Convective Outlook Valid for Today, Tonight and Tomorrow

Regional Impacts

 

Nova Scotia… Slight risk of an isolated thunderstorm over southwestern Nova Scotia early this afternoon, with the main risk being heavy downpours of 15 mm/h.

 

Convective Discussion

 

A small shortwave moving over New England this morning will cross the Maritimes this afternoon. This feature, in conjunction with a 55 knot 500 mb jet south of Nova Scotia will provide ample lift for some isolated convection over portions of southwestern Nova Scotia early this afternoon. Models have been under-forecasting dewpoints in this part of the province this morning by about 5 degrees, so MUCAPE values may exceed the 500 J/kg that the models are currently forecasting, increasing the likelihood that convection may occur. One of the limiting factors will be bulk shear values increasing to over 60 knots by late this afternoon, which may shear apart any convection that does fire earlier in the day. Any storms that do develop are likely to be short lived, with heavy downpours of 15 to 20 mm/h being the main hazard.

 

A colder airmass over northern Quebec will move south tonight, cooling the mid to upper levels. Provided ample clearing tomorrow morning, a lightning strike or two cannot be ruled out over central Nova Scotia, where there is more wind shear to work with. Thus said, the risk remains very low.

 

Thunderstorm Outlook for Today

 

 

Copp

Convective Outlook Valid for Today , Tonight and Tomorrow

Regional Impacts

 

Newfoundland and Labrador… Low probability of embedded thundershowers with rainfall rates  of 15mm/hr. Over southeastern NL there may be a prolonged period of 20+mm hr rainfall rates.

 

Nova Scotia… low probability of TCU generating showers over central NS. Very low chance of thundershowers.

 

Convective Discussion

 

There will be a low probability of embedded thunderstorms in the moisture plume that is slowly making its way across Newfoundland today. Most unstable CAPE values are marginal,  total precipitable water values are between 30 and 35mm in highest areas over NL, so it is unlikely that the highest rates will be realized unless convection becomes more prevalent. There is a very slight chance that convection may also be deep enough for thunderstorms in the low pressure trough over Labrador, for the trough over NS it is extremely unlikely to see deep convection but there is a very slight chance that a TCU or two might generate a lightning flash over central NS, but not probable. The dynamics which are quite good, may actually inhibit convection do to shearing of tcu, preventing growth.

 

Beyond or day one issues it looks like things are likely to dry out with high dew point temperatures returning later next week. It may very well be quite quiet over the next several days.

 

Thunderstorm Outlook for Today

 

 

Convective Outlook Valid for Today , Tonight and Tomorrow

Regional Impacts

 

NB… Moderate rainfall with amounts likely 15 to 25mm for most areas. For northwest parts of the province there is a slight chance that severe thunderstorm forming late this afternoon, with potential strong winds and small to moderate hail.

 

NS… There are likely multiple plumes of moisture with some rainfall associated with them. There is potential for 20mm/hr rainfall rates with heavier rainfall early today of SW NS then gradually moving eastward late this afternoon and through the evening.

 

PEI…The heaviest rainfall rates should not impact PEI but the rates of 15 +mm /hr could bring locally heavier rainfall this afternoon and this evening.

 

NL…The main plume will make its way into NL tonight with heavy rain along the south cost forecast, tomorrow there is a chance that convective rainfall rates will intensify over the southeast. Rainfall rates could exceed 25mm/hr for some areas, depending on the duration of the event local flooding may be

 

Convective Discussion

 

A slow moving low pressure system with an elongated trough over central Quebec will gradually move eastward over the next couple of days. Ahead of this low a deep southerly flow will continue to advect a moisture plume over the Maritimes today and over Newfoundland overnight and tomorrow. With some instability present it is possible to see rainfall rates over 20mm for areas in areas highlighted in yellow with a moderate risk. The feature should be moving quickly enough that flooding concerns will be localised. Some of the heaviest hit areas could see local amounts near 50mm.

 

Dynamically conditions are favourable for severe thunderstorm development over Northwest New Brunswick just ahead of an approaching upper trough. The timing of the upper troughs approach will be important in determining the severity of the thunderstorms with updraft potential being fairly weak, but of course there  is some uncertainty in the CAPE values, but they are expected to be near 500 J/kg. Deep layer shear and low level shear are both quite good aiding to the potential for damaging wind should intense cells develop. In this scenario with low likelihood watches will not likely be issued until thunderstorms begin to develop and intensify.

