Day 1 Convective Outlook for Atlantic Canada Valid for July 10, 2018

Convective Discussion:

A front east of an occluded low west of Labrador will move east across the Big Land. Ove the Maritimes the same front will drop south over New Brunswick this afternoon, spreading thundershowers south across the province and into Prince Edward Island. There is a possibility of significant weather, mainly downdrafts, for New Brunswick and parts of Southern Labrador, and small hail for Southern Labrador

Technical Discussion:

Currently the cold front is over the St. Lawrence River heading south, and there is another weak trough over Central New Brunswick that has triggered a few cells along and east west line.  Most of the tephis over NB show a weak cap that should break sometime this afternoon with temperatures into the low 30’s, and enhancement with both the trough and front should organise things. Shear values near 50 knots over NB diminishing to about 35 over NS, should stay steady or increase. CAPEs should be healthy, in the 500 to 1000 J/kg range. Moisture is more marginal, with dewpoint  values near 14-17 for NB and PEI. Also precipitable water amounts are in the 20-30 mm range, so not huge.  The big risk here is for downdrafts, probably from mid-afternoon until near sunset. Midlevel cooling as the main front approaches should enhance thing sin the middle to late afternoon as well.

Labrador is similar, with CAPEs in to 500-1000 J/kg range, and shear a little lower. Over the mountains south of Lake Melville the relatively low freezing levels give a risk for hail. This may affect the Trans-Labrador Highway.

The Avalon may see isolated showers into the overnight periods

Regional Impacts:

New Brunswick: Downdrafts possibly to 90 km/h. Thundershowers.

Labrador south: Downdrafts possibly to 90 km/h. Small hail, especially over higher terrain.

Avalon Peninsula: Chance of thundershowers into the overnight period.

Elsewhere: nothing significant

Doug Mercer

Day 1 Convective Outlook for Atlantic Canada Valid for July 9, 2018

Convective discussion

A weak low/frontal feature west of Labrador will move across tonight to lie east of Labrador Tuesday morning.  This system has a cold front associated with it that will drop down over the Norther Gulf  tonight. This will give some thundershowers, and over western Labrador may give significant wind gusts.

Technical discussion

A weak low near the surface is just west of Labrador, with some support from a weak trough extending to mid-levels just west of a thermal ridge. This is moving through western Labrador and cooling aloft is occurring behind it. Moisture is currently marginal, but there are precipitable water amounts of 25-35 mm near the front and low.  Also, an axis of 18 degree surface dew points are approaching from the southwest (south of the cold front), and should start moving into Western Labrador by late this afternoon.

Shear is strong, and there are two midlevel jets approaching western Labrador, one in the extreme southwest and the other further north. Shear values are between 45 to 70 kts.  Based on the soundings and forecast afternoon temperatures, CAPEs in the 500-1000 J/kg range are expected for western Labrador, dropping to 250 over eastern Labrador.

The main threat is downdrafts, possibly reaching 90 km/h for Churchill Falls and west, followed by short intense downpours. Freezing levels of near 10,000 feet allow the possibility of small hail over the western higher terrain and the mountains, but again, moisture levels are limited.

Regional impacts

Labrador:  possibly severe downdrafts Churchill Falls and west . Elsewhere isolated to scattered thundershowers.

New Brunswick/ Northern Gulf of St. Lawrence/Western Labrador Coast/ Atlantic slope waters: isolated thundershowers.

Elsewhere: none.

Doug Mercer

Day 1 Convective Outlook for Atlantic Canada Valid for July 8th, 2018

 

Convective Discussion

In general it will be a quiet day across most of the region with a showers expected along the warm front that will push its way through Labrador, Eastern Quebec and into Western Newfoundland later today. In the short term convection will be somewhat suppressed do to the thermal ridge aloft and fairly low dew point temperatures. Later this afternoon however the dynamics and thermodynamics will become more favourable over western Labrador.  Aloft  the 250 mb jet and 500 mb jets will begin to strengthen with strong diffluence at the upper levels. As well the thermal ridge will continue to track east and we will get into some cooling aloft later today in the order of 2- 4 degrees at 500 mb, this along with the low level moisture advection will aid to destabilize the atmosphere. With the strength of the low level winds >40 kts and the impending thunderstorms, it is not unlikely that wind gusts will exceed warning criteria west of Labrador. Close attention should be paid to the speed and timing of thunderstorms that may move into western Labrador early this afternoon or evening. Should thunderstorms be very active wrt lightning watches may be required.

 

 

Regional Impacts

 

For Labrador

 

Strong winds and heavy rain possible should thunderstorms approach Western regions later today.

 

For PEI, NB, NS and Newfoundland

 

No Impacts expected.

 

Mel Lemmon

 

 

Day 1 Convective Outlook for Atlantic Canada Valid for July 6, 2018

Convective discussion:

 

A cold front will sweep across the region today. Embedded thundershowers with frequent lightning and downpours are expected ahead and near the front.

 

Technical Discussion:

 

A cold front with embedded thundershowers will track across Atlantic Canada today and tonight. Abundant cloud near and ahead of the front is limiting Cape values to below 500 J/kg. Precipital water values of 40-50 mm are producing torrential downpours. Frequent lightning is also a concern. A low level southwesterly jet of 60 knots may give gusty winds in thundershowers.  

 

Regional Impacts:

 

New Brunswick, Nova Scotia and Prince Edward Island: Heavy downpours and frequent lightning. Gusty southwest winds.

 

Newfoundland: Chance of thundershowers.

 

Labrador: Nil

 

 

 

 

 

 

 

Day 1 Convective Outlook for Atlantic Canada Valid for July 5, 2018 - Resent for missing valid date

 

Convective discussion:

 

Some isolated thunderstorms are possible this afternoon and evening associated with a warm front extending from central Quebec to the Gulf of St. Lawrence pushing northeastward as well as within the warm sector over New Brunswick and Prince Edward Island.

 

Technical Discussion:

 

A low pressure system over Hudson Bay will slide eastward today. Showers with embedded thundershowers are occurring this morning over Quebec extending southeastward towards northern New Brunswick and will continue to push to the northeast In the afternoon and evening, these showers with the risk of thundershowers are possible as the front moves into southwestern Labrador stretching southeastward into the Gulf of St. Lawrence. The cold front will quickly approach southwestern Labrador by midnight tonight and will reach northwestern New Brunswick by early Friday morning. Heavy downpours are possible with this cold frontal passage but as it is moving through the area in the overnight hours, the severe potential is less.

 

The Maritimes remain in a hot and humid air mass today.  Conditions will be unstable this afternoon with interior temperatures expected to be in the low 30s. The adjusted KCAR sounding is showing CAPE values between 1500 and 2000 J/kg,  however conditions remain fairly dry in the low levels and there is a lack of sufficient low level wind shear and upward forcing for there to be any severe potential... A sea breeze is expected to set up this afternoon which will likely be the main enhancement to help start up convection over eastern New Brunswick. Precipital water values of 35mm are the area, so heavy downpours are possible should convection initiate.

 

Regional Impacts:

 

New Brunswick and Prince Edward Island PEI: brief heavy downpours and gusty winds in isolated cells.

 

Nova Scotia: Brief showers mainly along the spine.

 

Labrador: Heavy downpours and gusty winds, especially along the cold front overnight for southwestern Labrador.

 

Newfoundland: Nil.

 

 

Roberta McArthur