of showers could impact the area this evening as a weak through moves through. Updrafts with this feature will be very weak, however there may be just enough depth to these storms to produce some lightning.
Finally across western New Brunswick we are looking at TCU’s and CB’s forming early this afternoon, the more intense cells will likely remain over Maine. However, some could impact SW New Brunswick as well. CAPE values will generally be as high as 500 J/kg and locally heavier downpours are possible in the 15mm to 20mm range should they affect NB. Some stronger gusty winds and small hail could be generated as well. However, warning level criteria are not expected.
Mel Lemmon
Convective Discussion
The main feature today is an intensifying low pressure system southwest of Nova Scotia which is expected to track just south along the Atlantic coast of Nova Scotia today and stall east of Cape Breton Island Saturday morning. Heavy rain with embedded thunderstorms has moved and are continuing to affect parts of southern Nova Scotia this morning as the warm front continues to push eastward. Embedded thunderstorms are expected to continue mainly for Nova Scotia and southern marine waters for today and overnight, leading to locally heavier rainfall amounts (Kejimkujik has already received over 50 mm since rain began early this morning). The Yarmouth sounding verifies this with precipitable water values of 45 mm. Rainfall warnings are in effect for Southern New Brunswick, most of mainland Nova Scotia, and Prince Edward Island where 50 to 75 mm is forecast. These rainfall warnings also extend into western and central Newfoundland as heavy rain fills in for them this evening. Convection however, is expected to remain offshore south of Newfoundland.
There is also an area of instability associated with an upper level low spinning over western Labrador. Convection has already initiated south of Churchill Falls where areas remained more cloud free this morning. An area over northwestern Labrador where some positive vorticity advection will help to trigger some afternoon convection.
Regional Impacts
Nova Scotia: Embedded thunderstorms will give lightning and periods of enhanced heavy rainfall today.
New Brunswick and Prince Edward Island: Mainly heavy downpours today for southern New Brunswick and Prince Edward Island.
Newfoundland and Labrador: Lightning and brief downpours for parts of western Labrador. Heavy downpours for marine areas south of Newfoundland overnight and into Saturday morning.
Roberta McArthur
Convective Discussion
For today, we have a broad trough extending from the Great Lakes northeastward toward central Labrador. Within this trough there are a couple of upper level features over central Quebec and central Labrador. Convective temperatures are not expected to be reached, especially given the amount of cloud cover currently in the region but there will be a decrease in temperatures aloft, leading to further instability along with the upper level features as a trigger to initiate some non-severe convection this afternoon and evening. Areas south of Labrador City to Goose Bay over southern Labrador into North Shore Quebec could see CAPE values close to 500 J/kg and with 0-6km shear values nearing 40 kts, some thunderstorms may organize further into lines or weak bow echos where locally heavy downpours and brief gusts are possible.
Meanwhile, a low pressure system south of the Great Lakes will progress northeastward over New England today. The system is then expected to intensify offshore near Cape Cod overnight into Friday morning and track over Nova Scotia Friday afternoon. This system will likely have embedded convection overnight along the warm front which will push into New Brunswick this evening and into Friday morning for Prince Edward Island and western Nova Scotia. Heavy rainfall will be the biggest threat with this system as it taps into tropical moisture streaming northward from the southern Gulf stream. Rainfall warnings have been issued for southern New Brunswick and northern Nova Scotia and will likely be extended eastward this afternoon.
Regional Impacts
Newfoundland and Labrador: Lightning, locally heavy downpours and brief gusts for parts of central and western Labrador.
Maritimes: Embedded thunderstorms will give lightning and enhanced heavy rainfall beginning late this afternoon and evening over northern New Brunswick and overnight and into Friday morning for western Nova Scotia and Prince Edward Island.
Roberta McArthur