Convective Outlook Valid for Today, Tonight and Tomorrow

Regional Impacts

Today: Isolated severe supercell structure is possible along colliding boundaries across western NB this afternoon. There is an elevated risk of rotation within these cells. Damaging winds, heavy rain, and hail are also NB hazards today. Across NS, storms are also likely to fire along “the spin” of eastern mainland. These cells are capable of producing moderate rainfall.

Tonight: With a tropical push of moisture and enhanced lift, scattered-numerous strong thunderstorms are forecast across mainland NS during the overnight hours. These storms bring a significant rainfall risk. A special weather statement has been issued to provide the public with advance notice of this event.

Tuesday: As the forcing pulls away over the marine district, embedded thunderstorms are still possible midday. The hazards are diminishing at this time.

Convective Discussion

--Potentially Impactful Next 24hrs--  There are 2 events that are shaping up to affect the Maritimes district. (Nfld and Labrador are quiet for this forecast period.)

1. Supercell development is possible in western NB this afternoon driven by high moisture and high shear. Pwats support deep convection, and there is a 65kt Jet supporting the activity. Effective shear is over 55kts at max daytime heating today. Sig tor and EHI flag tornadic potential, similar to yesterday but with stronger signals. This poses a wind risk, hail risk, and heavy rain risk. The yellow area on today’s outlook conveys this message. Alerts may be required. The dynamics upstream in Maine are being closely monitored at the typing of this report. Organized storms are also possible across inland NS – but these storms don’t have the same enhanced shear dynamics to work with. Mositure across NS is impressive this morning, dewpoints over 20, with fog patches.

2. Tonight brings even more moisture into the equation, from the remnants of what was once Tropical Storm Chantal interacting with a longwave nearly-stationary trof. This stream of juicy air is entering our AOR this evening. As such, Pwats soar over 65 in places, and columns are fully saturated by midnight. The main hazard tonight is heavy rainfall. The yellow area on tonight’s graphic depicts this risk area. KF Rainrate products are depicting hourly rainfall rates of 60 mm/hr. This seems potentially too high. But it cannot be fully ruled out. The system has tropical influence and a history of producing locally very heavy downpours. If storms are training or slow moving, then higher precip amounts are possible. Forecasters will be paying close attention to this tonight.

Thunderstorm Outlook for Today

Thunderstorm Outlook for Tonight

Thunderstorm Outlook for Tomorrow

Forecaster: Tirone.

Convective Outlook Valid for Today , Tonight and Tomorrow

Regional Impacts

Labrador … A few thunderstorms possible along cold front today, heavier showers and some wind gusts possible.

NFLD… No thunderstorms expected.

NB…Thunderstorms this possible this afternoon and this evening, locally heavy downpours and strong wind gusts are possible.

NS and PEI… Some thunderstorms could affect PEI and northern NS this evening.

Convective Discussion

Unsettled weather will continue over the next few days. An approaching cold front is expected to trigger some surface-based convection today over Labrador and into New Brunswick. While instability in Labrador is not particularly high, the fast-moving upper trough may destabilize the atmosphere more than models currently suggest. Some gusty winds may be possible in stronger cells, but they are expected to remain below warning criteria.

Over New Brunswick, strong wind shear, high moisture, and moderate CAPE could support the development of some intense cells. With strong linear shear in place, cells are expected to move quickly — roughly east to southeast at 25–30 knots — which may help limit the overall heavy rainfall potential. However, rainfall rates could still be very high, with some potential for localized flash flooding.

Although some convection is possible this afternoon, the more likely scenario is for deeper convective cells to move into western parts of the province by early this evening. If severe thunderstorm alerts are issued they would likely be later in the day.

Thunderstorm Outlook for Today

Thunderstorm Outlook for Tonight

Thunderstorm Outlook for Tomorrow

 

Mel Lemmon

Convective Outlook Valid for Today, Tonight and Tomorrow

Regional Impacts

Today: Some scattered thunderstorms possible across Atlantic Canada today. Main hazards are locally heavy downpours, small hail, and some strong wind gusts.

Tonight: Risk of some isolated thundershowers tonight through the central Maritimes. Locally heavy downpours are the main hazard.

Tomorrow: Some scattered thunderstorms possible over central Newfoundland tomorrow afternoon. Main hazards are locally heavy downpours, small hail, and some strong wind gusts.

Convective Discussion…

A cold front that moved through New Brunswick yesterday continues to produce some thunderstorm activity over the Gulf of St. Lawrence and eastern PEI this morning. Bulk shear values over 40 knots here under the 500 mb jet will aid in sustaining these storms through the day with land impacts likely to only be felt in the Magdalen Islands and northern Cape Breton early this afternoon, and extreme western Newfoundland later this evening. Heavy downpours are the main risk with these storms due to some higher PWAT values over 35 mm here, though some small hail is also possible with the strongest cells.

