Day 1 Convective Outlook for Atlantic Canada Valid for July 17, 2018

 

Convective Discussion

 

An intensifying low pressure system over central Quebec will track northeastward into western Labrador by this evening. Showers and thundershowers have already developed ahead of the cold front this morning over Maine and northwestern NB. Showers with the risk of severe thunderstorms are possible beginning near noon and continuing into the afternoon and early evening as the front progresses eastward. Besides frequent lightning, heavy downpours and strong gusts will be a threat along and ahead of the cold front this afternoon and tonight. Some thundershower activity is likely over western Labrador this afternoon and evening in the vicinity of the low. For our marine area, there is the risk of thundershowers over the Grand Banks today and tonight.

 

Technical Discussion

 

For New Brunswick: The cold front is currently over the St. Lawrence river valley and should reach northwestern NB by this evening. KCAR sounding this morning indicates a capping inversion in the low levels however this is expected to break by the early afternoon with some insolation. There remains quite a bit of cloud cover already pushing into central NB so max heating will be a critical factor of whether severe thunderstorms could develop ahead and along the cold frontal passage. Cooling aloft with the approaching cold front will help to destabilize and continue convection over NB this afternoon ahead of the front. With dew points in the upper teens over northern NB, there will be ample low level moisture for thunderstorms to draw from. Sufficient CAPE values of above 1000 J/kg and moderate shear values of 25KT increasing to 35KT by this afternoon will also help maintain thunderstorms should they continue to develop. With upstream precipital water values of 30-50mm, torrential downpours are possible and some cells could reach severe criteria of 25mm/hr or total rainfall exceeding 50mm. With freezing levels expected to be maintained above 14,000ft, the risk for hail >2cm seems minimal.

 

For western Labrador: There is the slight risk for thundershowers near the vicinity of the low this afternoon but the system will begin to weaken this evening as it moves into central Labrador. With a 30Kt LLJ moving into the area, there is the possibility of gusts reaching 70 km/hr.

 

For the southern marine district: There is a risk for convection over the southern Grand Banks today and tonight. Given the relativity cool SSTs, gusts above 35KTS are not likely to penetrate the stable marine layer.  With the approaching cold front, thunderstorms are possible beginning tonight and overnight for the southeastern marine waters.

 

Regional Impacts

 

For NB

Lightning, locally heavy downpours and brief gusts are possible. There is also the very slight risk of small hail.

 

For Labrador

Lightning, gusty winds and local downpours possible for western Labrador.

 

For the southern marine waters

Lightning, locally heavy downpours and brief gusts are possible.

 

Roberta McArthur

 

 

 

 

Roberta McArthur

 

Operational Meteorologist / Météorologue Opérationnel

Atlantic Storm Prediction Centre / Centre de prévision des intempéries de la Région de l'Atlantique

Environment and Climate Change Canada / Environnement et Changement climatique Canada

3rd Floor, 45 Alderney Drive / 3ième étage, 45 Alderney Dr.

Dartmouth NS B2Y 2N6

roberta.mcarthur@canada.ca

Telephone | 902-426-9200

 

Day 2 Convective Outlook for Atlantic Canada Valid for July 17, 2018

 

 

Convective Discussion

 

Upper level trough over central Quebec and southern Ontario today will be approaching the Maritimes tomorrow. An intensifying low pressure system currently near James Bay is expected to track northeastward into western Labrador by Tuesday evening. Some thundershower activity is likely over western Labrador again tomorrow along the warm from in the vicinity of the low. Warm and humid conditions will continue for the Maritimes ahead of the cold front which is expected to approach northwestern New Brunswick by  Tuesday evening and slowly progress eastward. Showers and thundershowers are expected to develop early afternoon over northwestern NB ahead of the front. Uncertainly lies with the amount of cloud cover ahead of the cold front which could inhibit max heating especially over extreme northwestern NB. Besides frequent lightning, heavy downpours and strong gusts will be a threat along and ahead of the cold front. With upstream precipital water values of 30-50mm, torrential downpours are possible and some cells could reach severe criteria.

 

Roberta McArthur

 

Day 1 Convective Outlook for Atlantic Canada Valid for July 15, 2018

 

Convective discussion:

 

Risk of non-severe thundershowers over southeastern Nfld. and much of New Brunswick.

 

Technical Discussion:

 

In New Brunswick cape values of up to 750 J/kg however very low shear values (ie. No high or low level  jets in the proximity). High freezing level of 12K feet will reduce any hail size.

A convective temperature of 24 degrees is required however there is currently considerable low level cloud in NW New Brunswick  limiting solar heating. No real trigger to initiate convection either. If/when convection is realized the main threat is localized rainfall due to the light flow. Precipital water is 30-40 mm.

 

Regional Impacts:

 

New Brunswick: Possible thundershowers. Possible high local rainfall amounts in heavier showers.  

 

Nova Scotia and Prince Edward Island: Nil

 

Newfoundland: Chance of a thundershower.

 

Labrador: Nil.

 

 

 

 

 

 

 

Day 2 Convective Outlook for Atlantic Canada Valid for July 14, 2018

Convective Discussion

Slight risk for convection later in the day Saturday over northwestern Labrador as a trough moves in from the west. Meanwhile a slow moving trough south of Nova Scotia may generate conditions favourable for thunderstorms over most southern waters of the maritime marine district and possibly into the Newfoundland’s southwestern Grand Banks.

Technical Discussion

An upper trough moving toward Labrador has a history of producing thunderstorms in its wake over northern Quebec. The timing for the passage of this feature over western Newfoundland is not that great and the risk for severe convection during the outlook period for western Labrador is low.  Over the southern Maritime Marine district similar instability remains and upper level triggered thunderstorms are possible with primary threats being wind gusts of 35 knots or slightly more if they form. Of course the mixing of these winds would be subject to the in-situ stability over the marine areas.

Regional Impacts

Newfoundland, Labrador, New Brunswick, PEI, Nova Scotia, and Labrador Marine: No severe convection expected.

Maritime and Newfoundland Marine: Slight risk of thunderstorms around the through over southern waters. Primary thunderstorm threats should they form would be wind gusts near gale force.

Marshall Hawkins

Day 1 Convective Outlook for Atlantic Canada Valid for July 13, 2018

Convective Discussion

No severe convection is expected across Atlantic Canada today with the exception of and increased risk for severe convection over  the southern Maritime  and Newfoundland marine districts.

Technical Discussion

Multiple capping inversions across the region resulting from an upper ridge to the west and a slow moving upper trough over the region will not provide conditions favourable for the development of severe convection over the majority of Atlantic Canada. For the southern Maritime/Newfoundland marine districts, persisting upper level divergence associated ahead of the upper trough will assist in the continued development of convection throughout the period. Aside from lightning,  the primary risks with these cells would be wind gusts near 35-40 knots. The threat of wind is the largest where water temperatures are warmest in the shaded area above.

Regional Impacts

New Brunswick, Nova Scotia, PEI, Newfoundland, Labrador: No severe convection expected.

Labrador Marine: No severe convection expected.

Maritime Marine/Newfoundland Marine: Thunderstorm risk is largest over the southern waters. Primary threat – wind gusts near or exceeding 35 knots (stability dependent w.r.t sea surface temperatures), otherwise, no severe convection expected.

Marshall Hawkins