Day 1 Convective Outlook for Atlantic Canada Valid for August 4, 2018

 

Convective Discussion

 

There are a few areas of interest today that could see convective initiation this afternoon and evening for parts of the Maritimes and Newfoundland. Embedded thundershowers are possible in the heavy band of rain over central and northern New Brunswick, scattered showers and thundershowers are likely to begin in southwestern Nova Scotia as well as southern New Brunswick and track eastward this afternoon. There may also be a risk of a thundershower in interior Newfoundland this afternoon.

 

Technical Discussion

 

There is an upper level trough approaching New Brunswick from Quebec as well as a surface trough currently southwest of Georges Bank moving northeastward towards Nova Scotia today.

 

There is currently an area of heavy showers over central New Brunswick that will be moving slowly northeastward today. Given the sounding for Caribou this morning, there is little shear and CAPE values look to remain below 500 J/kg due to the cloud cover.  The main threat with that will be locally heavy rainfall with precipitable water values ranging from 30-40mm. There is the risk for an embedded thundershowers which could enhance the rainfall. Rainfall warnings and special weather statements have already been issued for parts of New Brunswick.  

 

Areas south of the heavy band in New Brunswick as well as Nova Scotia are clearing out of their low level cloud and are expected to reach convective temperatures of 25 to 30C. Altering Yarmouth’s 12Z sounding, CAPE values between 1000-1500 J/kg are possible. With upper level diffluence, sufficient low level moisture and moderate 0-6km shear, the environment is condusive for single to multicell thundershowers to develop this afternoon and continue into the evening. For Nova Scotia, the trigger may be a mix of surface heating, sea breeze convergence and the influence of the approaching surface trough. Should cells initiate,  they will likely begin early this afternoon in the southwest and track east-northeast. There could be heavy downpours, strong wind gusts that could reach 40kts and the possibility of small hail (<2cm).

 

For Newfoundland there is the slight risk of a thundershower this afternoon with some areas reaching convective temperatures and a weak LLJ over the area.

 

 

Regional Impacts

 

New Brunswick : Risk of an embedded thunderstorm within the area of heavy showers this afternoon. Stronger cell are more likely over Gaspe and southern Quebec but may approach western parts of Edmundston County. For areas south of the heavy showers, isolated to scattered thundershowers are possible giving heavy downpours and strong wind gusts.

 

Nova Scotia: Isolated to scattered thundershowers are possible starting this afternoon giving heavy downpours and strong wind gusts.

 

PEI: Showers and risk of thundershowers early this evening and overnight with heavy downpours possible.

 

Newfoudland: Mainly TCUs with the slight risk of a thundershower this afternoon.

 

Labrador: nil

 

 

 

Roberta McArthur

 

Day 1 Convective Outlook for Atlantic Canada Valid for Month Day, Year

 

 

Convective Discussion

Convective initiation is possible today over portions of the Maritimes. The best chances will again be north of NB (Gaspe region) where a 100kt  anti-cyclonic jet should support more robust cells (and to a lesser degree over the Highlands of Maine). The sounding from Caribou modified with T/Td (31,18) show the possibility of CB’s with tops to 280-300, low SSI and BR# suggests single to multiple cells – of the non-severe variety - that could still produce surface wind gusts of 30-40kt.

The strong Bermuda high is providing fairly a stable environment over the rest of NB, NS and PEI, but the water content of the airmass is significant with precipitable water values approaching 50mm. The Yarmouth sounding shows somewhat favourable lapse rates in the mid to upper level. Surface heating alone should provide CAPE in excess of 500 J/kg, however this will likely not be enough to overcome the stable layer between 850-700mb. Other lifting mechanism would be required and it doesn’t look like the weak seabreeze front will be enough. Regardless, a small area of “risk” is introduced over inland areas of NS north of the Atlantic coast.

Dry conditions are expected over northern half of Labrador with Cumulus/strato-cumulus prevailing in the south. Over Newfoundland, a weak frontal trough will maintain variable cloudiness with TCU’s (tops to 160) giving scattered showers over the southern half of the island.

 

 

Regional Impacts

 

PEI, Newfoundland and Labrador : No impacts expected,

 

Nova Scotia: Slight risk of a thunderstorm for inland areas (west and central). Downpours and gusty winds are the main threat.

