Convective Outlook Valid for Today and Tomorrow

Regional Impacts

Labrador: some isolated thundershowers expected today associated with a southeastward moving trough of low pressure. Gusts to 70 km/h are possible.

Central/eastern/northeastern Newfoundland: a slight chance of late afternoon and evening thundershowers.

Maritimes: a chance of thundershowers today and tonight in association with a fairly stationary upper level low.

 

Convective Discussion

Labrador:

There is a fairly strong 700-500 mb jet descending from Ungava Bay area (YVP has about 55 knots of shear based on the 12Z tephi). A surface frontal zone could trigger some isolated cells this afternoon and into this evening as daytime highs hit the upper 20’s to low 30’s. PWAT at YVP is close to 40, but the air mass is moving into a drier zone, so heavy downpours shouldn’t be an issue at this time. Given the dryness below 700 mb at YYR, some wind gusts in cells is a possibility. Right now gusts to 70 seem reasonable.

 

Central/eastern/northeastern Newfoundland:

There is a weak area of instability laying across the aforementioned areas on the island. Given the latest sat pix skies appear to be clearing so there is a risk of isolated cells this afternoon and into this evening. 12Z YJT tephi is not particularly interesting, but with enough solar insolation CAPE could reach 300. Given about 15 knots of 0-6 km shear, weak pulse cells are expected.

 

Maritimes:

Lastly, the stubborn upper area of low pressure west of the MRTMS. Yesterday the guidance was suggesting potential for some heavy downpours given PWAT was above 40mm and in an area of modest divergence aloft, but there was next to nothing. Lots of activity over the marine areas, but nothing at all over land. So given the history of the upper low only a slight risk of a TSRA is expected, and this would mainly apply to Nova Scotia tonight. Not included in the risk zone is northwestern New Brunswick. If given enough surface heating an isolated cell is possible. Right now I am expected mostly TCUs.

 

Thunderstorm Outlook for Day 1

 

Convective Discussion

The greyed area is a combination of the leftover upper low that will be weakening somewhat and the approaching frontal trough over Labrador. With the increasing shear associated with the trough some gusts to 70 km/h are possible over southern Labrador (of course where only the caribou roam).

 

Thunderstorm Outlook for Day 2

 

Jeremy

Convective Outlook Valid for Today and Tomorrow

Regional Impacts

Nova Scotia, PEI and southern New Brunswick: likely some elevated convection this afternoon and through the night associated with an upper level area of low pressure over New England. Heavy downpours are the only threat.

Newfoundland: a weak trough of low pressure may give an isolated thundershower to southeastern regions through the course of the day.

Labrador: nil.

 

Convective Discussion

The aforementioned upper area of low pressure has produced quite a bit of elevated convection to the east/southeast where the area of divergence is maximized. The sun has been warming the tops some this morning which has lead to a weakening in the lightning but things will continue gradually progress northeastward through the course of the day. There could be some locally heavy downpours later today and tonight. The PWAT is close to 40 mm. There is a slight risk of an embedded thundershower across portions of southeastern Newfoundland in association with a weak trough of low pressure. OTWZ nothing to talk about for the rest of the region.

 

Thunderstorm Outlook for Day 1

 

Convective Discussion

There will remain a risk of thundershowers over portions of SE NL and the MRTMS associated with the upper area of low pressure. Also a frontal trough will be moving southeastward across Labrador that may trigger a couple thunderstorms.

 

Thunderstorm Outlook for Day 2

Convective Outlook Valid for Today and Tomorrow

Thunderstorm Outlook for Day 1

 

Regional Impacts

New Brunswick: Some thunderstorms this afternoon, with heavy downpours, gusty winds, an a low chance for hail. In the northwest the winds and rain may reach warning criteria.

Newfoundland: Some thunderstorms this afternoon, with possible heavy downpours, some gusty winds, and a chance for hail. In the north, roughly from Grand Lake  east, there is a slight chance for severe weather, with rainfall followed by gusty winds being the greatest risk.

Nova Scotia: some thundershowers possibly this afternoon into this evening. Non-severe elevated convection, mostly.

Prince Edward Island: Possible non-severe thundershowers.

Western Labrador: Thundershowers this afternoon with some gusty winds.

 

 

Convective Discussion

The broad upper low has move southeast to northwest of the St. Lawrence River, with an associated broad surface low just north of Vermont. A trough stretching east over northern New Brunswick and continuing east over Newfoundland will be part of the trigger for this afternoon. Similarly,  an associated low/trough southwest of Yarmouth will give embedded elevated convection to the eastern part of Nova Scotia as it moves east today, and will start  impacting southern Newfoundland overnight tonight, and mat give a little more elevated convection to the southern Island. Southwestern Labrador will see some more afternoon into this evening.

