Day 2 Convective Outlook for Atlantic Canada Valid for June 2, 2018

Convective discussion:

Possible severe thunderstorms  for Monday afternoon into the evening, with a slight chance for supercells for New Brunswick.

Technical Discussion:

A low pressure system west of James Bay will move to lie over Ungava Bay Tuesday night.

For New Brunswick the warm front should pass north of the province tonight,  but then there will be increasing low level moisture and warm advection in the warm sector, along with increasing shear as the cold front from the low approaches. Current model tephis show an almost classic shotgun setup, with the inversion breaking between 18-21Z. Currently CAPES are forecast in excess of 1000 J/kg, precipitable water 30-40 mm. The shear start a bit light at 25 kts, but increases into the evening to 35 kts. Finally Storm Relative Helicity is in the 150-250 m^2/s^2.

On the other hand the steering flow will be west to west-southwest, so it could skim by to the north, and the trailing front will also be mainly in the north. Also, the Bulk Richardson Number indicates CAPE dominating shear, so it may be more of a multicell scenario than supercells, and in fact the guidance is hinting at a prefrontal line.

For Labrador the warm front will pass through western Labrador by noon, quickly followed by the cold front. In the  west, if the system passes a little farther north, there may be some convection with heavy rainfall.

Regional Impacts:

New Brunswick: heavy downpours, gusty winds, maybe small hail, and maybe a supercell. Wind will be an issue if a prefrontal squall line forms.

Labrador: Possibly heavy downpours.

Nova Scotia/Prince Edward Island/Newfoundland/Iles de la Madeleine: nothing significant.

Doug Mercer

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

Convective Discussion:

Generally fairly benign for today into tonight.  Some early morning thundershowers for Newfoundland have moved away. This afternoon isolated air mass  thunder showers for western Nova Scotia possible this afternoon. For today not much expected for New Brunswick at this time, but if things push east a little faster there may be thundershowers near the Maine border late this afternoon or tonight. Severe weather possible for New Brunswick into Monday.

Technical Discussion:

For New Brunswick we have warm temperatures and good low level moisture, with dewpoints in the 16-20 C range,  and precipitable water in the 30 to 50 mm range. However, there’s deep ridge  and thermal ridge approaching from the west,  and conditions might not improve until into the overnight period.  An upper feature may trigger convection overnight into Monday, which might be severe.

Regional Impacts:

Newfoundland/Labrador/Prince Edward Island: none.

Nova Scotia: Isolated thundershowers possibly giving  heavy downpours and some gusty winds.

New Brunswick: Low chance for today or this evening. Possibly something overnight or tomorrow.

Doug Mercer

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

Convective Discussion

Garden variety thundershowers possible for New Brunswick, and significant wind gusts and rainfalls for Northeastern Newfoundland.

Technical Discussion

The main risk is for Newfoundland, especially the northwest. Low level moisture is moving in with the west/southwesterlies, there will be good insolation as the cells move east, and there’s some PVA ahead of a trough approaching from the Gulf of St. Lawrence.  Also, dewpoints  at 12Z were in the 16-17 C range, especially  for Cantral and eastern areas. Precipitable water is in the 30-40 range from the 12Z soundings, and shear is about 40 kts. CAPE could be up to 500 J/kg, and possibly larger if daytime heating is higher than forecast. With dry midlayers and a fairly classic  daytime heating tephi profile, healthy downdrafts are likely, followed by good local downpours. With freezing levels between 110-130 hft, hail is unlikely, especially combined with the relatively low CAPE. Onset should be early to mid afternoon.

For New Brunswick: For this case precipitable water is about 25, the freezing levels are too high for significant hail, and while low levels will become unstable there is an inversion near 14,000 feet that probably will inhibit significant convection. Guidance hints at the inversion gradually eroding away and allowing  some convection to occur. If so, they will be classic popcorn, with some downdrafts and locally heavy showers. Preipitable water is near 24 mm, and shear is near 30 kts.  For Nova Scotia there is a bare chance of an isolated thundershower or two.

Regional Impacts

Labrador: none.

Newfoundland: Thunderstorms over much of the interior, with the northeast having a risk for strong and gusty winds, followed by local heavy downpours. Significant hail is unlikely.

New Brunswick: isolated shallow based cells capped near midlevels.

Nova Scotia: Isolated cells east.

Prince Edward Island: none.

Doug Mercer

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

Convective Discussion

Convective weather is possible for parts of every province, but possible severe weather should be restricted to New Brunswick and offshore waters, mainly for the Southern Scotian Shelf. For New Brunswick the main threats are gusty winds and local downpours. For the Scotian Shelf heavier rainfall is likely and there is a chance if significant gusts for southernmost waters.

Technical discussion

For New Brunswick there is a trough approaching from the northwest with  good insolation near it. While moisture aloft is low, and precipitable water is only about 25 mm, there is fairly good surface moisture. There is moderate straight line shear near 35 kts. If and when cells initiate, this should give showers that might reach warning criteria, and downdrafts, especially as the trough should be an organising feature. Conditions should improve towards evening as the insolation dies away. For Nova Scotia expect continued locally heavy showers. Downdrafts will be inhibited by a near surface inversion.

For the Scotian shelf There is a possibility of downdrafts over southern waters, which may become significant. There’s also precipitable water amounts near 40 mm, which may give significant downpours.

For Newfoundland the profiles are too stable for surface initiation. There may be isolated elevated cells ahead of the trough approaching from the west, combined with some uplift along the south coast.

For Labrador the main likelihood is for the west. Insolation looks good, and there is a trough moving through to enhance convergence. I tried some higher temperatures for La Grande 4 and Goose Bay, and they only give CAPEs to 100-200 J/kg.  If cells appear, weak dry microburst are possible.

Regional Impacts

New Brunswick: Local heavy downpours and gusty winds, gradually diminishing by evening.

Nova Scotia: Thundershowers, and occasional gusty winds

Prince Edward Island: Thundershowers.

Newfoundland: Chance of embedded elevated convection.

Labrador: Thundershowers with gusty winds possible, especially in western Labrador.

Doug Mercer

Day 2 Convective Outlook for Atlantic Canada Valid for June 29th, 2018

 

Day Two Convective Outlook

 

An increase in the low level moisture as well as a cold trough at 500 mb will act to destabilize the airmass tomorrow for central Labrador and much of the Maritimes. New Brunswick will see the highest potential for severe weather with surface CAPES over 1500 J/kg and Precipitable Water values > 30mm. It is likely that heavy downpours could become an issue  in thunderstorms and to a lesser degree there is some potential for squall line development through the afternoon which could bring strong winds as well. Areas that receive large amounts of rain tonight will be under the greatest threat should thunderstorms develop tomorrow.

 

Mel Lemmon