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.

 

Convective Outlook Valid for Today and Tomorrow

Thunderstorm Outlook for Day 1

 

Regional Impacts:

Labrador: generally non-severe convection, with some heavy showers and gusty winds.

Eastern Nova Scotia: A chance for topographic enhancement to trigger a thundershower or two.

Western Newfoundland: A chance for topographic enhancement to trigger a thundershower or two.

Southern waters: weak embedded convection from Sable south. It may get close to Cape Breton Island.

 

Convective Discussion

There’s a deep occluded low southeast of James Bay, and with a trowel  extending to a warm front-cold front system over western Labrador. The warn front extends east across Labrador, while the cold front extends south across the western Gulf of St. Lawrence and Nova Scotia through to Georges Bank. The frontal system is moving almost directly east, with the cold front also moving east across the Gulf.  The convection over Labrador is currently along the warm front-trowal line (the main trigger) and has some moderate moisture, shear in the 40-60 kt range, and marginal CAPE.  The most likely impacts are some heavy showers and possible gusty winds.  Southwest of the low and trowel there will be some cooling and drying aloft behind a deep thermal ridge. There will also be some PVA associated with the right exit of the upper jet, and the clouds . The only significant moisture source will be below 850 mb, with dewpoints maxing out near 12 C. This probably implies fairly gusty winds as the largest threat.

 

Ahead of the cold front over the Maritimes the moisture is great, but the profiles are closer to moist adiabatic with little cape. The font will be the primary lift mechanism, with heavier downpours. There’s a chance this afternoon for Cape Breton, and mor into the evening for western Newfoundland to have a couple of thundershowers enhance by topography.

 

 

 

Thunderstorm Outlook for Day 2

 

On Friday there will be atrough extending itself over southern Labrador, possibly giving some thundershowers mainly south of Goose Bay and west to Wabush in the afternoon. PWAT will be near 30 mm, surface based CAPEs near 500 J/kg, and with shear in the 20-30 kt range in the west and 10-15 kts stronger in the east, where there’s a slight chance for a severe thunderstorm. For Newfoundland, a chance for a final thundershower as things quiet down. For New Brunswick a similar setup as for southern Labrador, but a bit warmer. The Maritimes might have the highest risk for severe convection.

 

 

Convective Outlook Valid for Today and Tomorrow

Thunderstorm Outlook for Day 1

 

Regional Impacts

 

New Brunswick: Near-severe thunderstorms possible late this afternoon and evening mainly over northwestern regions giving localised heavy rainfall (20-30 mm/hr), moderate to strong wind gusts (80 km/hr) and small hail (<2 cm).

 

Labrador: Isolated non-severe thunderstorms over southern parts of the region early this afternoon and evening.

 

Convective Discussion

 

An upper level low over western Quebec will slowly move northeastward today. A weak cold front currently over southern Quebec will move over Maine and into western New Brunswick this evening. Meanwhile a broad warm front extends into southern Labrador. Convection has been ongoing along both features this morning. Embedded convection may continue into the early afternoon over southern and eastern Labrador.

 

Ahead and along the cold front extending into southern Quebec and western Maine, organized convection is expected which may continue into western New Brunswick in the early evening and overnight period. The main inhibitors for now will depend on the current cloud deck over Maine and western New Brunswick which is taking a while to burn off. The lapse rates over the area are also not ideal as the 500mb cold trough remains back over Ontario and there is not much further destabilization expected in the upper levels. However, there is ample moisture available with dew points reaching 20C. The 12Z Caribou sounding shows quite a warm nose around 900 mb and that cap is expected to be broken once the cloud clears. It also has a decent dry layer from 500-700mb, indicating a risk for strong downdraft gusts. The  0-6 km shear has increased to 23 KT with Should things align, there is the risk for convection to organize into lines which could make their way into northwestern New Brunswick this evening. Frequent lightning, locally heavy downpours and strong wind gusts are the main risks.

 

 

 

 

 

Thunderstorm Outlook for Day 2

 

The upper low somewhat stall over central Quebec tomorrow. The threat for any severe convection diminishes. Only daytime heating pop up TCUs in showers with a slight risk of a CB seems likely over New Brunswick and Newfoundland with some embedded convection over western Labrador where the warm frontal feature is stalled.

 

Convective Outlook Valid for Today and Tomorrow

Thunderstorm Outlook for Day 1

 

Regional impacts:

 

New Brunswick: Isolated to scattered thundershowers this afternoon giving localised heavy downpours (20 mm), moderate wind gusts, and small hail (1-2cm).

 

Nova Scotia: Isolated showers with possible non-severe thundershowers mainly over the Annapolis Valley and parts of northern Nova Scotia giving localised downpours.

 

Newfoundland: Isolated showers with possible non-severe thundershowers over parts of the interior and northeast giving localised downpours.

 

Labrador: Isolated thundershowers over southern Labrador with possible localized downpour, moderate wind gusts, and small hail.

 

Convective discussion:

 

A trough over Ontario will be slowly pushing eastward today, which will continue to bring in more humid air further east into Maine and eastern Quebec and the western Maritimes. Pulse type storms are expected for New Brunswick and possibly reaching southwestern Labrador by the evening.  The 12Z Caribou sounding shows that there is a lot of buoyant potential given the forecasted high temperatures today and increasing dew points with possible CAPE values in excess of 1500 J/kg. The area however remains in a weak area of shear with no real upper dynamics in the area to keep cells prolonged for too long. There is enough energy however that small hail may still be possible but heavy downpours are likely the biggest concern.

 

Sea-breeze convergence over mainland Nova Scotia could also kick up some TCUs with showers with a risk of CBs this afternoon. With shear remaining low and little flow aloft combined with precipitable water of 39 mm, heavy downpours will be the main concern but downpours should be relatively short lived. Daytime heating with a light southwest flow over Newfoundland may also give showers and possible CBs over the interior and northeastern regions this afternoon and evening.  

 

 

Thunderstorm Outlook for Day 2

 

A low pressure system moves into central Quebec today from Ontario with increasing moisture in the low levels. Ahead of a trough extending into southern Quebec and western Maine, organized convection is expected which may continue into western New Brunswick in the early evening and overnight. Given the timing, the highest risk of severe thunderstorms should remain over Maine and southern Quebec. A warm front will extends from central Quebec east toward the Lower North Shore. Along this front embedded convection is possible which may move into parts of southern Labrador early in the afternoon.