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.

 

Convective Outlook Valid for Today and Tomorrow

Regional Impacts

 

Southern Labrador: Scattered thunderstorms possible for southern Labrador this afternoon and evening giving localised heavy downpours, strong wind gusts and small hail.

New Brunswick: Isolated thunderstorms are possible also over northern New Brunswick giving localised heavy rainfall and strong wind gusts early this afternoon and evening.

Southern Maritime marine district: Isolated  Embedded thunderstorms today.

 

Convective Discussion

 

There are two features to consider today. An upper low resides over central Quebec with a warm front extending into central Labrador and southeast towards Newfoundland with some embedded convection this morning. A broad area of lower pressure extends from western Labrador and southwest into northwestern New Brunswick and Maine. There is also a low pressure system south of Nova Scotia It is within this area convection will likely pop up this afternoon and evening.

 

Over Labrador, the upper dynamics are favourable for convective initiation this afternoon. The area falls within the right entrance to a 250 mb jet and there is some destabilizing at 500 mb with a cold trough approaching western Labrador. That along with 0-6km shear of 47 KT and relatively high precipitable water levels, and low freezing levels will help prolong convection later into the evening and cells may form into organized lines of convection or possibly supercells. Heavy downpours, strong winds gusts (60-70 km/h) and small hail (1-2cm) are possible.

 

There is a capping inversion to break this morning over New Brunswick but with forecasted high temperatures are expected to be near the convective temperature needed to initiate lift. Additional lift will be aided by the higher terrain over northern New Brunswick. Some areas along the Bay of Chaleur that see a sea breeze this afternoon may also help to provide lift.  There is the potential for CAPE values to be in excess of 1500 J/kg but the area is lacking in upper dynamics. Moisture has been slowly increasing over the area so heavy downpours are possible as the steering flow is weak. 0-6km shear remains low as with Caribou sounding at 12Z so pulse or multi-cell type storms seems the most probable. Freezing levels are somewhat high so the risk of hail seems minimal. The threat of training heavy rainfall over some areas is have the highest impact.

 

Thunderstorm Outlook for Day 1

 

 

 

 

Thunderstorm Outlook for Day 2

 

Tuesday will be similar to today over New Brunswick with some afternoon convection possible over mainly northern areas. A trough over Ontario will be slowly pushing eastward, which will continue to bring in more humid air further east into Maine and western Quebec. Pulse type storms again expected for New Brunswick and possibly western Labrador. Sea-breeze convergence could also kick up some TCUs in showers over mainland Nova Scotia and Newfoundland.

 

 

Convective Outlook Valid for Today and Tomorrow

Regional Impacts

Risk of isolated thunderstorms producing downpours and wind gusts over northwestern NB, and northern Gulf of St. Lawrence regions.

Convective Discussion

This morning, the upper ridge at 500mb remains positioned from Hudson Bay to Maine. At the surface, a warm front extends from a low pressure area over northern Ontario to James Bay then southeastward through central Quebec, Maine and western Nova Scotia. Based on this mornings upper air soundings, the most likely areas for convective activity today is northwestern New Brunswick. Surface moisture is increasing over northern Maine and some of this moisture will have to advect over NB in order for convection to initiate.

Modifying the Caribou profile with a surface temperature of 32 and dewpoint of 18 results show:

·         Mixed layer cape values in excess of 1000 J per kg

·         A fair amount of this instability would be situated between 700-450 mb which would suggest a chance of hail (possibly around 1cm)

·         Slow moving storms could produce downpours with precipitable water in order of 30mm

·         And finally wind gusts 40kt or more.

 

On Monday, a non-tropical low pressure system east of the Carolinas will reach Georges Bank and bring isolated showers and thundershowers to the Atlantic coast of Nova Scotia. Further north, afternoon thunderstorms are possible.

 

Thunderstorm Outlook for Day 1

 

 

Thunderstorm Outlook for Day 2

Convective Outlook Valid for Today and Tomorrow

Thunderstorm Outlook for Day 1

 

Regional Impacts

 

Nova Scotia and New Brunswick: A risk of thundershowers this afternoon and early evening.

Newfoundland and Labrador and Prince Edward Island: None

 

Convective Discussion

 

Cape values of 500-1000 J/kg and shear of less than 20 kts will yield the potential for pulse storms along a weak trough over western NB and NS this afternoon and evening. If the convection does occur then the slow moving cells could give local downpours of 25 mm or higher.  The precipital water indicates 25-30 mm. Dry mid-levels will give the potential for some gusty winds if the convection does occur.  

 

Thunderstorm Outlook for Day 2

 

Pulse storms possible for northwestern NB on Sunday giving similar impacts as this afternoon.

 

 

 

Convective Outlook Valid for Today and Tomorrow

Thunderstorm Outlook for Day 1

 

Regional Impacts

 

Low risk of high based thunderstorms over the higher terrain in northern New Brunswick and western Newfoundland late this afternoon and evening. Convective downdraft gusts are the main concern should anything develop.

 

Convective Discussion

 

A very hot and slightly humid air mass continues to reside over the Maritime provinces today. A trough over southern Labrador into northern Newfoundland will slowly sink southward today and reach northeastern New Brunswick and the Gulf of St. Lawrence tonight.  Temperatures profiles over NB this morning suggest a very dry air mass but with a lot of buoyant potential.  An isolated thunderstorm cannot be ruled out, especially with the help of forcing from hills and where forest and other vegetation may provide the extra moisture and energy needed or with the help of a sea breeze convergence off the Bay of Chaleur. The forecast highs are such that the convective temperature of 37 could even be reached on its own. Given such a dry and warm profile at all levels, there may not be much in the way of precipitation but convective wind gusts could be a concern. There may be a similar situation off the higher terrain in southwestern Newfoundland.

 

 

Thunderstorm Outlook for Day 2

 

Daytime heating air mass type thunderstorms are possible mainly over Maine tomorrow afternoon but with the slight winds aloft from the west, some of these cells could reach into western New Brunswick in the evening. The risk remains very low as mixing in the low levels may inhibit development. A ridge of high pressure  sinks south into Newfoundland and Labrador tomorrow with no convective weather expected.

 

 

 

Roberta McArthur