 

New Brunswick : Risk of isolated thunderstorms mostly north of a line from southeastern Maine to Chaleur region. The development, if any, will occur around the middle part of the afternoon. Strong wind gusts is the main threat. Stronger cells are more likely to develop north and West of NB.

 

 

 

Jean-Marc Couturier

 

Day 1 Convective Outlook for Atlantic Canada Valid for August 2nd, 2018

 

Convective Discussion

Two different convective initiation scenarios are envisioned today. North of the Maritimes, thunderstorms will form in an area of surface convergence ahead of  a cold front analyzed from western Quebec to the Saguenay to Baie-Comeau. Based on the 12utc soundings from Sept-Isles and Maniwaki, the most energy available in this area is roughly 800-1200 J/kg which should be enough to push cumulonimbus development up to about 30,000 feet. The upper support points to multiple cells, and supercells are not out of the question given the available moisture and wind profile.

 

Over New Brunswick and Maine, a moisture-rich airmass with dew point temperatures in excess of 20 will probably generate isolated thunderstorms. Very warm surface temperatures will be required to over-come the low level stability, however, with a temperature of 31 and a dewpoint temperature of 22 the available energy spikes to 1,000 J/kg and pure convection develops leading to cumulonimbus with top near 40,000ft erupt. The storms are more likely to be widespread and stronger west of the region (i.e. over Maine)  

 

Over Newfoundland, the airmass is becoming increasingly more humid over Newfoundland with dewpoint temperatures now reaching 17 or 18 in the southwest. A 30knot low level jet will push this moisture to other areas of the island during the afternoon however due energy available development will be limited to TCU and ACC clouds. Further north over Labrador, a low pressure system will maintain a vast area of cloud and rain.

 

Regional Impacts

 

Nova Scotia, PEI, Newfoundland and Labrador : No impacts expected

 

New Brunswick : Risk of isolated thunderstorms mostly north of a line from southeastern Maine to Miramichi. The development, if any, will occur during the middle to later part of the afternoon. Brief downpours and strong wind gusts are the main threat. Stronger cells are more likely to develop over area adjacent to the St. Lawrence valley and over southern Maine (North and West of NB)  

 

Jean-Marc

 

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

 

Convective Discussion

Today the bulk of the convective weather will be over Labrador with favourable dynamics and thermodynamics expected. The 12Z sounding from Goose Bay doesn’t indicate much instability however Sept- Ilses is more favourable for convection and could be considered representative of the airmass over Southwestern Labrador in the warm sector south of the front. Looking at the situation today there are two possible modes of convection; the first more likely scenario is fairly quick moving pulse storms with the potential for downdrafts exceeding 40 knots. The second possibility is that storms become longer lived with an increased likelihood of strong winds along with larger hail near or above warning criteria. Cape values through Southern Labrador are generally expected to be near 1000 j/kg however some areas could near 2000 J/kg this instability combined with high shear values near 50 knots in the  0-6 km could lead to severe storms.

 

Regional Impacts

 

Nova Scotia, PEI, and New Brunswick : No impacts expected

 

Newfoundland and Labrador: Thunderstorms should develop early this afternoon and continue into early this evening. Potentially strong wind gusts do to hybrid microbursts, long lived severe storms are also possible and tracking the storms on satellite and with lightning will provide the most helpful guidance once storms develop.

 

Mel Lemmon

Day 1 Convective Outlook for Atlantic Canada Valid for July 31, 2018

Convective discussion:

 

Isolated to scattered thunderstorms will develop today across portions of Labrador associated with a weak cold front crossing the area..

 

Technical Discussion:

 

A weak cold front will trigger ISOL/SCT TS today as it swings across Labrador. 12Z YZV tephi shows a strong capping inversion at 600 mb, and given the weak dynamics and modest surface heating, that cap will be difficult to erode. No severe weather is XPCTD.

 

KCAR tephi looks like a “loaded gun” profile, but there is no trigger to develop any TS. SCT ACC/TCU is XPCTD across NB.

 

Regional Impacts:

 

Labrador: Isolated/scattered non-severe thunderstorms.

 

Newfoundland: nil

 

Nova Scotia: nil.

 

Prince Edward Island: nil.

 

New Brunswick: nil.

 

Jeremy