Thunderstorm Outlook for Today
A few near-severe thunderstorms are possible over parts of NB. Isolated non-severe thunderstorms for southern Maritimes and Labrador
Regional Impacts
NB: Northwest..Scattered thunderstorms developing upstream and migrating over the region with downpours likely, rainfall rates could result in amounts of 30 mm or more. Any longer lasting storms have the potential to produce small hail and wind gusts in excess of 70 km/h. Likelihood of thunderstorms is minimal over the northeast.
South NB, PEI, and NS.. rain spreading throughout with isolated embedded thunderstorms producing heavy downpours, rainfall amounts through Tuesday could exceed 50 mm locally.
Lab: Slight risk of a thunderstorms between Wabush and the power line towards Sept-Isles
Convective Discussion
The analysis this morning is showing a couple of interesting features, the first being a rather large cold pool associated with a 500mb low over Northwestern/central Quebec, and the second is a frontal disturbance currently east of Cape Cod and advancing towards the Maritimes. Widespread showers continue to spread across southern NB and northern NS. Radar is also detecting strong echoes in northern NB and the Lower St. Lawrence, however Lightning flashes have diminished since mid-morning but will likely increase once again this afternoon as the cold pool advances towards the St. Lawrence, Bands of Towering cumulus are currently forming over the Eastern Townships of Quebec and the Saguenay. In terms of dynamics for the afternoon, there is a departing cyclonic upper jet, advancing surface/upper trop with a moderate southerly flow ahead of it. In behind the trough there is a 30 westerly LLJ punching through the Highlands of Maine. The limiting factor for significant vertical development of convective cloud is the lack of strait sunshine, in fact Satellite is showing extensive cloud cover as far west as the Beauce, including Maine (north and south).
The most likely scenario will be for surface convection initiation to occur over the Lower St. Lawrence, Beauce and Maine north woods.. and then migrate east of the provincial/international border during the middle to late part of the afternoon. As for the mode of convection, the deep layer shear values will also be limiting (< 30kt) but using a convective temperature of 22 / dewpoint of 14 would suggest MLCAPE of 500 J/kg with CB tops above 10 km with a possibility of some isolated tops above 12km making some of the more persistent clusters near-severe. The main threat would be torrential downpours, but hail of up to 1cm and winds of 70km/h are also likely. We will defer until early afternoon before making a final decision on a severe thunderstorm watch.
The other area of concern is related to the frontal system, a large area of cold cloud top is currently advancing over the western marine district and there are at least three fairly active convective clusters embedded in that system. The steering flow aloft is south to southwest which could help in spreading heavier rain over NS.
Over NF an upper ridge will prevail today, as indicated by the dry conditions in both soundings from Stephenville and St. John’s. Satellite imagery shows open cellular Cumuliform cloud developing over Labrador and because of the diffluent flow aloft and the cold pool to the west - a few thunderstorms are likely. Goose Bay sounding is very dry in the low levels but if surface warming is maximised then there is a good possibility of strong wind gusts due to downdraft currents.
Thunderstorm Outlook for Tuesday
Slight risk of a thunderstorm over western NB, and also central and western Lab