 

Thunderstorm Outlook for Today

 

 

Thunderstorm Outlook for Tonight

 

 

Thunderstorm Outlook for Tomorrow

 

 

Mel Lemmon

Convective Outlook Valid for Today , Tonight and Tomorrow

Regional Impacts

 

Today:

NB: Scattered thunderstorms over central and southern regions. Some of these storms will likely be strong. The main threat will be very strong wind gusts of 70-90 km/h, and brief heavy downpours with rainfall rates of  20 mm/h

NL: Scattered thunderstorms mostly elevated over north-central regions of the Island ending this afternoon. Showers heavy at times with rainfall rates of 10-20mm/h

PEI: Risk of thunderstorms late this afternoon

NS: None

Lab: None

 

Tonight:

NB/PEI: Isolated thunderstorms ending

NL: Risk of thunderstorms over southern portions of the Island

NS: None

Lab: None

 

Sunday:

NS: Isolated thunderstorms developing with localized heavy showers

NL: Slight risk of thunderstorms over eastern regions

NB/PEI/LAB: None

 

 

Convective Discussion

 

At 12z this morning an upper trough extends from Hudson’s Bay to the lower Great Lakes, at the surface a low pressure system is located over the mouth of the St. Lawrence river with a warm front extending to central NF and a cold front extending southwestward over NE. Very warm and humid air prevails over the region with dewpoints ranging from 18-21 deg, and precipitable water between 30-40 mm. However dewpoints have been diminishing slowly over NW NB this morning, and this trend is expected throughout the area during the rest of the weekend. The low-level moisture was extending up to 300-400 m above ground this morning, but the latest satellite imagery is showing dissipation of the low level cloud. With full sunshine this afternoon, surface-based convection should initiate over southern NB where temperatures of 28-30 deg. are expected. In this modified environment, the potential energy will increase to about 1000 J/kg. Deep layer shear of around 30kt will likely lead to some clusters become somewhat organized into bands with a potential of low-key bow lines.

The convection over NF is associated with the warm front and much of the instability is above 850-700 mb level, CAPE of 400-500 J/kg are supporting this convection in an environment with precipitable water of ~40mm.

 

For Sunday, the weakening cold front should slowly move southeastward over NS and NF. The likelihood of thunderstorm activity is medium for Sunday afternoon

 

Thunderstorm Outlook for Today

 

Thunderstorm Outlook for Tonight

 

 

Thunderstorm Outlook for Tomorrow

Convective Outlook Valid for Today, Tonight and Tomorrow

Regional Impacts

 

Today:

NB: Near-severe thunderstorms possible over western and northern regions this afternoon and evening giving strong wind gusts (70 to 90 km/h), small hail and heavy downpours (20-40mm per hour rates).

NS/PEI/NL&LAB: None

 

Tonight:

NB: Thunderstorms possible overnight giving heavy downpours.

NL&LAB: Thunderstorms possible over southwestern Newfoundland and southwestern Labrador.

 

Saturday:

NB: Thunderstorms over central and southern regions this afternoon and evening giving heavy downpours.

NL: Thunderstorms over central to northern giving heavy downpours.

NS/PEI/LAB: None

 

 

Convective Discussion

 

An upper level trough extends from the Great Lakes northeast into the St. Lawrence River Valley with a surface low pressure developing this afternoon over central Quebec. Ahead of the trough over the Maritimes lies a warm and humid air mass with dewpoints remaining near 20C today over central to northern New Brunswick and precipitable water near 40 mm. Should there be sufficient clearing this afternoon, this may also provide additional surface based heat with temperatures into the high 20s with possible MUCAPE (upwards of 2000 J/kg) over central to northern New Brunswick. Caribou’s 12Z sounding indicates a capping inversion near 850mb but there is decent instability above.  With a 40KT 700mb jet and weak to moderate deep layer shear near 25KT, multicell development will likely occur over the higher terrain in north and central Maine (already ongoing this morning) and northwestern New Brunswick this afternoon. Cells may form into more organized lines as they track northeastward this afternoon and into the overnight. Given general northeast orientation of storm movement, there is the threat training and locally heavy downpours over the same locations. With sufficient mid-level instability, downdraft wind gusts and small hail with the more intense storms are also possible.

 

For Saturday, the trough inches slowly southeastward. The threat for severe thunderstorm development has decreased, but areas of central and southern New Brunswick could see heavy downpours in the afternoon and evening as well as northern Newfoundland.

 

Thunderstorm Outlook for Today

 

Thunderstorm Outlook for Tonight

 

Thunderstorm Outlook for Saturday

 

Forecaster: McArthur