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.

 

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