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

Day 1 Convective Outlook for Atlantic Canada Valid for June 18th, 2018

 

Convective Discussion

 

There are a couple areas of interest today one over Western Labrador the other over the southwestern portion of the Maritimes. Over Western Labrador the dynamics for severe weather are favourable this morning but becoming less of an issue later in the day.  Non the less there is still some potential for thunderstorms with Gusts in the 60-70 km/hr range mainly do due to the combination of developing downdrafts and mixing of stronger winds aloft. Just west of Labrador there is an indication that dry microbursts could develop but this airmass is not likely to move over Labrador until tonight. Convective rainfall is not expected to be an issue.

 

Over Southwestern NS and NB embedded TS are possible beginning later this evening and into tomorrow morning. This is likely cause some heavier rainfall amounts in thunderstorms and possibly localized flooding.

 

Regional Impacts

 

For Labrador

Gusty winds possible for western Labrador should TS develop this afternoon and early this evening.

 

NS and NB

Embedded TS causing heavier downpours in rain that will continue through tonight.

 

For PEI

No impacts expected.

 

For Newfoundland

Very low probability of Convective Showers becoming TS early today just north of the island.

 

Mel Lemmon

Day 1 Convective Outlook for Atlantic Canada Valid for June 27th, 2018

Convective Discussion

Most of the region will see stable conditions today with very little potential for convective weather to develop. There is however some favourable dynamics coming together over central Newfoundland for this afternoon and early this evening. The main two being the upper level Jet and the slow moving surface trough slowly crossing over the island later today. In general low level moisture is not overly favourable but afternoon dew points could reach 12 degrees which given sufficient solar insolation could generate some weak thunderstorms. The overall balance between shear and updrafts will likely not be sufficient for any organized convection however given the very low freezing levels (< 9000 ft AGL) small hail is likely. Thunderstorms should they develop should be concentrated just ahead and in the surface trough where convergence will be a contributing factor.

Regional Impacts

For New Brunswick.. Nova Scotia.. PEI and Labrador.

No Impacts Expected.

For Newfound

Should thunderstorms develop this afternoon no significant impacts are expected however local rainfall ~ 10mm with small hail is possible.

Mel Lemmon