Convective Outlook Valid for Today, Tonight and Tomorrow

Regional Impacts

 

Today:

NL/LAB… Some isolated thunderstorms possible in parts of central and eastern Newfoundland as well as western Labrador today. Locally heavy downpours of 10 to 20 mm/h.

NB…Risk of thundershowers in western New Brunswick with 10 to 20 mm/h rates.

NS/PEI… None.

 

Saturday:

NB…Some thunderstorms possibly severe for northern regions with locally heavy downpours of 25 mm/h, small hail and strong wind gusts.

LAB…Scattered thunderstorms over central and southeastern regions.

NS…Embedded thunderstorms possible with locally heavy downpours of 10 to 20 mm.

 

Convective Discussion

For today, a shortwave trough continues eastward across Newfoundland today while a broad trough moves across Hudson Bay into Central Quebec and western Labrador this evening. MLCAPE over central Newfoundland of 500 J/kg, bulk shear of 20kt and further cooling at 500mb could see some convection maintain in the afternoon. High precipitable water near 45 mm remains over the region, so locally heavy downpours are the main risk. Some TCUs in showers are possible for the Maritimes this afternoon under a hot and humid air mass. Precipitable water is around 30 mm but mean wind is rather weak, so any convection that develops may also give locally heavy downpours. There is a risk of a thunderstorms mainly where MLCAPE could reach 1000 J/kg over northern New Brunswick with a weak trough approaching from the west. There is only a slight risk that TCUs could breach the warm 500-700mb cap currently over Nova Scotia along the Annapolis Valley with the help of some sea breeze convergence.

 

For tomorrow, the broad trough continues over central and eastern Labrador and southwestward into northern New Brunswick. Convection could be organized given increasing moisture under a southwest flow in the afternoon with upwards of 1200 J/kg of MLCAPE, but upper dynamics are expected to be weak to moderate with bulk shear of 20 kt. Some organization of convection is possible into multicells are possible with brief strong wind gusts, locally heavy downpours and small hail being the main threats for northern New Brunswick. A second shortwave moves into New England tonight and into the Gulf of Maine by early Saturday into southwestern Nova Scotia. There is a risk of elevated convection and locally heavy downpours for Nova Scotia and southern Maritimes waters with an increasing 40kt 850mb jet as this wave continues eastward.

 

Thunderstorm Outlook for Today

 

Thunderstorm Outlook for Tonight

 

Thunderstorm Outlook for Tomorrow

 

 

Roberta McArthur

 

Convective Outlook Valid for Today, Tonight and Tomorrow

Regional Impacts

NS/NB/PEI… Some scattered thunderstorms possible over the central Maritimes today. Heavy downpours of 20 to 30 mm/h possible.

NL… Some isolated thundershowers possible today and tonight. Heavy downpours of 10 to 20 mm/h possible.

 

Convective Discussion

A broad upper trough will be the main focus for convection today across Atlantic Canada. A few isolated thundershowers are possible over parts of Newfoundland and Labrador this afternoon, however the main focus will be in the central Maritimes, where a few weak embedded shortwaves slightly increase the potential for some organized thunderstorms to develop today. 49.4 mm of PWAT was measured at the 12Z Yarmouth sounding this morning, with this number forecast to reach 50 to 55 mm across portions of western and central Nova Scotia, southeastern New Brunswick, and PEI this afternoon. Due to the high amount of low level cloud cover over the region, convection will rely heavily on upper level forcing today. There is a 500 mb jet over the area of interest, however it is quite weak, so convection may initially struggle to become organized until the 250 mb jet begins to intensify off the coast of Nova Scotia later this afternoon. With 20 to 30 kts of bulk shear today, the severe risk is low and storms are not expected to be long lived, however any thunderstorms that develop will have the potential to produce torrential downpours of 15 to 25 mm/h due to the high PWAT over the area.

As the upper trough moves east tonight, the risks exists for some embedded thundershowers over western Newfoundland overnight. Heavy downpours of 10 to 20 mm/h will be possible here, maybe higher over areas with topographic enhancement.

 

Thunderstorm Outlook for Today

Thunderstorm Outlook for Tonight

Thunderstorm Outlook for Tomorrow

Copp

Convective Outlook Valid for Today, Tonight and Tomorrow

Regional Impacts

NL… Some strong thunderstorms possible in parts of northern and central Newfoundland today. Main hazards are torrential downpours of 20 to 30 mm/h and strong wind gusts of 50 to 80 km/h.

NB… Some isolated thunderstorms possible over extreme northern New Brunswick this afternoon. Heavy rainfall of 10 to 20 mm/h possible.

NS/PEI… None

 

Convective Discussion

Two areas of interest exist today, the first being triggered by a weakening shortwave currently near the Strait of Belle Isle, the second being triggered by a broad upper trough over central Quebec, extending down into New England. The shortwave moving into western Newfoundland already has a history of lightning activity, with this expected to continue as it moves across northern Newfoundland today. MLCAPE is expected to build today in central Newfoundland as clear skies will allow surface temperatures to climb ahead of the approaching feature. In addition, decent dynamics (30 to 40 kts of bulk shear) will aid in sustaining some organized convection today across this region. This shortwave is forecast to weaken slightly as it traverses east across northern Newfoundland, however PWAT values of 40 to 45 mm may produce some torrential downpours of 20 to 30 mm per hour. Some strong wind gusts of 50 to 80 km/h are also possible in convection here today.

