Convective Outlook Valid for June 2nd 2019

Convective discussion

The general picture is for little no impacts for most of the forecast region. Impacts are expected mainly along the Atlantic coast of Nova Scotia today and tonight, and southern coastal regions of Newfoundland. There is also a risk for Northwest New Brunswick.

There are two main feature. A slow moving low south of Nova Scotia has a warm front extending well  to the east as a triggering feature, along with good moisture advection, precipitable water amounts in the 40 mm range, CAPES probably in the 500-1000 J/kg range, and above the marine inversion winds in the 55 kt range. While there is veering with height in the inversion, this should have little or no effect. The main impacts are rainfall, especially enhanced by topography near landfall, and gusty winds. With freezing levels in the 8000-10000 foot range, there is also a slight possibility for small hail.

For new Brunswick there is a line of weak convection across central main as an organising feature and a trigger. Mositure is relatively low, and the Caribou 12Z tephigram has zero CAPE. However, the  trigger temperature is near today’s daytime high, and CAPES of 300-500 J/kg are possible this afternoon, with shear near 38 kts. Again, the main impacts would be rain followed by gusty winds. With freezing levels in the 8000-10000 foot range, there is also a slight possibility for small hail.

Regional Impacts

Newfoundland: Rain and some gusty winds late today into tonight. A slight chance of hail. Topographic enhancement may give 25 mm/hr especially in the southwest.

Labrador: none

Nova Scotia: Rain and some gusty winds today into tonight. A slight chance of hail. Topographic enhancement may give 25 mm/hr especially in the east.

Prince Edward Island: none.

New Brunswick: Rain and some gusty winds, and a slight chance of hail.

Doug Mercer

Lead Meteorologist

Atlantic Storm Prediction Centre

Meteorological Service of Canada

45 Alderney Drive, Dartmouth

Nova Scotia, B2Y 2N6

Doug.mercer@canada.ca

Office: (902) 426-9200

Premier Météorologue
Centre de prévision des intempéries de la région de l'atlantique

Service météorologique du Canada

45 promenade Alderney, Dartmouth

Nouvelle-Écosse, B2Y 2N6

Doug.mercer@canada.ca

Bureau: (902) 426-9200

Cell: (902) 403-7951

Day 1 Convective Outlook for Atlantic Canada Valid for May 30, 2019

 

Convective Discussion.

 

This is the first convective outlook issued this year.

 

A slight risk of a TSRA across far northwestern NB later today as an upper trough of low pressure approaches the region.

 

Regional Impacts.

 

For New Brunswick: slight risk later today.

 

Elsewhere: nil.

 

Jeremy

Day 1 Convective Outlook for Atlantic Canada Valid for September 3rd, 2018

 

Convective Discussion

·         A frontal disturbance over Quebec may bring a  few potentially severe thunderstorms to northwesternmost areas of New Brunswick late in the day-evening and night

·         The same disturbance may trigger isolated thundershowers over parts of Labrador during the day

·         Elsewhere, a quasi-stationary ridge will prevent any convection.

 

Technical Discussion

 

High pressure system continues to prevail over southern Maritimes and Newfoundland. A frontal disturbance west of the region is triggering elevated thunderstorms over southern Quebec and central Labrador this morning. The  thermal ridge associated with this feature will slowly advance over NB today with un-modified SWEAT indices suggesting non-severe thunderstorms. However moisture content will continue to increase in response to a 25-30kt southwesterly low-level jet so that by the end of the day modified indices are indicating a potential for severe thunderstorms especially during the evening and night period. Precipitable water estimates from this morning sounding are already in the 40-50mm range in parts of the risk area (25-30mm over Labrador) resulting in a potential for bouts of heavy showers with amounts approaching warning thresholds.

 

Regional Impacts

 

Northwestern New Brunswick:  Isolated thunderstorms possible during the evening and night.  Heavy downpours are possible (25+mm).

 

Labrador:  Isolated thunderstorms possible during the afternoon and early evening. Brief heavier showers and wind gusts to 70km/h are possible.

