Day 1 Convective Outlook for Atlantic Canada Valid for July 30, 2018

 

Convective discussion:

 

Scattered thunderstorms will persist today over central and eastern LAB under the influence of an upper level trough of low pressure. Scattered thunderstorms will develop early this afternoon over central and east/northeast NL in association with a surface trough and 500 mb short wave.

 

Technical Discussion:

 

Conditions look pretty good for near severe thunderstorms across portions of NL this afternoon. From 15-21 UTC there is a strong 250 jet mb with divergence over SE NL, placing the risk area above in the left exit region (which will promote ageostrophic motions). 12Z tephi from YJT shows good shear at close to 45 knots, good drying in the mid levels which would lead to possible strong wind gusts at the surface. Looking at SAT PIX there appears to be an area of PVA moving into WRN NL (associated with a somewhat decent short wave trough at 500 mb) which will move eastward and add to the potential for thunderstorms once the convective temperature is reached. 12Z surface analysis shows a surface trough near WRN NL (which should act as a trigger). Adjusting the YJT surface temperature to 27 degrees produces close to 1000 CAPE, with fairly good moisture near the surface. In addition instabilities are pretty good (LI’s of minus 4 to minus 6). All these ingredients point to the potential for severe thunderstorms, likely from the Buchans/Millertown area to regions north and east. For now near severe cells are expected, but this area will have to be monitored this afternoon and into early this evening.

 

Over LAB scattered TS will continue today east of the upper trough. Looking at the 12Z YYR sounding, freezing levels are pretty low, but the shear is really weak so no severe weather is expected. Small hail is almost a given under some of these cells.

 

Regional Impacts:

 

Newfoundland: Scattered afternoon/evening thunderstorms from central and east/northeast. Slight risk of severe.

 

Labrador: Scattered non-severe thunderstorms. Small hail likely.

 

Nova Scotia: nil.

 

Prince Edward Island: nil.

 

New Brunswick: nil.

 

Jeremy

 

 

 

 

 

 

 

Day 1 Convective Outlook for Atlantic Canada Valid for July 29, 2018

 

Convective discussion:

 

Scattered thunderstorms associated with an upper level short over the Maritimes will move over WRN NL and SE LAB this afternoon and evening. Isolated thundershowers over parts of CNTRL/WRN LAB in the vicinity of an upper low will become more widespread this afternoon. Scattered thunderstorms will develop over ERN NB, ERN and NRN NS, PEI and Cape Breton this afternoon.

 

Technical Discussion:

 

A short wave trough and associated PVA triggered some early morning thunderstorms across CNTRL/NERN NB, and this area will move into WRN NL and SE LAB this afternoon. Upper air analysis shows a strong 250 mb jet axis (around 100 knots) and also a strong 500 mb jet (around 55 knots) with decent divergence. This area will not necessarily need lift from the surface, so cloud cover shouldn’t inhibit convection. There is an upper trough at 500/700 mb progressing slowly eastward, so falling heights and clearing skies should trigger some cells over portions of the MRTMS this afternoon. In addition a surface trough/cold front (analyzed at 12Z to be crossing NB) should aid in thunderstorm development. There is ample moisture near the surface, but the question is how quickly the surface dewpoints drop. 12Z tephi analysis shows good shear across the region (35-45 knots) but the instability is not as impressive as yesterday. In addition, adjusting the profiles for a Tmax of 26-27 produces about 500-700 CAPE, so the Buoyancy will not be that great either. Some storms could produce gusts to 40 knots with torrential downpours (precipitable water is 30-40 mm). Hail will not be a concern given the low buoyancy and associated weaker updrafts in cells. That being said small hail is possible within the stronger storms. Severe weather is not anticipated, however a very isolated severe cell in ERN NB, ERN/NRN NS and PEI cannot be ruled out.

 

Regional Impacts:

 

New Brunswick: Scattered non-severe thunderstorms across the east this afternoon. Slight risk further to the west.

 

Prince Edward Island: Scattered non-severe thunderstorms this afternoon.

 

Newfoundland: Scattered afternoon/evening thunderstorms in the west. Slight risk in the east.

 

Labrador: Scattered non-severe thunderstorms.

 

Nova Scotia: Scattered non-severe thunderstorms this afternoon in the north and east (as well as Cape Breton).

 

Jeremy

 

 

 

 

 

 

 

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