Day 1 Convective Outlook for Atlantic Canada Valid August 8, 2019

Convective Discussion

A trough of low pressure and an associated upper jet will approach from the west today. Over the Maritimes embedded/non-surface based thundershowers are possible with the associated area of cloud and rain this afternoon and/or tonight.  Over western Labrador, surface based thundershowers are possible this afternoon and into the evening ahead of the approaching area of cloudiness. The energy is weak and the shear is minimal so just a risk is expected.

Regional Impacts

Maritimes…

Risk of thundershowers.

Newfoundland and Labrador…

Risk of thundershowers .

Andy Firth

Convective Outlook Valid for August 7, 2019

Convective Discussion

Today there are three things going on. For Labrador a combination of a surface low under a weak diffluent upper  jet will give non-severe thunderstorms which may persist into this evening. For New Brunswick air-mass thundershowers are possible ahead of a slowly approaching low, with a southerly flow brining in an axis of moisture up to al least 700 mb. Finally there is a low crossing the scotia shelf and giving embedded thundershowers.

For Labrador the upper trough moving in from the west is diffluent with a weak jet on the south side. Shear is around 25-30 kts, with CAPEs possibly reaching 500 J/kg, but more likely less.

Dewponts will increase slightly this afternoon, and if they reach 16 C, the 500 CAPE may occur. Downdrafts are possible, but shouldn’t be more than 60- kph.

For western New Brunswick, surface to 700 mb dewpoints are increasing and may reach 17+ C from he surface, and good insolation is expected early this afternoon with mainly clear skies in the northwest. PWATs are healthy and between 30-40 mm. Shear is low, arguing for airmass pop-ups. The only worry is the slow speeds of the potential thundershowers, which may give 25 mm/hr.

Regional Impacts

New Brunswick: mainly a chance of  non-severe thundershowers into this evening, with a chance for local downpours near or possibly exceeding 20 mm.

Labrador: a chance of non severe thundershowers, with showers up to 15-20 mm and some gusty winds.

Southern Waters: Embedded thundershowers with occasional gusty winds.

Doug Mercer

Convective Outlook Valid for August 6, 2019

Convective Discussion

 

There are residual low clouds associated with a departing low pressure system east of Newfoundland. Elsewhere across the Atlantic provinces, the upper ridge continue to prevail today preventing any convective cloud to form.

Two upstream systems may impact the parts of the region in the medium term:

A frontal wave over northwestern Quebec will advance towards western Labrador tonight and Wednesday bringing a slight risk of thundershower;

Another frontal wave, over the Gulf stream will push a humid air mass over Nova Scotia offshore waters bringing scattered thundershowers along with a chance of downpours and locally gusty winds.

 

Regional Impacts

 

Newfoundland: Nil

 

Labrador: Slight risk of thundershowers early Wednesday

 

NB and PEI: Nil

 

NS: Downpours and higher gusts near thundershowers for offshore waters

 

 

Jean-Marc

 

Convective Outlook Valid for August 5, 2019

Convective Discussion

 

At the surface, a cold front stretches north-south over eastern portions of Newfoundland. An upper cold low will remain parked over the lower Quebec North Shore today, eventually moving east of Labrador Tuesday.

A cluster of lightning associated with the cold pool persists this hour over southeastern Labrador, and this activity is expected to continue well into the afternoon. An upper trough extending southward from the upper low along the west coast of the island will move across central regions this afternoon triggering isolated thundershowers ahead of it. The main threat with these storms is brief heavy downpours and gusts up to 70 km/h.

 

Tranquil weather conditions are expected across the rest of the Atlantic provinces this afternoon.

 

Regional Impacts

 

Newfoundland: potential for brief heavy downpours (20 mm) for northern, central and eastern regions of the island

 

Labrador: potential for brief heavy downpours (20 mm) over southeastern regions

 

NB, NS and PEI: Nil

 

 

Jean-Marc

 

Convective Outlook Valid for August 4, 2019

Convective Outlook.

 

There are a couple areas of concern today: one over central NL and the other along portions of NS. First in NL, elevated morning convection giving locally heavy downpours should weaken in the coming hours. If things clear in the SW after lunch there is plenty of CAPE (1000 j/kg or more) so cells could fire fairly quickly and move northeastward through the afternoon and into the evening. Based on the 12Z YJT tephi the shear is low, but freezing levels are fairly low as well. Could be some strong wind gusts and small hail, but the main threat would be torrential downpours with 25 mm/h possible.

 

Now for NS. After analyzing things from the surface to 250 all the ingredients are there for a potentially stormy afternoon (of course, from experience working the desk this not always pans out). There is a 90 knot 250 mb jet observed in YQI placing NS in the left exit region. There is a strong 50 knot jet along SW NS with an approaching upper trough to the west, with 40-50 knots of shear along with falling atmospheric heights. There is also an approaching 700 mb trough. Sat pix shows plenty of sunshine to aid convection at the moment (need to keep an eye on some mid-level cloud sneaking into SW regions). There is a thermal ridge at 850 pushing into the province and an approaching surface cold front, that at 12Z, was analyzed to be somewhere near western and northern portions of the province. Dewpoint temps should be in the upper teens this afternoon proving the moisture. There were a couple severe cells in NB yesterday with Tds several degrees cooler, so confidence is fairly high that we see a couple strong storms this afternoon and into this evening. In addition, CAPE values could be up around 1500 j/kg. The instability won’t be that strong as LI’s will be around -4, but with all the other ingredients, this shouldn’t be of concern as a limiting factor at all. Torrential downpours and strong winds are the main threats…though some small hail is also possible.

 

Regional Impacts.

 

Newfoundland: potential for very heavy downpours with cells today across central regions of the island…25 mm/hr.

 

Nova Scotia: potential for very heavy downpours and strong winds from Queens County through to Pictou and Antigonish (including all counties along the ATL coast of NS) as well as Cape Breton.

 

PEI: slight risk of a thundershower for eastern portions of the Island.

 

NB/LAB: nil.

 

 

Jeremy