Day 1 Convective Outlook for Atlantic Canada Valid for Month Day, Year

 

Convective Discussion

 

Isolated thundershowers are occurring over northern Quebec this morning. Smaller cumulus clouds are widespread across Labrador. Also, Radar is currently detecting an area of rain over southeastern Newfoundland.

 

Technical Discussion

 

A surface low and its associated frontal wave east of Hudson Bay will advance over northern Quebec today. A 20 knot southwesterly low-level jet is supplying ample moisture and instability to support thunderstorms southeast of the low. Temperatures in the 20-24 degree range and dewpoints approaching 15 should be enough to fuel isolated thundershowers with CAPE values in excess of 500 J/kg west of Labrador this afternoon. Some of these storms could propagate over Western Lab with some help from a 40-50 knot westerly upper jet.

 

A frontal band continues to maintain a very moist air mass over southeastern Newfoundland. The sounding from St John’s West indicates a moist profile from the surface up to 850mb and again from 500mb and up, with precipitable water of 47mm! A series of vorticity centres will support isolated thundershowers along the frontal band throughout the day.

 

Regional Impacts

 

New Brunswick, Prince Edward Island and Nova Scotia

Nil.

 

For Newfoundland

Heavy downpours for the southeast.

 

For Labrador

Lightning and possible gusts for western Labrador.

 

 

Jean-Marc Couturier

Day 1 Convective Outlook for Atlantic Canada Valid for July 18, 2018

 

Convective Discussion

 

Convection is occurring ahead of a cold frontal trough stretching from central Labrador south through eastern Nova Scotia toward the southern marine district. The trough will continue to push slowly eastward today to lie over Newfoundland by early tomorrow morning.  Rainfall rates have been the primary concern with this front as it brought close to 50 mm over central New Brunswick yesterday and this morning a heavy band of rain and embedded thundershowers moved over mainland Nova Scotia giving total rainfall amounts greater than 40 mm. Once the band moves off of Nova Scotia early this afternoon, it is expected that the heaviest rainfall rates  will now remain over the southern marine districts but may reach portions of southern Newfoundland.

 

Technical Discussion

 

Over central Labrador and Newfoundland: The dynamics near the low and along the cold front have weakened overnight. There is a strong 250mb jet but is not well aligned to help enhance any convection.  However with cooling heights aloft there is still the risk for non severe thundershowers this afternoon as the front continues to approach Newfoundland. The highest rainfall is forecasted to remain over the marine district but there may still be some high rainfall rates that make it into southern Newfoundland today and overnight. Precipitable water values near 40 mm have already streamed northward toward YJT.

 

For eastern Nova Scotia and the southern marine districts: Heavy showers with embedded thundershowers will continue to push into Cape Breton in the next hour. The heaviest showers are aided by a 30KT LLJ over eastern NS extending northward towards Port Aux Basques. There is a weak low pressure area developing in the trough southwest of Georges Bank and is forecast to move northeastward into the southern slope waters late this evening. There is already frequent lightning near and ahead of the low as it lies still within the warmer gulf stream waters. That activity may continue into the southern slope waters this afternoon and overnight where SSTs continue to be warm. Gusts to 35 KTS are possible over the southern slope waters of Georges Bank and West Scotian Slope  The convection will likely become more isolated as it approaches our cooler waters further north an low level stability increases, however cloud top cooling may provide enough upper level energy to keep convection going overnight. Over the southeastern Grand Banks toward East Coast, there is a bit of upper level instability this morning firing up a few thundershowers but they are not expected to be severe and should diminish during the day.

 

Regional Impacts

 

For eastern Nova Scotia and the southern marine waters

Lightning, heavy downpours and gusts are possible.

 

For Newfoundland

Lightning and heavy downpours for the south coast.

 

For Labrador

Lightning and brief gusts for central and eastern Labrador.

 

 

Roberta McArthur

 

 

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.