 

 

Jean-Marc

 

Day 1 Convective Outlook for Atlantic Canada Valid for September 2nd, 2018

Convective Discussion

·         A frontal disturbance over Quebec/New England may bring isolated/non-severe thunderstorms to northernmost areas of the Maritimes later today

·         A cold front over Labrador may trigger thundershowers over southeastern portions this afternoon

·         Scattered thundershowers are continuing over the Gulf Stream mainly south of the Scotian Slope waters

·         Elsewhere, a quasi-stationary ridge will inhibit significant convection.

 

Technical Discussion

 

High pressure system prevails over southern Maritimes and Newfoundland. A frontal disturbance west of the region is triggering elevated thunderstorms (eastern Ontario and Western Quebec this morning). The  thermal ridge associated with this feature will slowly advance towards northern NB later today with K indices suggesting isolated thunderstorms becoming likely especially during the tonight period. Precipitable water estimates from this morning sounding are already in the 30-40mm range in the risk area, and 40-50mm near the thermal ridge resulting in a potential for bouts of heavy showers, especially tonight.

 

The cold front over Labrador is generating some instability with towering cumulus tops extending well above the freezing level, but with limited penetration into the glaciation levels. However, the combination of mediumsurface heating - extra lift provided by the mountainous areas - and the active front could generate a small cumulonimbus capable of giving brief heavier showers and wind gusts to 70km/h. Behind the cold front, westerly gusts of around 80km/h are likely to persist.

 

For slope waters, clusters of convection mainly south of forecast waters will continue. Scattered thundershowers are expected.

 

Regional Impacts

 

Northwestern New Brunswick:  Isolated thunderstorms possible during the late afternoon and evening. Brief heavier showers are possible.

 

Labrador:  Slight risk of a thunderstorms possible during the afternoon and early evening. Brief heavier showers and wind gusts to 70km/h are possible. Very windy in the westerlies behind the cold front with gusts to 80 km/h.

 

Slope waters of the Scotian Shelf and Southern Grand Banks: Scattered thundershowers with no severe impacts expected.

 

 

Jean-Marc

 

Day 1 Convective Outlook for Atlantic Canada Valid for August 23, 2018

 

 

Convective Discussion

A cold front approaching Labrador from the west may give a few thundershowers to westernmost regions this evening. There may be some gusts and small hail. Over most other areas a QS ridge will inhibit significant convection. For slope waters, there will be scattered thundershowers for much of the day and tonight.

 

Technical Discussion

 

The cold front will start becoming unstable in western Labrador in the evening (with a slight possibility for late afternoon). PVA will be strongest near 00Z, and some weak lift from the left entrance of the 250 mb jet will help. An associated thermal trough from 700 mb up will give some slight cooling to midlevels as well, but not enough to give significant CAPE. Shear will be on the high side, at 40+ knots, especially relative to low CAPE values expected. However, if cooling aloft is stronger than predicted, the CAPEs may be high enough for a few hundred Joules/kg, giving some local and brief heavy showers of 5-15 mm, and some gusts to 70 kph.  Rainfall estmates are based on a fast moving cell and 30 mm of precipitable water. Freezing levels will be about 9500 AGL (11500 ASL), giving a possibility of small hail.

 

For slope waters, there’s an area of weak embedded convection mainly south of forecast waters, but occasionally creeping into our region. There is low to moderate shear, and about 20-30 mm of precipitable water. The main trigger is weak troughing south of the ridge. Isolated to scattered thundershowers are expected, but, no hail, and wind gusts are inhibited by a warm nose near 850 mb.

Regional Impacts

 

Labrador:  Isolated thundershowers possible mainly this evening west of Churchill Falls and from near Wabush to the north. Possible wind gusts to 70 kph, and a small chance of small hail.

 

Slope waters of the Scotian Shelf and Southern Grand Banks: Isolated to scattered thundershowers. No severe impacts expected.

 

Elsewhere: no thundershowers expected.

 

Doug Mercer