Day 1 Convective Outlook for Atlantic Canada Valid for August 9th, 2018

 

Convective Discussion

 

Lots of fuel remains over the Maritimes for the development of severe convection, the problem – as usual – is what will assist in the triggering of thunderstorms. For Newfoundland and Labrador in the very short term a ridge of high pressure will inhibit the development of severe convection.

 

Technical Discussion

 

An upper trough west of the region is helping support a surface low that will move through New Brunswick today. Ambient moisture remains high from the surface through to the mid levels and will continue to generate heavy shower producing TCU and embedded ACC today. Ahead of the approaching cold front relatively weak low level jets of near 30 knots or so below 500 hPa  will not likely favour super cell development, but afternoon thundershowers may become severe given the amount of available moisture. Freezing levels remain similar to yesterday at near 13000-14000 feet so again, without the generation of supercells, significant hail seems unlikely. Similar again to yesterday modifying the Yarmouth sounding as well as other upstream soundings above 900 hPa does produce CAPE values between 700 to 1200 J/kg, and precipitable water is near 50 mm. The curvature of the hodograph in the warm sector would favour right moving cells but in the absence of deep vertical shear, the development of super-cells would likely be hindered. Primary threats to the Maritime Public and Marine regions include heavy embedded showers ahead of the warm front with isolated, high precipitation thunderstorms, and similar conditions in behind the warm front, in the warm sector ahead of the passing cold front where marginal winds may serve to generate peak gusts near 70 to 80 km/h. In addition, the low level jets at 850 and at 700 have left exit regions that are expected to move over the bay of Fundy this afternoon, this may serve as additional support for longer lived cells just ahead of the cold front. With the current information available, evidence to support wind gusts in excess of 90 km/h is low because the majority of wind levels below 500 remain sub 45 knots; however, all cells are expected to be able to produce torrential rain.

 

Over Newfoundland and Labrador a ridge of high pressure will slowly yield to the system discussed above, and as the warm front moves through in the overnight hours, cloud top cooling may assist in the development of isolated thundershowers/thunderstorms primarily over the west coast and gulf regions. Embedded ACC giving locally heavy downpours likely.

 

Regional Impacts

 

Newfoundland and Newfoundland Marine: As the warm front passes through isolated nocturnal thunderstorms or thundershowers are possible. Mainly over the west coast (extra lift) and over the Gulf regions.

 

Nova Scotia, New Brunswick, PEI: Heavy showers, isolated thundershowers and thunderstorms. Torrential rains in the heaviest cells with rates exceeding 25 mm / hour.

 

Nova Scotia Marine: Squalls possible but stability over the region may make it difficult for 35 knot winds to reach the surface. 30 knots as an upper end is a more probable gust.

 

Labrador and Labrador Marine: Nil significant convective weather expected.

 

 

 

Marshall Hawkins

 

Operational Meteorologist – Atlantic Storm Prediction Centre, Meteorological Service of Canada

Environment and Climate Change Canada / Government of Canada

Marshall.Hawkins@canada.ca  / Tel: 902-426-7779

 

Météorologue opérationnel – Centre de prévision des intempéries - région Atlantique, Service météorologique du Canada

Environnement et Changement climatique Canada / Gouvernement du Canada

Marshall.Hawkins@canada.ca / Tél. : 902-426-7779

 

Day 1 Convective Outlook for Atlantic Canada Valid for August 7th, 2018

 

Convective Discussion

 

Technical Discussion

 

Another day with high potential for severe thunderstorms over New Brunswick.  Conditions are even a bit more favourable today then yesterday.  An upper level jet over northern New Brunswick will help as a lifting mechanism for tstorm formation.  Mid level jet of 35 kts should help tilt updraft and help keep tstorm organized and longer lived.  At the low levels, ample and deep moisture will help fuel tstorms and a trough over NB will help kick start and maintain storms.  Surface temps in the high 20’s to low thirty will also be a big aid for tstorm formation.  Low level helicity values are expected to be good this afternoon with the presence of the trough, so supercell formation over parts of NB are likely. 

