Convective Outlook Valid for Today and Tomorrow

Regional Impacts

For Nova Scotia: there is a slight risk of a severe thunderstorm across portions of the eastern mainland into early this evening. The main threat is heavy rainfall. Some gusty winds are also possible but below warning threshold (90 km/h).

For New Brunswick: scattered thunderstorms are expected to develop this afternoon.

For Newfoundland: there is a risk of isolated elevated/nocturnal  thundershowers tonight

 

Convective Discussion

As mentioned above, there is a slight risk of a severe thunderstorm across the eastern mainland of Nova Scotia this afternoon. Looking at the parameters (MIST): we have the moisture with dewpoint temperatures into the low 20’s and a moist tongue at 850; the instability is there with model LI’s in the minus 5 to minus 8 range; analysis from the 12Z YQI and YAW tephis show 30-40 knots of shear in the 0-6 km level; and lastly a weak trough offering up a trigger mechanism once the convective temperature is reached early this afternoon. Upper air analysis doesn’t show much at 250 mb as the main jet is well to the north of NS. There is a 40 knot jet at 500 mb, but not really an indication of at least minimal falling atmospheric heights. The upper level trough at 500/700 is well to the west and won’t be a factor until D2 as it approaches Maine and New Brunswick. One thing noted on the 12Z tephis is the warm, stable air aloft. This will be the determining factor. Modifying the 12Z soundings for temperatures of 30, and dewpoints of 22 gives about 1000 mixed-layer CAPE. The MUCAPE is way overdone and ignored. I expect TCUs to form shortly after the lunch hour with isolated-to-scattered TS not long thereafter. Once again, the stability aloft may prevent any potential cells from becoming severe (25+ mm of rain in one hour).

 

Scattered cells are likely to develop across northern New Brunswick this afternoon…there is a very little chance in seeing anything close to severe limits. Locally heavy downpours can be expected.

 

Lastly, there is a risk of elevated convection tonight across southern Newfoundland.

 

Thunderstorm Outlook for Day 1

 

Thunderstorm Outlook for Day 2

Convective Outlook Valid for Today and Tomorrow

Thunderstorm Outlook for Day 1

 

Regional Impacts

New Brunswick: Heavy downpours for most of the province likely, with strong downdrafts and possibly a wet microburst. The main threat area is the southwest.

Nova Scotia: Poissibly a similar situation to New Brunswick, but mainly for the Cobequids and the west inland for the worst impacts.

Prince Edward Island: Scattered thundershowers possible, with local heavy showers and gusty winds.

 

Convective Discussion

There’s a cold front from north of the Gaspe and extending southwest over northwestern Maine, slowly moving in this afternoon . There’s ample low moisture in the warm sector with PWATs near 45 mm, a moist tongue over southern New Brunswick, and a dry layer at 700 mb near and behind the cold front. Bulk shear is 40+ kts, but there is little storm relative helicity. This argues for multicells with the front as an organising feature.  There’s already some weak prefrontal lines approaching the northwest, and conditions should intensify this afternoon, with some cooling aloft as a thermal ridge moves east of the porvince and we get some daytime heating. A diffluent flow aloft associated with an upper low to the northwest should also help.

 

For Nova Scotia conditions will occur later this afternoon or this evening with the approach of the low. Conditions should moderate into this evening.

 

 

 

Thunderstorm Outlook for Day 2

 

 

Convective Outlook Valid for Today and Tomorrow

Thunderstorm Outlook for Day 1

 

Regional Impacts

New Brunswick and western Nova Scotia: Depending on cloudiness, possibility of thunderstorms east and severe thunderstorms west, with heavy downpours the greatest threat, followed by gusty winds.

Northern Newfoundland and southeastern  Labrador: chance for non severe thunderstorms,

 

 

Convective Discussion

The synoptic situation is similar to yesterday, with a low near Hudson Bay and a second over the Labrador Sea, with an extended east-west trough between them and over Labrador. This also produces slight ridging over Quebec and New England.  For Newfoundland and Labrador there is some support for non-severe convection near the Strait of Belle Isle.  Some weak PVA from a jet right exit, combined with daytime heating may give a few cells. PWAT is only about 20 mm., but CAPEs in excess of 200 J/kg with a slight chance of 500 J/kg are possible.  Gusty winds and brief heavy showers are the main issues, with a slight chance for hail with YYR having freezing levels below 8000 ft.

