Convective Outlook Valid for Today, Tonight, and Tomorrow

Regional Impacts

NS: Embedded thunderstorms today, torrential downpours of 30-50 mm in convection.

NB: Isolated thundershowers possible this afternoon, locally heavy downpours of 10-15 mm.

PEI: Embedded thundershowers possible this afternoon, locally heavy downpours of 10-15 mm.

NL: Isolated thundershowers possible over extreme northern Labrador, locally heavy downpours.

 

Convective Discussion…

A stream of tropical moisture has made its way into southwestern NS this morning. Embedded convection is expected to continue along this stream as it slowly moves northeast over the next few days, bringing torrential rain across much of the Atlantic coast. The 12Z UA sounding from Yarmouth indicated PWAT values of 42 mm, with this forecast to increase to near 50 mm by this afternoon between Yarmouth and Halifax. Rainfall warnings have been issued along the Atlantic coast of NS for total rainfall amounts of 40 to 70 mm, with possibly higher amounts in areas of more intense convection by tomorrow night.

 

Tomorrow, an additional area of interest exists in NW NB, associated with a low pressure system that brought widespread severe weather to southern Ontario yesterday. This low is forecast to slowly track northeast along the St. Lawrence river tomorrow. Significant cooling aloft and ample moisture in NB will allow for SBCAPE values to build to 2000 J/kg tomorrow afternoon, with convective temperatures around 23 degrees, making this a suitable environment for severe weather. Bulk shear values of 30-40 kts will allow for some sustained updrafts, with the potential for 1 to 2 cm hail with these storms. Strong wind gusts to 70 km/h and locally heavy downpours of 20-40 mm are also possible.

 

Thunderstorm Outlook for Today

 

Thunderstorm Outlook for Tonight

 

Thunderstorm Outlook for Tomorrow

Copp

Convective Outlook Valid for Today, Tonight, and Tomorrow

Regional Impacts

NS: Pop-up thundershowers possible in central NS this afternoon. Locally heavy downpours of 10-15 mm.

NB: Pop-up thundershowers possible in SE NB this afternoon. Locally heavy downpours of 10-15 mm.

PEI: None.

NL: Embedded thundershowers possible for south-central NFLD. Heavy downpours of 20-40 mm.

 

Convective Discussion…

The main threat for severe weather today exists with an embedded low currently moving into Banquereau waters towards NFLD. Strong upper dynamics and mid level instability will support convection as this low approaches NFLD this afternoon. No lightning has been detected, however the feature remains highly convective with rainfall rates over 25 mm/hour possible. This feature is forecast to quickly weaken once it reaches the southern coast of NFLD later today.

 

A few pop-up thundershowers are also possible in central NS and SE NB this afternoon. Warm, moist air will bring SBCAPE values near 1500 J/kg which will aid in the development of convection this afternoon, a lack of upper level support however, should hinder the life span of any storm that initiates. Locally heavy downpours of 10-15 mm will be the main threat with these cells.

 

On Friday, another stream of moisture from the tropics will move into southwestern NS. Embedded convection within this moisture stream will lead to locally heavy downpours. PWAT values are forecast to build to 40-50 mm by the afternoon, so rainfall rates upwards of 25mm/hr are not out of the question.

 

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: Embedded thunderstorms with locally heavy downpours >25 mm/hr possible today.

NB: Isolated thunderstorms over NW NB, 70 km/h wind gusts, 1-2 cm hail, and locally heavy downpours >25 mm/hr possible this afternoon.

PEI: None.

NL: Isolated thundershowers over Labrador and western NFLD today, embedded thunderstorms with locally heavy downpours >25 mm/hr possible over the Port-Aux-Basques region this evening.

 

Thursday

NS: None.

NB: None.

PEI: None.

NL: Embedded thunderstorms possible over central NFLD tomorrow, with locally heavy downpours >25mm/hr possible.

 

Convective Discussion…

Two main areas interest for potential severe weather exist today. The first threat area is associated with a trough moving through Nova Scotia today. Within this trough we are likely to see some embedded convection, with heavy downpours being the main risk. PWAT values from the Yarmouth sounding this morning indicated 45 mm, this is forecast to build to 50-60 mm by this afternoon east of Halifax. Bulk shear values of 40-50 kts will aid in sustaining this convection today as it moves east, reaching the Port-Aux-Basques region of NFLD by this evening. Total rainfall amounts may be in the range of 30-60 mm in areas of strongest convection, with rainfall rates of >25 mm/hr possible.

 

The second area of interest is in NW NB, which is associated with a small vorticity centre that is forecast to track along the Gaspe Peninsula this afternoon. This vort max has already spawned a severe thunderstorm near Saguenay, QC, with some weaker echoes popping up to the south near Maine. A relatively strong 250 mb jet over NW NB should aid in the intensification of some of these southern cells as they approach the NB border this afternoon. SBCAPE values upwards of 1500 J/kg and bulk shear values of 40-50 kts in this region will allow for some of these storms to potentially become severe as they enter NB. Small hail (1-2 cm), gusty winds (70 km/h), and brief but intense downpours (>25mm/hr) will be the main threats associated with these cells.

 

Tomorrow, the aforementioned trough moving through NS today will bring the threat for some embedded convection with locally heavy downpours to central NFLD. PWAT values are expected to be slightly lower tomorrow, in the 40-50 mm range. The main threat will once a gain be heavy rain, with rainfall rates of >25mm/hr possible.