 

Northwest New Brunswick and north-central Newfoundland have measurable risks for severe convective impacts, mainly for strong gusts and heavy downpours. For New Brunswick, there is a little dynamic forcing with some weak upper PVA near the trough, and PWAT is higher than yesterday at around 30-35 mm. Adjusting the Caribou and Gray  tephis for this afternoon’s max temperature gave CAPEs between 700-1500 J/kg. assuming the clouds break up a bit. The shear is low, and we’re not expecting supercells, but some heavy downpours and strong downdrafts are possible. A watch will be issued shortly. For Newfoundland the ingredients are weaker, but there is still a possibility forstrong downdrafts and some local heavy showers.

 

 

 

 

Thunderstorm Outlook for Day 2

 

Northern Labrador may have some weak thundershowers in the afternoon. Newfoundland may see some embedded convection associated with the trough from Nova Scotia moving into the southern island. For Nova Scotia and souther New Brunswick  a second disturbance may give some embedded convection.

 

 

 

Convective Outlook Valid for Today and Tomorrow

Thunderstorm Outlook for Day 1

 

Regional Impacts

Nova Scotia, New Brunswick and Prince Edward Island, and Newfoundland:  a few thundershowers with a slight risk of small hail, some gusty winds, and brief downpours.

Labrador: similar to the Maritimes, but a bit weaker.

 

Convective Discussion

Currently there is a broad occluded low west of Labrador with a deep cold upper trough extending south to southeast across the eastern Gulf of St. Lawrence. In addition there is a weak broad east-west trough across most of Atlantic Canada.  Currently there are strikes along the deep upper trough from Gulf - Port au Port to the vicinity of Wabush. The ingredients are fairly moderate for most regions, with PWAT generally between 20-30 mm, with most of it below 700 mb, freezing levels not far from 10,000 ft, giving a possibility of small hail, and dry air above 700 mb, giving some gusty winds. Shear is a bit more variable. Over Labrador it varies from 15-25 kts, which probably will inhibit hail. For Newfoundland it increase from 15 near Stephenville to near 45 kts near St. John’s, and with some partially clear skies over the island may give some small hail and good but non-severe wind gusts as you move east.  Surface dewpoints range from 8-12 for Labrador and 10-15 for Newfoundland. The CAPEs for Newfoundland should stay below 500 L’kg, and for Labrador below 250.

 

For the Maritimes there is little in the way of upper support except for the weak broad trough near New Brunswick.  Clear skies over most of the Maritimes will give good insolation, except for New Brunswick where some cloud is moving in.  For the Maritimes PWAT is similar, with dewpoints ranging from  13-16 C  for New Brunswick and 15-18 for the rest of the Maritimes.  The shear is between 25-35 kts, and mean layer CAPEs may exceed 500 J/kg, especially for eastern Nova Scotia. Again, freezing levels near 10,000 ft may give small hail, as seen yesterday in New Brunswick. For tonight a trough will approach from the southwest and may give some nocturnal convection.

 

 

 

Thunderstorm Outlook for Day 2

 

Roughly similar areas, with somewhat different dynamics. There will be a  broad low over Maine and extending north, with a trough extending east to Newfoundland, part of which will push north twards Labrador, initiating some thundershowers.  A second trough, mentioned above, will continue to  move across Nova Scotia tomorrow.  The freezing levels will be a bit higher tomorrow, reducing the chance for hail.

 

 

Convective Outlook Valid for Today and Tomorrow

Thunderstorm Outlook for Day 1

 

Regional Impacts

 

New Brunswick: strong wind gusts likely and near severe possible, possibly small hail, and some brief heavy downpours.

Nova Scotia and Prince Edward Island: chance for weak thundershowers, with some gusty winds and brief downpours.

Labrador: strong wind gusts likely and near severe possible, possibly small hail, and some brief heavy downpours.

 

Convective Discussion

The situation is roughly similar to yesterday, with an occluded low southeast of James Bay and a trough extending from it east to the Great Northern Peninsula of Newfoundland. A weak low has developed northeast of the Island, with yesterday’s cold front extending south. The low is moving away and will no longer be of concern.  For the Maritimes there is a deep cold north-south trough near western Maine which will slowly drift east, destabilising mid to upper levels.  The associated upper jet over New Brunswick is not aligned to help with lift, however.  There is a moderate but no generous amount of moisture, with PWAT in the 20-30 mm range, and mainly between 700-900mb, with surface dewpoints near 15 C being advected in from the southwest. So the main lift mechanism is daytime heating, a bit of PVA from the trough near west and northwest New Brunswick, and enough moisture to make things interesting. With the dry mid to upper layers, strong downdrafts are possible. With the freezing level near 10,00 ft, there is a chance for small hail.  For Nova Scotia there is less dynamic lift, so marginal thundershowers are possible. For Labrador, condifitons are similar to New Brunswick, but with the freezing level higher and some weak PVA associated with the east-west trough.

 

 

 

 

Thunderstorm Outlook for Day 2

 

For the Maritimes basically a repeat from yesterday, with the area of highest threat moved southeast into parts of Nova Scotia. The low freezing levels from yesterday persist, so there is a chance of small hail, and gusty winds are also likely.