Convective Outlook Valid for Today, Tonight and Tomorrow

Sunday, July 16, 2023 11:49 AM

Regional Impacts

 

Today

NB: Heavy downpours, with intense rainfall rates of 25 to 50 mm/h and locally strong wind gusts.

Labrador: Scattered thunderstorms over central and southeastern Labrador giving heavy downpours, small hail and strong winds.

NS: None.

PEI: Risk of downpours for Prince County.

 

Tonight

NB: Heavy downpours with isolated embedded thunderstorms. Rainfall rates of 25 to 50 mm/h.

NS: Heavy downpours possible for the Atlantic coast and western regions.

 

Monday

NS: Heavy downpours likely for western and central regions with rainfall rates at 25 to 50 mm/h.

NB: Isolated to scattered thunderstorms with intense rainfall rates of 25 to 50 mm

 

Convective Discussion

An upper low currently over James Bay will slowly weaken and track eastward as a quasi-stationary ridge holds over Newfoundland extending towards the Grand Banks. The Maritimes remains under a tropical air mass with precipitable water between 40 and 50 mm. An area of heavy showers this morning over the Bay of Fundy into southern New Brunswick will continue to push northward this afternoon. The trough associated with the low over James Bay is giving multiple rounds of heavy rainfall in southern Quebec extending south that will slowly push into western New Brunswick overnight and into Monday over PEI and western NS Monday night. Persistent cloud cover over New Brunswick is not likely to allow any surface based convection today, but there is 25KT 0-6km shear from KCAR so along with heavy rainfall, some isolated strong wind gusts are possible if more organized lines develop.

 

Over central and southeastern Labrador today, some clearing of cloud this morning will allow temperatures to reach the convective temperature based off Goose Bay’s 12Z sounding and expected high reaching 30C with MUCAPES of near 1000 J/kg. Deep level shear is fairly week, so not expecting strong organization of any convection that develops, but with precipitable water also in the low 30’s so heavy downpours will be a factor.

 

Behind the trough on Monday, there air mass remains unstable with isolated thunderstorms possible for western New Brunswick and western Labrador.

 

Thunderstorm Outlook for Today

 

 

Thunderstorm Outlook for Tonight

 

 

Thunderstorm Outlook for Tomorrow

 

Convective Outlook Valid for Today , Tonight and Tomorrow

Regional Impacts

 

Today

NB: Scattered thunderstorms, slight risk of severe in multi-clusters/features in the north. Rainfall rates in excess of 25mm/h, gusts 60km/h, small hail and frequent lightning.

NF&LAB: Frequent thunderstorms over central and southern Labrador. Rainfall rates in excess of 15mm/h, gusts 70km/h, small hail and frequent lightning.

NS: Risk of thunderstorms producing downpours

PEI: None

 

Tonight

NB: Isolated thunderstorms possible  giving localized downpours.

 

Sunday

NB and Lab: Thunderstorms possibly giving strong wind gusts, heavy downpours, and frequent lightning.

 

Convective Discussion

At 12z the two large scales features of interest are an upper low on the western shores of Hudson’s Bay and a ridge along the NF coast. The thermal ridge is holding strong over NF where fair conditions continue to prevail. Convective temperatures for interior regions is 30+ which will likely not be reached however Cu/Tcu should develop. Over Labrador, a weak area of convergence will advance over central regions/Churchill Valley and become the focus for surface-based convection. If the extend of the cloud cover currently over the lower Churchill valley diminishes, warm temperatures will yield increase instability with cape values ranging from 500-800 J/kg, LI -3, but deep layer shear are less than 20 kt. The main threat would be downpours and high wind gusts.

Over the Maritimes, the tropical air mass continues to prevail with VIL values for today between 40-50 mm (up to 65 mm  over New England). Some of the clusters of thunderstorms have produced significant amounts of rainfall even during the night and that will continue today with the focus for the afternoon over northern NB. The steering flow will be southwest around 20kt.

 

Thunderstorm Outlook for Today

 

 

Thunderstorm Outlook for Tonight

 

 

Thunderstorm Outlook for Tomorrow

 

Convective Outlook Valid for Today , Tonight and Tomorrow

Regional Impacts

 

Today

NB: Scattered thunderstorms developing this afternoon and evening, risk of severe thunderstorms in organized bands/features in the NW. Rainfall rates in excess of 25mm/h, gusts to 80km/h, and hail diameter near 2cm.

NF&LAB: Isolated non-severe thunderstorms in western Labrador, also south coast of the Island.

 

Tonight

NB: Isolated thunderstorms possible  giving mainly heavy downpours.

