Day 1 Convective Outlook for Atlantic Canada Valid for July 3, 2018

Convective discussion:

Some isolated thunderstorms expected today and tonight associated with a weakening cold frontal trough that’s slowly crossing the Maritimes and Newfoundland.

Isolated thunderstorms possible for southeastern Labrador this afternoon and evening associated with a fairly strong short wave trough with some PVA.

Technical Discussion:

A weakening cold frontal trough will cross southern New Brunswick and western Newfoundland today to lie from Nova Scotia to southeastern Newfoundland this evening.

Isolated morning convection is occurring along the aforementioned frontal trough and this will likely occur at anytime today and tonight in its vicinity. KGYX 12Z tephi shows about 35 knots of shear with decent low-mid level lapse rates, as well as deep low level moisture. Modifying the profile generates between 500 and 800 CAPE, however the instability is marginal given the weakening state of the front…so just garden-variety pop up cells are expected with a very low threat of severe weather. Locally heavy downpours and some gusty winds can be expected. Freezing levels are quite high posing very little chance for hail.

In behind the front (southeastern Labrador) there is a considerable increase in wind shear, falling 500 mb atmospheric heights, drying at the mid levels, a short wave trough (and associated PVA) and a favorable left exit 250 mb jet. However, what’s lacking is the low-level moisture and instability. Isolated non-severe thunderstorms will possibly develop towards the mid afternoon today.

Regional Impacts:

New Brunswick, northern Nova Scotia (including Cape Breton), PEI and Newfoundland: locally heavy downpours and gusty winds.

Southeastern Labrador: Gusty winds.


Jeremy

Day 1 Convective Outlook for Atlantic Canada Valid for July 2, 2018

 

 

Convective discussion:

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

Some isolated to scattered thunderstorms possible for western Labrador. Isolated thundershowers over the Southern Grand Banks.

 

Technical Discussion:

A low pressure system near  James Bay will move to lie over Ungava Bay tonight.

 

For New Brunswick 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 15-21Z. Currently CAPES are forecast in excess of 1000 J/kg, and precipitable water about 30-40 mm. Surface and low level dewpoints are also good, with 20 C likely this afternoon at the surface, and a couple of degrees lower at 850 mb. It gets dry near and above 700 mb, which will support downdrafts. The shear starts 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.

 

If a supercell does develop later this evening, strong downdrafts are likely with DCAPES in the 1200 J/kg range  based on KBUF, KGYX, and WMW upstream. There will likely be heavy downpours, and nickel sized hail is also possible.

 

On the other hand the steering flow winds will be west to southwesterly, 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. In that case, the downdrafts and rain is still likely, but freezing levels of 15,000 ft, make hail unlikely.

 

For Labrador the warm front will pass through western Labrador early this afternoon, 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.

 

For the Southern Grand Banks, a low slowly moving away will five isolated thundershowers until the system passes east overnight tonight.

 

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.

 

Southern Grand Banks: Wind gusts and local heavy showers ending overnight.

 

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

 

Doug Mercer

 

Lead Meteorologist, Meteorological Service of Canada

Environment Canada / Government of Canada

doug.mercer@ec.gc.ca / Tel:902-426-9200

 

Météorologue principal, Service météorologique du Canada

Environnement Canada / Gouvernement du Canada

doug.mercer@ec.gc.ca / Tél: 902-426-9200

 

 

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