Day 1 Convective Outlook for Atlantic Canada Valid for August 17th 2018

Convective Discussion

 

No Thunderstorms are expected today for the Atlantic Provinces as a sharp change in the airmass with cooler temperatures and lower dewpoints across the region.

 

Regional Impacts

 

Non expected.

 

Mel Lemmon

 

Day 1 Convective Outlook for Atlantic Canada Valid for August 16th, 2018

 

Convective Discussion

 

Convective activity is expected to be confined to the southern Maritimes and southern Newfoundland today. Elsewhere the passage of a cold front mitigates convection for today. The primary threat zone is Nova Scotia from Halifax east. Heavy downpours of rain are the most likely impact. However, strong wind gusts less than 90 km/h and small hail are also possible.

 

Another area to watch is southern Newfoundland where heavy rain is a concern. There is an area of heavy rain and  t-shwrs offshore heading toward the Avalon and Burin Peninsulas that will arrive early this afternoon. Heavy showers and tshwrs will persist into the evening.

 

Technical Discussion

There is an approaching mid level trough and associated cooling aloft that will destabilize the column this afternoon and this evening over the threat area. There is 30 knots of unidirectional shear over the threat area. The 250 mb winds are around 50 knots over the threat area. CAPEs are in the 600-900J/kg range once initiation occurs.

Regional Impacts.

 

Srn NS and srn NB.

 

Hatt

 

Day 1 Convective Outlook for Atlantic Canada Valid for August 15th, 2018

 

Convective Discussion

 

Thunderstorms are expected to form ahead of a cold front approaching from Quebec. There is potential for very heavy downpours or rain, strong winds and possible hail over northern New Brunswick this afternoon and to a lesser extent over central New Brunswick early this evening. Elsewhere some thundershower activity is either occurring or possible today in the areas highlighted. Thundershowers over Labrador are giving significant lightning and possibly gusty winds. However, these thundershowers won’t likely be severe.  

 

Technical Discussion

There is a 250 mb Jet of 75 to 90 knots extending from the Gulf of Maine to northern NB to southern Labrador. There is a deep layer thermal ridge extending from Cape Hatteras to Newfoundland. The associated low level Jet is 20 to 30 knots. The primary triggers are sunshine and the cold front over southern Quebec. Cape values are over 1000 J/kg over the primary threat area. However, there is a cap at 750 – 800 mb that needs to broken before significant thunderstorms can form. Rainfall rates over northern NB could exceed 25mm in an hour due to the fact the airmass is very moist and steering flow is moderate around 20 to 25 knots. There could also be some training of cells due to unidirectional flow from the sfc to 500 mb.

Regional Impacts.

 

Primarily NB, but also Lab.

 

Hatt

 

Day 1 Convective Outlook for Atlantic Canada Valid for August 14th, 2018

 

Convective Discussion

 

Isolated thunderstorms along a frontal band which extends from James Bay to Labrador.

 

Technical Discussion

Surface ridge over the Gulf of St. Lawrence and Newfoundland will prevail today. A surface low over NY state is pushing moisture over NB and western NS, however with no trigger to speak of and limited daytime surface heating,  instability will be limited to elevated Cu,  isolated TCU and even some ACC. The Caribou and Yarmouth soundings show potential instability but a warm nose at 750mb and cloud cover prevent any surface development.

A frontal feature tied to a surface low near James Bay and very strong dynamics aloft, continue to produce organized clusters of thunderstorms again this morning. An area of high precipitable water (30+mm) stretches from the low into Labrador and is indicative of potential brief heavy showers. Storm motion is eastward under a 100 knot slightly anti-cyclonic upper jet. This core of this feature will drift eastward during the next 24 hours or so bringing a risk of thunderstorms to central and western Labrador.

Regional Impacts.

 

NB, PEI, NS and Newfoundland: none

 

Labrador:  Isolated thunderstorms producing brief heavier showers and 60km/h wind gusts over central and west portions.

 

 

Couturier

 

 

Day 1 Convective Outlook for Atlantic Canada Valid for August 13th, 2018

Convective Discussion

Isolated thunderstorms associated with a warm and humid airmass over New England will slowly propagate northward during the next day.

Technical Discussion

Surface ridge will prevail today across the region. A surface low south of New England will continue to push moisture, however with no trigger to speak of, surface-based instability will be limited to cumulus and perhaps a few towering cumulus. The Gray sounding from 12z showed some potential for CB’s but due to the incoming mid/High cloud cover coming in surface heating will not be sufficient.

A frontal feature near James Bay, supported by very strong dynamics aloft, is producing organized clusters of thunderstorms this morning. This feature will drift southeastward during the next 24 hours or so bringing a slight risk of thunderstorms to western Labrador tonight.

Regional Impacts.

Atlantic Provinces: none

Marine waters: Isolated thunderstorms for Gulf of Maine possibly reaching Lurcher late in the forecast period

Couturier