A line of severe tstorms had already formed this morning over northern NB and currently over the Napadogan to Blackville area giving strong winds, hail and heavy downpours.  Expected that tstorms will develop again this afternoon and begin to ease with the setting of the sun tonight.  Tornado formation is a probability (but low) over central NB due to the expected formation of supercells.

 

Regional Impacts

 

New Brunswick – Severe thunderstorms expected within the orange threat area of the map.  These thunderstorms could produce winds gusting up to 100 km/h, nickel sized hail and rainfall amounts of 25 mm/hr.  The threat is less over the yellow threat areas but still possible.   

 

Nova Scotia – Lower probability of severe thunderstorms along the valley and stretching out to New Glasgow.  Climatologically, tstorm generation over NS is much more difficult.

 

Labrador – risk of non severe thundershowers over grey threat area.

 

Newfoundland – None

 

 

Barrie MacKinnon

 

Day 1 Convective Outlook for Atlantic Canada Valid for August 6th, 2018

Technical Discussion

Risk of severe thunderstorms today for NB and perhaps parts of NS.  An approaching upper level jet will help enhance vertical development.  A passing thermal trough could mean destabilization this afternoon as upper levels could cool slightly.  No clouds in the area right now so solar insolation will be maxed out this afternoon and temps in the high 20’s to low 30’s will be a significant contribution to Tstorm development.  Add to that, a weak trough will move over northern NB this afternoon which will help as a lifting mechanism and also could create high helicity values at low levels.  Low level moisture is quite high but not significantly deep as 850 mb dew points are quite low…so this could be a limiting factor on development.  Precipitable water is only around 35 mm.

The weak through mentioned earlier will meander towards southern NB late this afternoon or evening, so to does the treat of severe tstorms.

 

Regional Impacts

NB – Supercell development is possible over northern NB this afternoon and southern NB late this afternoon or early evening.  Large hail, winds gusting to above 90 km/h and heavy localised rain are all possible.  A low probability of a tornado is not out of the question giving the possibility of supercell development.

 

NS – Heavy rain and hail possible, mainly along the valley and stretching towards New Glasgow.  Northern NS could see the potential this evening as tstorms move in from NB.

 

PEI – Heavy rain, high winds and hail possible as tstorms move in from NB late this afternoon or evening.

 

Labrador – risk of non severe thunder showers.

 

Barrie MacKinnon

 

Day 1 Convective Outlook for Atlantic Canada Valid for August 5, 2018

 

Convective Discussion

 

Showers at times heavy and isolated thunderstorms are expected today over eastern Nova Scotia, parts of western and southern Newfoundland and over the central and southern marine district today and tonight. An isolated thundershowers is also possible this afternoon over central and western New Brunswick.

 

 

Technical Discussion

 

An upper level trough over the Maritimes will push eastward today towards Newfoundland. There are a couple of surface low pressure areas over the Gulf of St. Lawrence and another over LaHave Bank. Convection has been firing up this morning ahead and near the vicinity of these lows where the air mass remains quite moist and unstable. There is a tropical moisture feed in the southerlies ahead of this trough and some areas of Nova Scotia have already seen heavy downpours (Nappan saw 29mm in one hour) this morning. There is ample low level moisture with precipitable water values near 45mm and dew points between 18-20C, so the concern will continue to be locally heavy downpours with some area exceeding rates of 25mm/hr. Adjusting the 12Z sounding at Stephenville, CAPE values could reach 500 J/kg if they get a bit of solar insolation this afternoon and with shear values in the mid to low 20KT range so there is a low threat for damaging winds.

 

For areas over central and western New Brunswick, skies are relatively clear this morning and daytime convection may initiate some pop up showers or thundershowers this afternoon. Shear is relatively low so should anything develop they will be slow moving so heavy downpours are possible.

 

 

Regional Impacts

 

Eastern Nova Scotia, eastern PEI and Newfoundland: Heavy showers in TCu at time and thundershowers giving lighnting and frequent downpours.

 

New Brunswick: Pop up Tcu or CB this afternoon possible which may give heavy downpours.

 

Marine areas: The southern Maritime marine district may see gusts to 30KT where the SST are warmer.

 

 

 

 

Roberta McArthur

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