Convective Outlook Valid for Today, Tonight, and Tomorrow

Regional Impacts

 

Today:

NS: Embedded thunderstorms this evening west of Halifax, heavy downpours of 30-60 mm.

NB: Risk of embedded thunderstorms along the Fundy coast this evening, heavy downpours of 30-60 mm. Chance for an isolated thunderstorm in extreme NW NB this evening, locally heavy downpours of 25-50 mm.

PEI: Chance for embedded thundershowers to move into PEI before midnight, locally heavy downpours of 10-15 mm.

NL: None.

 

Tonight:

NS: Embedded thunderstorms along the Atlantic coast between Halifax and Cape Breton Island overnight, heavy downpours of 25-50 mm.

NB & PEI: Slight chance for embedded thundershowers overnight, locally heavy downpours of 10-15 mm.

NL: None.

 

Convective Discussion…

An upper low over southern Ontario this morning will slowly move east, interacting with a stream of tropical moisture off the eastern seaboard today. Embedded thunderstorms have been observed over New England and to the southeast of Cape Cod this morning. These storms will move northeast today, reaching the southwestern shores of the Maritimes by this evening. Ample jet support wrapping around the aforementioned upper low will aid in sustaining this convection as it approaches the Maritimes, and is expected to persist along the Atlantic coast of NS tonight. The 12Z Yarmouth sounding this morning indicated precipitable water values of 50 mm, with this number expected to build to 60-65 mm over much of NS this evening. With a high volume of moisture in the atmosphere, any significant instability could lead to torrential downpours in the area of interest. Currently, MUCAPE values are forecast to build upwards of 500-750 J/kg by this evening along the Atlantic coast. With these numbers in mind, torrential downpours will be the main threat, with 30-60 mm and rainfall rates upwards of 25 mm/h possible. These amounts may end up being higher if the mid-level instability increases, depending on how much upper and mid-level cold air wraps around the upper low over Ontario, so this will be something to closely monitor.

 

A second smaller area of interest closer to the upper low exists in extreme NW NB. Here, an isolated thunderstorm may develop over Maine where stronger dynamics are present, however significant instability may be a challenge to reach. Thus, there is only a slight risk for severe weather, with the main threat being locally heavy downpours of 25-50 mm.

 

Thunderstorm Outlook for Today

 

Thunderstorm Outlook for Tonight

 

Thunderstorm Outlook for Tomorrow

 

Copp

Convective Outlook Valid for Today, Tonight, and Tomorrow

Regional Impacts

 

Today:

NS/NB/PEI/NL: None.

 

Tomorrow:

NS: Embedded thunderstorms on Friday in western NS, locally heavy downpours of 30-60 mm.

NB: Scattered thunderstorms on Friday along the Fundy coast on Friday, locally heavy downpours of 30-60 mm.

PEI: Isolated thundershowers possible on Friday, locally heavy downpours of 10-20 mm.

NL: None.

 

Convective Discussion…

Not much going on today in terms of convective weather, although a few lightning strikes are possible over the Atlantic ocean, south of Sable Island.

 

The main story will be with an incoming stream of tropical moisture tomorrow. Rain and some isolated thunderstorms are forecast to move into the Maritimes by tomorrow morning along a warm front, before more organized convective activity along the cold front will move through Nova Scotia and southern New Brunswick on Friday evening into Saturday night. PWAT values will build to near 60 mm by Friday afternoon for Nova Scotia and southern New Brunswick, with ample mid-level instability upwards of 500-750 J/kg allowing for some organized convection along the cold front by Friday evening. Storms are expected to be more isolated Friday morning and afternoon, before becoming more organized on Friday evening and overnight into Saturday as the 250 and 500 mb jets approach the Maritimes. The main threat with these storms will be locally heavy downpours of 30-60 mm. Rainfall rates in excess of 25 mm/h are possible.

