Convective Outlook Valid for Today and Tomorrow

Thunderstorm Outlook for Day 1

 

Regional Impacts

Burin and Avalon Peninsulas of Newfoundland: showers heavy at times giving amounts near 50 mm.

 

Convective Discussion

This morning’s 12z analysis is showing the 500mb upper cold low anchored off the South Shore of Nova Scotia. Some convection is evident on the Satellite pictures south of the low and the Scotian slope, and lightning is limited to areas south of 40N. East of the low, very strong southerly winds are pushing a moisture plume originating from around 35N all the way to the Gulf of St. Lawrence and Newfoundland. The 12z sounding from YJT is showing nimbostratus up to 27,000ft and precipitable water of 35mm. The YYT sounding is showing a 50 knot low-level-jet and layers of instability in the mid-levels suggesting ACC’s which could result in rainfall rates of 10mm or more per hours later in the day and tonight. The probability of thunderstorm is not zero over NF, but the risk was left out of the outlook. The very moist airmass will continue to move northward over southeastern Labrador today and tonight.

 

Thunderstorm Outlook for Day 2

Convective Outlook Valid for Today and Tomorrow

Thunderstorm Outlook for Day 1

 

 

Regional Impacts

Burin and Avalon Peninsulas of Newfoundland: slight chance of embedded thundershowers, which may give some gusty winds over land.

Maritimes including marine waters: nothing significant expected.

 

Convective Discussion

Basically the same reasoning as for Sunday. There was some convection well south of Newfoundland last night, but no threat resulted for the island. The low/trough is basically stalled near and south of the Cabot Strait, and will gradually intensify and drift north. CAPE’s are marginal or non-existent with current guidance, but could become strong enough to fire off a cell or two if the models are off a little. If so, it will probably be over eastern Newfoundland in onshore flow, where the shear is weak enough (near 30-35 kts) to allow TCus or CBs  to develop. Over western Newfoundland the shears are 65+kts, eliminating the chance for CBs.

 

 

Thunderstorm Outlook for Day 2

 

 

Burin and Avalon Peninsulas of Newfoundland: slight chance of embedded thundershowers, which may give some gusty winds over land. Barely possible.

 

Convective Outlook Valid for Today and Tomorrow

 

Thunderstorm Outlook for Day 1

 

Regional Impacts

 

Southern Avalon Peninsula and marine waters to the south and southwest: Chance of embedded thundershowers this afternoon into the overnight period. No significant impacts expected.

 

Convective Discussion

A cold front from a weak low on the Gulf of St. Lawrence will slowly move east today and will then stall south to southwest of Newfoundland on Monday. North of the low and west of the cold front conditions will quickly dry out, and as the ridge to the west moves in convection will be inhibited, with thunderstorms unlikely. Embedded thundershowers near Georges banks will move east over slope waters today, and may approach the Avalon this afternoon and again overnight as the storm stalls and intensifies. CAPE’s are expected to be low,  well below 500 J/kg. In addition the deep layer shear will be too strong southwest of Newfoundland, shearing off the cells, but will drop a bit towards the Avalon allowing a chance for some isolated thundershowers and possibly gusty winds over land.

 

Thunderstorm Outlook for Day 2

 

Slight chance of embedded thundershowers continuing into Monday for eastern and northern Newfoundland. No significant impacts expected.

Convective Outlook Valid for Today and Tomorrow

Day 1 Thunderstorm Outlook

 

Regional Impacts

For New Brunswick...

There is a slight chance that severe thunderstorms could develop giving strong wind gusts and potentially heavy downpours.

Elsewhere..

A slight chance of embedded thunderstorms giving locally heavier rainfall.

 

Convective Discussion

Overall there is not of lot of instability in the airmass but with cold air advection aloft and some daytime heating surface based CAPE’s should exceed 500 j/kg by this afternoon over New Brunswick. In terms of the dynamics, conditions are very favorable ahead of the approach cold front that will pass over New Brunswick today, with about 50 knots of deep shear to support supercell development. In addition there will be convergence and enhanced low level vorticity along the front. It is possible that supercells could develop with the potential to produce bowing segments leading to strong surface winds. Watches may be required should organized storms begin to develop.

 

Threat Table for reference

Threat	Rainfall Amount	Wind Gusts	Hail Diameter	Tornado Probability
Minor	< 25 mm	< 70 km/h	< 2 cm	Unlikely
Moderate	25 – 50 mm	70 –89 km/h	2 – 4 cm	Low
Severe	50– 100 mm	90– 119 km/h	4 – 6 cm	Moderate
Extreme	100 mm or more	120 km/h or greater	6.0 cm or greater	Elevated
Risk of Thunderstorms: Thunderstorms that develop in this area will be less organized and will not likely meet warning criteria. Severe Thunderstorm Watches will not likely be required.
Slight Risk of Severe Thunderstorms: There is a low probability that weakly organized thunderstorms will develop, these thunderstorms may reach warning criteria. Severe Thunderstorm Watches may be required
Moderate Risk of Severe Thunderstorms: There is a moderate probability that organized thunderstorms will develop, thunderstorms are likely to reach warning criteria. Severe Thunderstorm Watches will likely be issued.
High Risk of Severe Thunderstorms: There is a high probability that well organized thunderstorms will develop, these thunderstorms are likely to meet or exceed warning criteria. Severe Thunderstorms Watches will be issued.

 

Day 2 Thunderstorm Outlook

Convective Outlook Valid for Today and Tomorrow

Hello All,

 

Although we have had an active start to the week with respect to severe weather today marks the official start of the ASPC’s Summer Severe Weather Desk. We have decided to include the Day and the Day 2 outlook in this one email, unless this proves to challenging due to the weather. If you have any comments about the format we are open to suggestions and feel free to contact me at mel.lemmon@canada.ca.

 

 

 

Regional Impacts

 

For Newfoundland and Labrador..

Heavy downpours and strong wind gusts nearing warning criteria are possible for western Labrador. Most areas are more likely to see weaker storms with less frequent or isolated thunderstorms.

 

For New Brunswick..

Strong thunderstorms are possible this evening with activity possibly extending into the overnight hours. Frequent lightning and some downpours less than 15 mm are possible. However should storms develop earlier in the day severe cells are possible over western parts of the province.

 

Convective Discussion

 

A significant low pressure system just west of Labrador will be the main driver of convective weather for the next couple of days. This low will move eastward today and along with the low the cold front extending southwest into southern Quebec and eastern Ontario will also begin to track eastward. The most favourable thunderstorm dynamics will be just ahead of the cold front that will extend into western Labrador by this evening. However CAPE values for Labrador will be low and not expected to exceed 500 J/kg. There is a significant amount of deep shear and lower level shear which could lead to supercells and bowing segments which could generate strong surface gusts. The probability of this is scenario is low but careful attention will be paid to the satellite and lightning detection to monitor thunderstorm development.

 

Over New Brunswick Cape values may exceed 1000 j/kg however the lack of a trigger could push back thunderstorm potential well into the evening as thunderstorms initiate well ahead of the cold front over New England and eventually move over New Brunswick. There is a low probability that a severe thunderstorm could impact the region but the most likely scenario is that thunderstorms will occur late after maximum daytime heating with weakened updrafts.

 

 

 

 

Mel Lemmon