In addition, an upper low centred over northern Maine this morning will track into southern New Brunswick early this afternoon and continue to move east through the day. Modest MLCAPE values of 500 to 1000 J/kg through southern New Brunswick and central/eastern Nova Scotia will support the development of some thunderstorms out ahead of the upper low, in areas of enhanced vorticity advection. Bulk shear values are a bit lower than yesterday here, in the 25 to 35 kt range, which should limit storms from becoming severe or very long-lived. PWAT values in the 20 to 30 mm range and low freezing levels hint at the risk for some locally heavy downpours and small hail. Some of these storms may sustain themselves overnight around the upper low centre as it moves through the central Maritimes. Tomorrow, a similar environment is in place for central Newfoundland with the same upper low forcing convection.

Thunderstorm Outlook for Today

Thunderstorm Outlook for Tonight

Thunderstorm Outlook for Tomorrow

Forecaster: Copp

Convective Outlook Valid for Today, Tonight and Tomorrow

Regional Impacts

Today: Organized strong to severe thunderstorms across much of New Brunswick this afternoon. Main hazards are strong wind gusts, large hail, and heavy downpours. Also the chance for some isolated thundershowers over central Newfoundland.

Tonight: Elevated thundershowers with locally heavy downpours possible over central Atlantic Canada tonight.

Tomorrow: Risk of scattered thundershowers with some strong wind gusts and locally heavy downpours over central Atlantic Canada tomorrow.

Convective Discussion…

A potent day is in store for much of New Brunswick today as an upper low spins into the region. A very unstable airmass in combination with an approaching cold front has triggered some strong to severe thunderstorms this morning along the St. Lawrence river valley. This front will move east, reaching western New Brunswick early this afternoon. The 12Z Caribou sounding from this morning indicated the potential for upwards of 2000 J/kg of MLCAPE given sufficient surface heating, with some fairly steep lapse rates due to the cold upper low. Moderate bulk wind shear of around 35 to 45 knots will be conducive for organized thunderstorms to continue through the afternoon and into this evening. Shear remains fairly linear, as such the main storm mode is expected to be multicellular, though an isolated supercell is possible early on in the afternoon. Dry mid-levels will help contribute to hail growth this afternoon, which could reach up to 2 cm in size.

Elsewhere, a very slowly moving frontal boundary off the Atlantic coast of Nova Scotia has been triggering some isolated thundershowers overnight, aided by a moderate 500 mb jet. Later this afternoon, as the left exit to this jet reaches Newfoundland, a few isolated thundershowers may trigger over southwestern portions of the province and progress northeast later this afternoon. Modest MLCAPE values of 500 to 1000 J/kg should be enough for some of these thundershowers to develop, but weaker bulk shears will keep anything from getting too organized. As such, the main hazards with these storms will be locally heavy downpours.

Thunderstorm Outlook for Today

Thunderstorm Outlook for Tonight

Thunderstorm Outlook for Tomorrow

Forecaster: Copp

Convective Outlook Valid for Today , Tonight and Tomorrow

Regional Impacts

Today: risk of a thundershower near the ATL coast of NS as well as portions of NF. Scattered afternoon thunderstorms likely for southeastern LAB giving some gusty winds and possibly small hail. Slight risk of a thunderstorm this afternoon in NB.

 

Thursday: likely numerous afternoon and evening thunderstorms for a good portion of NB giving strong winds, hail and torrential rainfall. Isolated thunderstorms possible for western LAB, the interior of NF and some regions of the MRTMs.

 

Convective Discussion

Frontal trough crossing eastern NS and NL produced a couple isolated cells early this morning but have since weakened. Given the history there remains a chance of an elevated thundershower or 2 as the trough continues moving eastward. Another weak trough-like feature will likely spark some scattered thunderstorms across southeastern LAB this afternoon that could give some gusty winds, small hail and brief heavy downpours. The other area that could see some lightning is in NB, but I am having trouble finding a trigger. We have the heat and humidity, with good shear but lacking that trigger and instability. So just the slightest risk of a thunderstorm is expected.

 

Thursday could be active on the desk. A colleague once told me “I never met an upper low I could trust”. And that is what we will have approaching the region from Quebec in the afternoon and evening. All the ingredients will be in play so expect some strong to severe thunderstorms for most of NB. The activity could spill well into the evening hours.

 

Lastly there once again is no D3 prog sent this morning, but as that upper low/trough continues to dive E/Seward there could be strong to possibly severe thunderstorms for some regions of the MRTMs on Friday. Today’s potential outcome upstream in ON and QC should give an idea of what to expect in NB tomorrow. And what happens tomorrow will give a feel for what to anticipate on Friday. It’s the gift that keeps on giving…

 

Thunderstorm Outlook for Today

 

Thunderstorm Outlook for Tonight

 

Thunderstorm Outlook for Tomorrow

 

Jeremy