Convective Outlook Valid for Today, Tonight and Tomorrow

Regional Impacts

NB… Slight chance for some isolated thundershowers this afternoon and evening. Main threat is locally heavy downpours of 10 to 20 mm/h.

NS/PEI/NL… None.

 

Convective Discussion…

An upper low currently sitting over the St. Lawrence Valley will be the main trigger for convection today. The low will slowly begin to move east this afternoon, with convection expected to fire in Maine before crossing the border as the low moves into western New Brunswick. Instability is expected to be quite limited today, with only up to 500 J/kg of MLCAPE forecast this afternoon. Modest bulk shear of 15 to 25 knots should inhibit most storms from becoming too organized, so severe weather is not expected today. Nonetheless, the upper low should be enough to fire a few weak thunderstorms later this afternoon in western New Brunswick once convective temperatures (20 C) are reached, with locally heavy downpours of 10 to 20 mm/h being the main risk.

 

This low will continue to move east tonight, to lie over Anticosti Island by tomorrow afternoon. As such, most of the convection will be centred over the Gulf of St. Lawrence tomorrow. As the low moves into the Gulf early tomorrow morning, relatively warm waters of 18 to 20 C in the area of interest, and the cold upper low, should provide enough instability and vorticity for convection. With this, there exists a slight chance for a waterspout off the Acadian coast of New Brunswick in the morning hours.

 

Thunderstorm Outlook for Today

Thunderstorm Outlook for Tonight

Thunderstorm Outlook for Tomorrow

Copp

Convective Outlook Valid for Today , Tonight and Tomorrow

Thunderstorm Outlook for Today

 

Thunderstorm Outlook for Tomorrow

Convective Outlook Valid for Today , Tonight and Tomorrow

Regional Impacts

Maritimes… Isolated thunderstorms giving 15-30mm, some potential for torrential downpours today. Some stronger wind gusts possible.

Newfoundland… Isolated thunderstorms giving 15-30mm, some potential for torrential downpours today. Some stronger wind gusts possible. Heavier downpours along south coast possible overnight.

 

Convective Discussion

Most of the region will remain under this moist airmass for today and into tomorrow for Newfoundland. The most probable areas for isolated severe storms have been highlighted but it should be noted that intense rainfall could set up in various locations today and warnings ,if required, would be very short fused. There are a few areas that lend themselves to a slightly higher potential due to severe thunderstorms, including interior Newfoundland and central parts of the Maritimes. The break up of the cloud is already happening in these areas and along with the solar heating there are potential triggers for convective initiation in these areas. Surface based CAPE is likely to exceed 1000 j/kg in these areas and with the approaching upper trough at 500 there is ample shear of >50 kts for longer lived thunderstorms. Despite some of these values, updrafts are still somewhat weak with thin CAPE that could be sheared off initially, possibly impeding the deeper convection somewhat.

 

The other area of interest will be likely be along the south coast of Newfoundland tonight as the atmospheric river sets ups just over the eastern part of the island, the instability may be high enough to bring rainfall rates over 40mm/hr, radar rainfall estimates and convection will have to be closely monitored through the evening and overnight.

 

Thunderstorm Outlook for Today

 

Thunderstorm Outlook for Tonight

 

Thunderstorm Outlook for Tomorrow

Convective Outlook Valid for Today, Tonight and Tomorrow

 

Regional Impacts

 

Today

NS/PEI/NL…Heavy downpours with rainfall rates of 20 to 40 mm/h with slight risk of embedded convection.

NB…Slight risk of a thundershower over northwestern regions.

 

 

Tonight

NL/LAB…Heavy downpours associated with Ernesto to affect southeastern Newfoundland.

NB…Locally downpours with risk of embedded thunderstorms possible.

 

 

Tomorrow

NB/PEI…Locally downpours with risk of embedded thunderstorms possible.

NS/NL/LAB…None.

 

 

Convective Discussion

The centre of Ernesto is beginning to track into the southern marine waters and northeast towards the Avalon Peninsula of Newfoundland tonight. Some bands of heavy rain with a few embedded thunderstorms have begun to move north into Nova Scotia this morning and southern Newfoundland well ahead of Ernesto. With precipitable water near 55 mm over this area, rainfall rates could be high at times today. As the actual storm moves south of the Avalon tonight, rainfall rates are expected to be high for a short period of time. There is a slight risk of embedded thunderstorms with the passage of this storm as well today and overnight.

 

A shortwave trough over southern Quebec and New England will slowly move eastward to lie over New Brunswick tomorrow then stall across the region. Heavy downpours are possible with this especially over northwestern New Brunswick as this trough slows further.

 

Thunderstorm Outlook for Today

 

Thunderstorm Outlook for Tonight

 

Thunderstorm Outlook for Tomorrow

 

 

Roberta McArthur

 

 

 

Convective Outlook Valid for Today, Tonight and Tomorrow

Regional Impacts

 

Today

NL/LAB… Scattered thunderstorms possible across Labrador today with downpours of 15 mm/h.

NS/NB/PEI… None.

 

Tonight

NS…Heavy downpours with risk of embedded thunderstorms possible giving rainfall rates of 20 to 40 mm/h.

 

Tomorrow

NB…Slight risk of a thundershower over northwestern regions.

NL/LAB…Heavy downpours ahead of Ernesto to affect southern Newfoundland.

NS/PEI…Risk of heavy downpours but thunderstorm risk is low at this time.

 

 

Convective Discussion

An upper trough stretches from southern Ontario northeastward into Labrador that is slowly moving eastward but will slow down as it begins to interact with Tropical Storm Ernesto over the next few days as the storm tracks over the southern marine waters. A shortwave trough over southern Ontario has been fairly active with convection yesterday and continued overnight into this morning. This shortwave will slowly drift eastward and weaken somewhat today. A secondary trough south of Ungava Bay this morning will help stretch moisture into western Labrador. MLCAPE values in southern Labrador are forecast to rise upwards of 1000 J/kg today along with mainly linear bulk shear values of 20 to 30 knots. With precipitable water forecast to increase to 30 mm, the main hazard today will be torrential downpours of 15 mm/h but brief strong wind gusts are possible if discrete cells organize into lines.

 

As Hurricane Ernesto continues northward and begins to enter the southern maritime waters early on Monday, an area of heavy downpours is possible well ahead of the circulation that will affect the southern maritime provinces on Monday with a slight risk of embedded thunderstorms mainly for overnight and early Monday with the help of cloud top cooling. Precipitable water is forecast to increase above 55 mm across Nova Scotia then east towards southern Newfoundland by Monday night.

 

Thunderstorm Outlook for Today

 

Thunderstorm Outlook for Tonight

 

Thunderstorm Outlook for Tomorrow

 

 

Roberta McArthur