Day 1 Convective Outlook for Atlantic Canada Valid for July 22, 2018

Convective discussion:

High probability of thunderstorms in western and central Labrador. Heavy rain, gusty winds and possible hail would likely accompany these thunderstorms. There is a slight possibility of severe conditions.

Thundershowers will likely persist south of NS and NF with the southern Avalon remaining in the risk of non-severe thundershowers.

Technical Discussion:

CAPE values could exceed 1000J/kg in western Labrador. In addition, 0-6 km shear would be in the 40-50 knot range. Supercells and squall lines are possible.

Regional Impacts:

New Brunswick: Nil

Nova Scotia and Prince Edward Island: Nil, except a very slight risk northern mainland NS if a period of prolonged sunshine occurs this afternoon.

Newfoundland: Chance of a thundershower on the Avalon.

Labrador: Likely.

Day 1 Convective Outlook for Atlantic Canada Valid for July 21st, 2018

Convective Discussion

Isolated thundershowers are occurring near Wabush/Lab City this morning along a frontal boundary that’s sitting across the region. As the boundary slowly progresses east-northeastward today, this activity should continue to progress along that zone. Later today (just south of the front) convection is likely to become more widespread once surface heating is maxed. A weak upper low and associated surface low south of NS will gradually move northeastward today and tonight giving isolated thundershowers to portions of NS and NL.

Technical Discussion

There is a slight risk of severe thunderstorms for extreme western Labrador this afternoon and early evening. Upper air analysis shows a fairly strong jet streak, placing WRN LAB in the right entrance region (though there is no diffluence aloft) There is a weak upper 500 mb trough north of the region but that really doesn’t advect into the region as the day progresses. There is a decent tongue of moisture at 850 analyzed from the Great Lakes into south-central Quebec. The question is how far northeastward will this stretch through the course of the day. The instability is there and 12Z tephis shows about 30-40 knots of shear, in addition the surface boundary being the trigger. Moisture will be the key ingredient to generate strong storms this afternoon. YZV shows good lapse rates in the low-mid levels with drying conditions about 700mb. Adjusting the surface T and Td can generate CAPEs of close to 1000 J/Kg and associated DCAPEs of around 600-800. If cells develop, strong wind gusts (close to 80 km/h), small hail and heavy downpours will be the concern.

The upper level low and associated PVA south of NS will give some isolated thundershowers as it progresses to the northeast (although it will be weakening over time). Precipitable water is quite high so the only concern would be torrential downpours. Even in the presence of embedded TCU, heavy rain can be expected.

Regional Impacts

For Labrador

Possible strong winds, small hail, frequent lightning and heavy downpours for the YWK area. OTWZ isolated thundershowers within the grey zone.

For Nova Scotia and Newfoundland:
Isolated thundershowers giving torrential downpours are possible.

For New Brunswick and PEI:

Nil.

Jeremy

Day 1 Convective Outlook for Atlantic Canada Valid for July 20th, 2018r

 

Convective Discussion

 

Isolated thundershowers are occurring west of Labrador (halfway to James Bay) again this morning. This activity should slowly move eastward during the day. Cumulus clouds are becoming more widespread over central and eastern Labrador. Isolated thunderstorms will likely develop especially over higher terrain.

 

Technical Discussion

 

The cold pool associated with the upper low northeast of Labrador will continue to support convective clouds today. The main focus will be over southeastern regions where surface temperatures will reach 25. Convection will be enhanced by higher terrain especially south of Lake Melville. Moisture in this airmass is limited so the main threat would be brief high winds (gusts around 70km/h or even a little more are possible).

To the west, a low over northern Ontario with a warm front extending eastward continues to support some elevated convection with a help a weak (80kt) anticyclonic upper jet. Some of this dynamic might approach western Lab late in the day but the prime area for surface-based convection will likely remain south of the frontal feature (and out of Lab) where higher wind gusts appear to be the main threat.

 

Regional Impacts

 

New Brunswick, Prince Edward Island, Nova Scotia and Newfoundland

Nil.

 

For Labrador

Lightning and possible gusts for eastern Labrador.

 

 

 

Jean-Marc Couturier

 

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