 

For the Maritimes things are trickier. Cloudiness will be the key. There’s some upper support from a 500 mb jet near southwestern New Brunswick with some associated lift, with plenty of moisture below 700 mb. PWATs range from 40-60 mm, with surface dewpoints near 20 C,  and 850 dewpoints up to 15 C , with a moist tongue poking into southern New Brunswick. If things clear, CAPEs of up to 1000 J/kg or maybe higher are possible. Freezing levels near 15,0000 ft suggest large hail is unlikely. Very heavy rainfall is the largest risk, followed by gusty winds that may reach warning level, and finally some small hail. For the rest of New Brunswick and parts of Nova Scotia, the impacts will be somewhat weaker, but rainfall followed by winds as the main risks.

 

 

Thunderstorm Outlook for Day 2

 

Convective Outlook Valid for Today and Tomorrow

Thunderstorm Outlook for Day 1

 

Regional Impacts

New Brunswick: Possibility of non-severe thundershowers in the northeast if the thick cloud there burns off this afternoon. Possibility of Severe thundertorms in the extreme southwest if the cloud deck moves away or burns off,  with high wind the largest risk, followed by heavy downpours.  

Newfoundland: thunderstorms beginning this afternoon with possible heavy downpours, especially with the slow movement of the cells.

 

Convective Discussion

 

For Newfoundland: there’s a low of the coast of Labrador  from 250 mb down, with some upper dynamic support with PVA approaching western Newfoundland.  There’s some good moisture with dewpoints of up to 17 C and PWATs near 24 mm, with most of it below 850 mb, Shear and storm motion is relatively low,  suggesting plenty of time for rain to accumulate, and mean layer CAPEs suggest possibly reaching or exceeding 500 J/kg near and west of Gander, if they can get some clearing. Small hail is also possible, with freezing levels around 10-11,000 ft.

 

For New Brunswick:  there are some good ingredients, especially in the far southwest, but the big inhibitor is cloudiness.  The most likely place for convection is in the northeast, where things may start clearing this afternoon, but with non-severe cells likely. The general ingredients are near 50 kts of shear, PWATs in the 30-45 mm range, with surface dewpoints near 19 C and 850 dewpoints near 13 C moving in from the west.  If things clear in the north, CAPEs could reach 200-500 J/kg.  Gusty winds and local heavy downpours are the main threats, but the cells should move fast enough to limit therainfall from any given cell.

 

For the southwest the main risk would be wind gusts possibly reaching warning level, followed by local heavy downpours. Again, this is assuming some surface insolation this afternoon.

 

 

Thunderstorm Outlook for Day 2

 

Convective Outlook Valid for Today and Tomorrow

Regional Impacts

Maritimes – Heavy rainfall with small hail and strong gusts with the strongest likelihood along coastal NS but eastern inland NS and eastern NB also possible.

Newfoundland and Labrador – risk of thundershowers with slight chance of severe over central and northeast.

 

Convective Discussion

Potential for thunderstorms today for some regions.  Upper dynamics will become favourable for tstorm formation today as the Maritimes enters the left exit area of an upper jet and area of diffluence.   Cooling aloft through the first part of the day will help destabilize the upper atmosphere.  Closer to the surface, there is quite a bit of moisture at the surface but it does not extend significantly up to 850mb.  There is some higher moisture values moving in today but again…not significantly high.  Precipitable water values of around 25 to 30 mm are expected.  0-6 km shear values as high as 35 kts so this will be enough to maintain tstorms if/when they form.  500 to 1000 J/kg of CAPE can be expected today for the Maritimes.  The one thing that seems to be lacking is a significant synoptic feature to kickstart tstorms.  What we will have is coastal convergence along sea breeze fronts that are expected to develop this afternoon.  That will be the main area of concern but inland areas could get tstorms if there are some weak areas of convergeance in the area or if coastal storms advect.  The highest impact would be heavy rainfall as storms along the coast will likely be moving slowly and/or training could develop. Newfoundland is farther from the favourable dynamics today but still have a risk of thunderstorms especially over central and northeast.

 

Thunderstorm Outlook for Day 1

 

Thunderstorm Outlook for Day 2

 

Barrie MacKinnon