The second feature being monitored will approach western Labrador and parts of northern New Brunswick later this afternoon. Although MLCAPE values are forecast to climb above 500 J/kg, weaker bulk shear (15 to 25 kts) keeps the risk for any organized thunderstorms low. Low level cloud cover this morning is also expected to limit the available CAPE in northern New Brunswick, as such the risk for thunderstorms is lower here than in western Labrador, where clear skies are allowing CAPE to better build. The broad nature of this upper trough should keep the risk for any severe weather near nil due to the lack of shear and cold air advection. Nonetheless, some brief heavy downpours of 10 to 20 mm/h are possible in any thunderstorms here today.

Tomorrow, this upper trough will move into central New Brunswick. Again, bulk shear values remain quite low (10 to 20 kts), however PWAT values are expected to climb upwards of 45 mm by tomorrow afternoon over the southwestern Maritimes. Torrential rainfall rates of 15 to 25 mm/h will be the main hazard here in any thunderstorms that do develop here.

 

Thunderstorm Outlook for Today

Thunderstorm Outlook for Tonight

Thunderstorm Outlook for Tomorrow

Copp

Convective Outlook Valid for Today , Tonight and Tomorrow

Regional Impacts

Today.

Scattered thunderstorms expected for central and western LAB as well as most of NB. There is a chance that a couple of the storms will become severe across northwestern NB giving strong wind gusts, small hail and torrential downpours.

 

Tonight.

Scattered thunderstorms are possible across most of LAB. Isolated thunderstorms possible for NE NB and most of NL.

 

Wednesday.

Scattered thunderstorms likely again for a portion of LAB, central and NE NL and northern NB. Some of these storms could approach severe limits.

 

Convective Discussion

A couple features today will be responsible for some thunderstorm activity – the first one being an upper level low/trough currently over ME that will drift northeastward and the second a frontal trough ahead of the upper low that is currently just south of NS. The upper trough could give a severe thunderstorm or 2 to northwestern NB this afternoon and into this evening. Adjusting upstream TEP over New England gives 700-1000 MLCAPE. Wind shear is in the 30-35 knot range with PWAT values of 35-40 mm. The only thing that could limit the growth of the cells would be the available sunshine and associated instability. If things clear sufficiently it will allow for a fairly buoyant atmosphere to exist which would give way to some strong to severe cells. Also of note is some storms will be fairly slow-moving owing to a threat for locally heavy rainfall amounts. Right now things are pretty overcast so time will tell. Slightly falling atmospheric heights and a surface trough should be enough to trigger some non-severe thunderstorms across central and western LAB this afternoon and evening. Locally heavy downpours seem to be the main threat.

 

Isolated thunderstorms are expected for northeastern NB and a portion of NL tonight as that frontal trough ahead of the upper low/trough moves through the region. High PWATs will give way to locally intense rainfall rates. There is a little history of lightning south of NS associated with this feature. Scattered thunderstorms could persist into tonight across LAB associated with the surface trough.

On Wednesday that same upper trough impacting NB today will cross central/northeastern NL giving some scattered thunderstorms with some approaching severe limits. Of course this is based on model information and what happens today in NB will be a better gauge on what could happen tomorrow. Lastly another upper trough will approach NB and LAB on Wednesday giving some scattered afternoon/evening thunderstorms.

 

 

Thunderstorm Outlook for Today

 

Thunderstorm Outlook for Tonight

 

Thunderstorm Outlook for Tomorrow

 

Jeremy

Convective Outlook Valid for Today , Tonight and Tomorrow

Regional Impacts

 

Labrador… A risk of severe thunderstorms over western areas with some potential for significant wind gusts and heavy downpours.

Maritimes… Some embedded thunderstorms tomorrow in showers and rain moving in from the south during the day.

 

 

Convective Discussion

 

The potential for severe thunderstorms edging into western Labrador still persists for today. It is possible that the most severe storms will remain in Quebec, but should storms track into Labrador there is a reasonable chance that some will produce very strong wind gusts. Lightning and satellite should be monitored closely.  Weak upper ridging could be sufficient to limit convection early in the day, however wind shear is favourable for the development of some discrete supercells, with strong shear to 3 km it is likely that strong rear flank downdraft could develop on some of these storms.

 

For the remainder of the Atlantic provinces today there is little to know risk of thunderstorms, however there is a risk of some embedded thunderstorms in the moisture plume moving into the region tomorrow. At this point the instability isn’t looking high enough for torrential downpours but with PWAT values near 50mm there is still some potential for heavy rainfall rates should thunderstorms develop.

 

 

Thunderstorm Outlook for Today

 

 

Thunderstorm Outlook for Tonight

 

 

Thunderstorm Outlook for Tomorrow

 

 

Mel Lemmon