Convective Outlook Valid for Today, Tonight and Tomorrow

Regional Impacts

 

NS/NB/PEI/NL… None

 

Convective Discussion

 

As an upper trough stalls over the Maritimes today, cold dry air will usher in from northern Quebec dropping dewpoints to the low to mid single digits. Dry low levels in conjunction with minimal wind shear will likely inhibit any thunderstorm development today across the Maritimes. A warm nose above 600 mb will inhibit much vertical development over central NS, however some isolated TCU are still possible. Extreme western New Brunswick may also see some isolated TCU this afternoon, but extremely dry low levels will likely prevent any development. The only area that is likely to see any lightning will be well offshore, closer to the 500 mb jet southeast of Nova Scotia.

 

Similar dry conditions are expected again tomorrow under the stalled upper low, with no thunderstorm development expected.

 

Thunderstorm Outlook for Today

 

 

Thunderstorm Outlook for Tomorrow

 

 

Copp

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