 

Thunderstorm Outlook for Today

 

Thunderstorm Outlook for Tonight

 

Thunderstorm Outlook for Tomorrow

 

Copp

Convective Outlook Valid for Today, Tonight, and Tomorrow

Regional Impacts

NB: Isolated thundershowers possible over NW NB this afternoon. Locally heavy downpours of 10-15 mm.

NL: Isolated thundershowers possible over western Labrador this afternoon. Locally heavy downpours of 10-15 mm.

NS & PEI: None.

 

Convective Discussion…

A few areas of interest exist today, however no severe weather is expected. The first area is associated with a small, weak embedded upper low moving through the St. Lawrence river valley. This upper low will approach NW NB this afternoon bringing the chance for a couple isolated thundershowers. While SBCAPE values of around 750 J/kg are forecast, this number may be too high as extensive cloud cover is present over the area of interest. In addition, a lack of bulk shear (10-15 kts) should prevent any severe potential. As such, the main hazard with these cells will be locally heavy downpours of 10-15 mm. 

The second area of interest is within an upper trough moving into western Labrador this afternoon. Although bulk shear values are forecast to increase to 30-40 kts this afternoon, meager CAPE values of 250-500 J/kg should keep convection non-severe, with the main risk being locally heavy downpours of 10-15 mm.

 

Thunderstorm Outlook for Today

 

 

Thunderstorm Outlook for Tonight

 

 

Thunderstorm Outlook for Tomorrow

 

 

Copp

Convective Outlook Valid for Today , Tonight and Tomorrow

Regional Impacts

Interior Newfoundland and Northern New Brunswick.. Low probability of isolated showers or thunderstorms giving 10-15mm.

Southern NS.. 25-25mm possible in embedded thunderstorms.

 

Convective Discussion

Overall instability is somewhat diminished from yesterday which doesn’t bode well for thunderstorm potential, however many areas likely received 10-15 mm yesterday which is similar to todays potential for some showers. No severe thunderstorms are expected in the general airmass thunderstorms and the main focus of interest is with the broader synoptic area to the southwest of NS. Most of the potential for high rainfall should pass just south of NS but rainfall rates of 25-35mm could just clip southwest parts of the province tonight. However this system seems to have less potential for heavy downpours when compared to some of the recent systems which tended to have significant convection and thunderstorm activity, most of that type of weather will remain further to the south. This system does have high precipitable water values approaching 50mm but the instability is not expected to be significant, these weaker updrafts are less likely to realize the full potential rainfall rates, nevertheless some uncertainty remains and this system could track slightly further north than expected.

 

Thunderstorm Outlook for Today

 

Thunderstorm Outlook for Tonight

 

Thunderstorm Outlook for Tomorrow

Convective Outlook Valid for Today , Tonight and Tomorrow

Regional Impacts

Maritimes.. Isolated thunderstorms giving 20mm rain with small hail and gusts to 50 to 70 km/hr possible.

NF.. Embedded thunderstorms with amounts 20-40 mm.

 

Convective Discussion

There are two main areas of concern once again, that associated with the low, and the unstable airmass behind that system. The system will likely impact some areas of Newfoundland with amounts in the 20 to 40mm in the heaviest areas of rain. This system is likely to be slightly weaker than is was as it passed over Nova Scotia which saw some amounts over 50mm. Over the Maritimes there are a couple factors coming together that could make this a possibly impactful day. Overall CAPE values are moderate with mixed layer values near 1000 J/kg, and sfc dew points approaching 20C. PW values are somewhat less than they have been but still sufficient for thunderstorms producing moderately heavy rainfall. In addition the dynamics are somewhat favourable with mid level wind shear values above 30 knots and some low level helicity in the vicinity of the surface trough. Later this afternoon the upper level jet could also aid in the development of severe storms over eastern NB.

 

It should be noted that although updraft are weak, there is some potential for severe weather to occur and it is unlikely albeit not impossible that waterspout or land spout tornado could occur along some coastal waters including the Northumberland Straight and Cape Breton shores.

 

Thunderstorm Outlook for Today

 

Thunderstorm Outlook for Tonight

 

Thunderstorm Outlook for Tomorrow

 

Mel Lemmon