 

Thunderstorm Outlook for Today

 

Thunderstorm Outlook for Tonight

 

Thunderstorm Outlook for Tomorrow

 

Copp

Convective Outlook Valid for Today and Tomorrow

Tuesday, July 18, 2023

Regional Impacts

 

Today

NB: Gusty winds, small hail and downpours, rainfall rates >25mm/hr.

NS: Downpours begin this evening, rainfall rates >25mm/hr.

LAB: Isolated lightning

 

Tonight

NB: Downpours, rainfall rates >25mm/hr

NS: Downpours, rainfall rates >25mm/hr

PEI: Downpours, rainfall rates >25mm/hr

Magdalen Islands: Downpours, rainfall rates <25 mm/hr

LAB: Isolated lightning.

 

Wednesday

NS: Downpours, rainfall rates> 25mm/h

NL: Downpours, rainfall rates> 25mm/h

LAB: Scattered lightning

NB: isolated lightning.

 

Convective Discussion

UA’s from Caribou, Yarmouth and Maniwaki indicated only modest PWATs of 26, 32 and 20 mm respectively at 12z. Nevertheless moisture is anticipated to rebuild ahead of a weakening cold front in NW NB throughout the afternoon. Thunderstorms are expected to initiate ahead of this feature in eastern Quebec and Northern Maine and move into Madawaska and western parts of Restigouche County late this afternoon. Storms will have SBCAPEs of 1000-1500 J/kg to work with following peak insolation (assuming minimal impacts from smoke in the vicinity). Bulk shears of 35-45 kts, of primarily linear character will allow storms to sustain themselves and grow into multi-cells that could present the odd strong downdraft to bring gusty winds. In addition PWATs are expected to build to ~ 40mm by late this afternoon yielding primary hazards of rainfall rates, and gusty winds in this region of NB. Small hail is also possible, however the hail threat will be limited by less than ideal buoyancy through the HGZ, given high freezing levels, despite the favourable shear character.

 

A broad upper trough well to the west of the region has begun to take on a negative tilt, this will guide a new stream of tropical moisture into southern NB and western NS by the evening hours. Models suggest PWATs building up to 65mm during the overnight hours. Moist unstable profiles will be aided by a broad ascent region associated with the right entrance of an upper jet. Lightning with this stream of moisture will initially be isolated, but should become more frequent overnight with cold top cooling as this band of moist unstable air slowly moves east. Elevated MUCAPES of 500-1000 J/Kg, should yield torrential downpours, with localized flooding being possible throughout the overnight in southern NB, parts of PEI and the western half of mainland NS.

 

On Wednesday this same area of moisture will present a similar downpour threat for eastern NS and western NL, although lightning frequency may be lower during the daytime hours as cloud tops warm. Northern NB and much of Labrador can expect air mass thunderstorms of isolated to scattered frequency respectively due to a cooler air mass aloft moving eastward from James Bay.

 

 

Thunderstorm Outlook for Today

 

Thunderstorm Outlook for tonight:

 

 

Thunderstorm Outlook for Wednesday:

 

 

 

 

Allen

Convective Outlook Valid for Today and Tomorrow

Monday, July 17, 2023

 

Regional Impacts

 

Today

NB: Isolated thunderstorms possible producing gusty winds.

LAB: Isolated lightning

 

Tonight

NB: Isolated lightning

 

Tuesday

NB: Thunderstorms with downpours with rainfall rates of 15-25mm/h

LAB: isolated lightning

 

Convective Discussion

A broad area of ascent driven by a surface trough, vorticity advections and favourable upper jet dynamics will drive isolated storms later this afternoon across parts of northwestern Labrador.

In the Maritimes today a trough will continue to advect eastward throughout the day. Elevated instability exists near and east of the trough, with high PWATs thin MUCAPE profiles of 200-300 J/KG, however due to the proximity of a mid tropospheric thermal ridge very high freezing levels exist. As a result, buoyancy looks to be limited through the ICN, thereby limiting charge separating and lightning. Regardless downpours may produce rainfall rates of 15-25mm/hr, despite no lightning being expected along this feature.

 

Behind the trough in NW NB, clearing and residual moisture in the PBL will yield MLCAPEs near 1000 J/Kg, with moderate shear profiles. However beyond surface heating there is a lack of a dynamic triggering mechanism which should temporarily delay initiation and limit the number of storms that do develop. The Mid to upper troposphere does dry significantly by the late afternoon and early evening, supported by observations upstream (12z Maniwaki UA). This favours a wind hazard for storms (DCAPEs 700-1000 J/Kg), while limiting the risk for heavy downpours. Storms will weaken late this evening, but may exist just after midnight before dissipating.

 

On Tuesday deep moisture rebuilds ahead of an shortwave moving eastward from Southern Quebec, this will yield a downpour threat for NW NB. Despite lower CAPEs, this synoptic trigger should allow for a greater prevalence of storms and lightning compared to today.

Cool air aloft and differential heating in the terrain in northern Labrador may also produce some localized lightning.

 

Overnight Tuesday into Wednesday this deep moisture will move eastwards along a trough and any storms that develop will bring the risk of heavy downpours.

 

 

Thunderstorm Outlook for Today

 

Thunderstorm Outlook for tonight:

 

 

Thunderstorm Outlook for Tuesday:

 

 

Thunderstorm Outlook for Wednesday:

 

 

 

Allen