Convective Outlook Valid for July 21, 2019

Convective Discussion

 

The focus for severe weather today will be regions adjacent to southeastern Maine, and also inland portions of western Nova Scotia where hot and humid conditions prevail. Strong thunderstorms have migrated from Quebec over Maine this morning and are now entering Charlotte county in southwestern NB. These clusters of thunderstorms have a history of producing hail the size of a ping pong ball. The latest radar presentation suggests that these clusters are becoming better organized and are beginning to look more like early stages of squall lines. One of these features extends from Oromocto Lake to St. Stephen, and another one stretches NE-SW upstream from Bangor.

 

Convection should initiate during the first half of the afternoon over inland areas of Nova Scotia. The best forcing is expected over western regions but any storms that form could migrate to other regions of the province during the later part of the day. Buoyancy will increase throughout the afternoon and some of the forcing mechanisms will become better aligned. Using a Ts of 34 and Td of 24 is enough generate CAPE values in excess of 1500 J/kg pushing the lifted index to -5 or less. These large cumulonimbus would then become long-lived in this moisture-rich environment where deep layer shear of 30-40 knots are expected.

 

Regional Impacts

 

New Brunswick…For southernmost regions, strong thunderstorms with clusters of frequent lightning, widespread downpours, hail and locally severe winds.

 

Nova Scotia…For western regions, strong thunderstorms with clusters of frequent lightning, widespread downpours, hail and locally severe winds.

 

Newfoundland and Labrador…Isolated non-severe thunderstorms over southeastern regions.

 

PEI…None

 

 

Convective Outlook Valid for Month Day Year

Convective Discussion

 

A strong upper level jet stream (150 knots) runs from the Southern Prairies-Northern Ontario-Central Quebec-and then drops southeastward over the Gulf of St. Lawrence and then south of Newfoundland. Cloudy and cool conditions prevail in the airmass northeast of this feature, while hot and humid conditions prevail southwest of it.

This mornings upper-air soundings from CAR GYX and YQI are showing strong capping around 600mb, and a definite lack of forcing mechanism in the upper levels. The hot humid conditions will not be enough to trigger surface-based convection this afternoon. For this evening and overnight, some airmass thunderstorm activity from the lower Great Lakes and Northern New England may reach Maine and spread into western NB.

Regional Impacts

 

New Brunswick…Risk of a thunderstorm with localized downpours this evening and overnight

 

PEI and Nova Scotia…None

 

Newfoundland and Labrador…None

 

 

Convective Outlook Valid for July 19th, 2019

Convective Discussion

 

A short wave trough is approaching New Brunswick from central Quebec today. An increasingly more hot and humid air mass is also approaching the region this afternoon over western New Brunswick as winds begin to shift to the southwest on the  backside of a ridge of high pressure. This trough is expected to bring showers and thundershowers to northern New Brunswick this afternoon and continue to track eastward towards Nova Scotia and western Labrador by tomorrow morning.

 

The air mass will become increasingly warmer in the low levels which will lead to further instability this afternoon. As the trough approaches northwestern New Brunswick, 0-6km wind shear will increase somewhat but remain moderate, with divergence aloft and the approach of a strong 250mb jet to help maintain convection. Only modest CAPE values of 200-500 J/kg are expected, except perhaps for extreme northwestern New Brunswick which could exceed 500 J/kg. It is expected that multicellular storms will be the dominant type of convection with brief heavy downpours and strong wind gusts possible with some cells possibly developing stronger lines of convection over northwestern New Brunswick this evening. Freezing levels remain high enough that hail is not expected to be a main threat with cells that develop today.

 

Regional Impacts

 

New Brunswick…Lightning, locally heavy rainfall amounts, and brief strong wind gusts are possible for northern and central New Brunswick this afternoon and evening.

 

PEI and Nova Scotia…Scattered showers with the slight risk of a thundershower maintaining into the late evening possible.

 

Newfoundland and Labrador…None

 

 

Roberta McArthur

 

 

Convective Outlook Valid for July 18th, 2019

Convective Discussion

 

We are in for a mainly quiet day convective wise for Atlantic Canada. Two upper level lows had continued to trigger some non-severe elevated convection this morning over Funk Island Bank and the northwestern Labrador Sea. These features will continue to slowly pull further to the northeast today. Some towering cumulus in showers will remain over coastal regions of Labrador toward the northern peninsula in Newfoundland in an unstable NW flow.

 

 

Regional Impacts

 

None

 

 

Roberta McArthur

Convective Outlook Valid for July 17th, 2019

Convective Discussion

 

There is the potential for marginally severe thunderstorms to develop for parts of Atlantic Canada as a trough of low pressure swings through the region today and overnight.

 

A low pressure system over northern Quebec will move into Labrador by this evening. A cold front extends southward from this low and will track through all of the Atlantic provinces today into tomorrow morning. The primary concern for severe weather today will be ahead of and along the cold front as it moves over parts of central Labrador and central New Brunswick this afternoon and evening.

 

For New Brunswick, there will be plenty of low level moisture with dew points already reaching 20C and precipitable water values approaching the low 40s in southern Maine and over southwestern Nova Scotia. There is an area of showers starting to push into western New Brunswick but the cloud already over the area may inhibit some daytime heating. Looking at areas father north which have had clearer conditions as well as the Caribou 12Z sounding, convective temperatures near 28C are expected to be reached. CAPE values could reach the  1000-1500 J/kg range. Wind shear is mainly unidirectional but moderate enough to sustain potentially severe lines of convection with strong wind gusts possibly reaching 90 km/hr and heavy downpours. Freezing levels are quite high at 14,000ft but given the high CAPEs there could also be the risk for small hail <2cm.

 

Convection has also begun to initiate over northern Quebec and is expected to continue into central Labrador. The precipitable water values are not as high nor is the mid level shear but the freezing levels are lower so there is the risk for small hail.

 

 

Regional Impacts

 

 

New Brunswick…Locally heavy rainfall, frequent lightning, small hail and strong wind gusts possible early this afternoon into this evening.

 

Nova Scotia and Prince Edward Island…Locally heavy rainfall and  brief wind gusts possible this afternoon and evening.

 

Labrador… Locally heavy rainfall, frequent lightning, small hail and strong wind gusts possible early this afternoon into this evening.

 

Roberta McArthur