Day 1 Convective Outlook for Atlantic Canada Valid for July 28, 2018

 

Convective discussion:

 

Scattered thunderstorms should develop early this afternoon over ME and advect into western NB. Scattered storms will move into central and western Labrador as well. There is a risk of thundershowers/storms along the warm front crossing NL today.

 

Technical Discussion:

 

There is decent support from the surface to 250 mb for the potential of severe thunderstorms across the western half of NB this afternoon. A fairly strong jet streak will move across the region this afternoon placing most areas within the yellow threat zone above in the right entrance of that jet. A fairly strong 500 mb jet of 45 knots was analyzed at 12Z and an associated upper trough over the eastern GRTLKS/WRN New England. There is quite a bit of moisture from 850 mb to the surface, and instabilities are good as well. The 12Z KCAR tephi shows upwards of 1000 CAPE if modifying for a surface temperature of 27 degrees C (CAPE is somewhat fat as well with decent low-mid level lapse rates). Precipitable water is close to 40 mm, and there is about 30-35 knots of shear from 0-6 km. 12Z surface analysis shows a slowly approaching surface front, which will act as the trigger, since atmospheric heights don’t really drop at all through the course of the day. If cells develop, local torrential downpours and strong wind gusts appear as the main threats. FZLVL are around 12,000 feet so small hail is also possible. Over western/central LAB non-severe cells are expected…however local torrential downpours and modest wind gusts are also a possibility with some storms. And isolated thundershowers embedded along a warm front will continue across gulf waters and extreme NW NL today.

 

Regional Impacts:

 

New Brunswick: Possible hail, downpours and gusty winds over western sections this afternoon/evening.  Risk of thundershowers for areas adjacent to and east of the severe threat zone.

 

Prince Edward Island: Slight risk for Prince County this evening.

 

Newfoundland: Slight risk in the far NW.

 

Labrador: Scattered thunderstorms across western/central regions.

 

Nova Scotia: nil

 

Jeremy

 

 

 

 

 

 

 

Day 1 Convective Outlook for Atlantic Canada Valid for July 27, 2018

Convective discussion:

 

Risk of thundershowers over northern New Brunswick this afternoon and evening. They could become severe over northwestern areas. Risk of non-severe thundershowers over southwestern Labrador today and tonight.

 

Technical Discussion:

 

There is a supportive upper level jet approaching northwestern N.B. this afternoon. It has a diffluent flow and an embedded approaching vorticity centre. The Caribou sounding is likely the most representative and it suggests CAPE values in the 1000 J/kg range, shear of 30 kts and precipital water near 30 mm. There are no clear trigger mechanisms other than afternoon surface heating in the relatively cloud free skies currently over that region and a potential subtle surface trough. Local downpours and hail are possible. The flow is generally uni-directional and the higher winds are quite elevated. Gusty winds are still a possibility with water loading. The mid-levels are fairly moist so evaporative cooling will not aid significantly in the wind generation.  

 

Regional Impacts:

 

New Brunswick: Possible hail, downpours and gusty winds over northwestern sections this afternoon/evening.  Risk of thundershowers northeast.

 

Nova Scotia and Prince Edward Island: Nil

 

Newfoundland: Nil.

 

Labrador: Chance of thundershowers southwest.

 

 

 

 

 

 

 

Day 1 Convective Outlook for Atlantic Canada Valid for July 26, 2018

 

Convective discussion:

 

There is the risk of heavy rain and embedded thundershowers over parts of the Maritimes today and tonight with the approach of an upper level trough tracking eastward. 

 

Technical Discussion:

 

The upper trough that has been persistent over the eastern United States and southern Quebec is beginning to slowly push eastward towards the Maritimes and weaken as it tracks across New Brunswick this evening and overnight.  There is the risk of isolated thundershowers along and ahead of the trough this afternoon and evening for New Brunswick and western parts of Nova Scotia. With increasing moderate wind shear of 40-45kt, CAPE values near 500 J/kg  as well as high precipitable water of 40-50mm, heavy downpours and brief wind gusts are possible. At this time thundershowers do not look to be severe but the concern will be heavy rainfall where total rainfall values could reach over 50mm in some areas especially over western New Brunswick and southern New Brunswick.

 

Regional Impacts:

 

 

New Brunswick and Nova Scotia: Risk of thundershowers this afternoon and evening with locally heavy downpours possible.  

 

Prince Edward Island: Locally heavy showers in TCu possible.

 

Southwestern marine waters: Thundershowers this afternoon.

 

Labrador: Locally heavy showers over central and eastern section in TCu.

 

Newfoundland: Nil

 

 

Roberta McArthur

 

 

Day 1 Convective Outlook for Atlantic Canada Valid for July 25, 2018

 

Convective discussion:

 

There is the risk for non-severe thundershowers over southern Labrador, in the Annapolis valley area of Nova Scotia and over the southwestern marine waters this afternoon and evening.

 

Technical Discussion:

 

A warm front currently over southern Labrador is lying near quasi-stationary today. There is the slight risk of embedded thundershowers along and north of the front today and into the  overnight hours where cloud top cooling may help to destabilize the air mass further. Precipitable water values are ranging between 40-50mm so locally heavy downpours are possible. The area from Goose Bay toward Cartwright is already under a rainfall warning.

 

There is a slight risk of a thundershower over Nova Scotia this afternoon, especially in the Annapolis Valley where clouds are expected to clear, allowing convective temperatures to be reached. There is no real trigger over the area other than ample moisture and moderate instability so anything that develops will not be long lived but given the high precipitable water levels, a locally heavy downpour is possible.

 

On the northwestern edge of the ridge, some thundershowers are occurring over West Scotian Slope and approaching Georges Bank and will likely continue to slowly push northward today.

 

Regional Impacts:

 

Nova Scotia: Slight chance of a thundershower.  

 

Labrador: Chance of thundershowers with local downpours.

 

Southwestern marine waters: Thundershowers this afternoon.

 

New Brunswick, Prince Edward Island and Newfoundland: Nil

 

 

 

Roberta McArthur

 

Day 1 Convective Outlook for Atlantic Canada Valid for July 24, 2018

Convective discussion:

 

Risk of non-severe thundershowers over southern Labrador today and tonight.

 

Technical Discussion:

 

A warm front will move into southern Labrador today. Abundant cloud is associated with the warm front so just a chance of embedded thundershowers near and north of the front are expected. Precipital water is 30-50 mm so local downpours in thundershowers are possible.

 

Regional Impacts:

 

New Brunswick: Nil.  

 

Nova Scotia and Prince Edward Island: Nil

 

Newfoundland: Nil.

 

Labrador: Chance of thundershowers with local downpours.