 

Saturday

NB: Isolated thunderstorms likely for a good portion of NB giving possible some hail, strong wind gusts and heavy downpours.

LAB: A slight risk of a thundershower over southern portions giving locally heavy downpours.

 

Convective Discussion

At 12z a surface low was located east of James Bay with a warm front extending eastward to Lake Melville, and a decaying cold front southward to New England. A couple of cold trough are rotating around a 500mb low located over Hudson’s Bay, one of which is linked to the surface frontal boundary. A 85kt cyclonic upper jet stretches from Long island-central Maine-Lower Quebec North Shore. Convection from yesterday has vanished for the most part, some lingering cells are detected north of the St. Lawrence. The cold front will enter the region later in the day and will be the focus for surface-based instability. Convective temperature in the upper 20’s should be reached fairly easily and robust dynamics should lead to some organized convection over northern Maine around the middle of the afternoon. The steering flow will promote propagation of the activity into Madawaska-Victoria and potentially into Carleton county. Surface-based CAPE of 1000-1500 J/kg and deep layer shear values of nearly 50kt indicates a potential for supercell development.

 

 

Thunderstorm Outlook for Today

 

 

Thunderstorm Outlook for Tonight

 

 

Thunderstorm Outlook for Tomorrow

 

Convective Outlook Valid for Today , Tonight and Tomorrow

Regional Impacts

 

Today

NS: There is a risk of a severe thunderstorm this afternoon and evening for a portion of the Valley, with torrential rainfall in the order of 25-50 mm/h the main threat. Elsewhere for NB and most of the rest of mainland NS, some isolated non-severe thunderstorms are possible.

 

Tonight

NW NB: Some isolated thunderstorms possible  giving mainly heavy downpours.

SW NL: A slight risk of an embedded thundershower giving locally heavy downpours.

 

Friday

NB: Isolated thunderstorms likely for a good portion of NB giving possible small hail, strong winds and heavy downpours.

SW NL/WRN LAB: A slight risk of a thundershower giving locally heavy downpours.

 

Convective Discussion

 

The main area of threat today will be the Valley of NS, with a lesser threat for areas of NB and most of the rest of the mainland of NS. The 12Z tep out of YQI looks decent for possible torrential rainfall and small hail. Adjusting for a surface temperature of 31 gives over 1000 j/kg of MUCAPE, with fairly fat CAPE below 500 mb owing to some decent updrafts. Tds are in the lower 20’s, so surface moisture if plentiful…as well at 850 mb. The instability is modest with LI’s of around -4 or so. The concern will be for training convection and hence locally very high rainfall rates in a relatively short period of time. The shear is very weak so wind gusts won’t be a big deal. Will have to see how things unfold this afternoon.

 

Some isolated thunderstorms are possible later tonight across NW NB as some development in ME might drift across the border. Also a slight risk of an elevated, embedded thundershower for SW NL associated with a very slow moving trough of low pressure.

 

More thunderstorms are likely for a good portion of NB and western LAB Friday afternoon and evening. Dynamically speaking, tomorrow is a better day for convection as the large scale trough to the west makes its way into our region. That aforementioned trough over NL will continue to give a slight risk of an embedded thundershower as well across SRN NL.

 

Thunderstorm Outlook for Today

 

Thunderstorm Outlook for Tonight

 

Thunderstorm Outlook for Friday

 

Jeremy

Convective Outlook Valid for Today , Tonight and Tomorrow

Regional Impacts

Today

NB…thunderstorms likely late this afternoon, beginning over northwestern areas. Small to moderate hails, gusty winds and torrential rain possible.

NS PEI and Labrador…low probability of thunderstorms, likely non severe. Rainfall less than 25mm.

Elsewhere…embedded thundershowers possible.

 

Tonight

Slight chance of lingering thunderstorms, no significant weather expected.

 

Tomorrow

NS, NB and PEI… Isolated thunderstorms, some locally heavy downpours possible greater than 25mm.

 

Convective Discussion

 

Thunderstorm development with moderately favourable dynamics and instability will come together this afternoon over New Brunswick. Upper dynamics are not overly good with only a very weak upper level jet moving over southern Quebec later today. However significant low level moisture, likely underdone by the models, will aid in the development of thunderstorms this afternoon. Mid level shear is moderate with about 30 knots in the 0-6 km layer. Mixed layer Cape will also be upwards of 1000 J/kg, what is less clear is if mid-level air will dry out this afternoon leading to an increased potential for downdrafts. There is however a strong possibility for torrential downpours and moderate hail with the thunderstorms. This will be a closely monitored situation and Severe Thunderstorm Watches are likely to be issued later today.

 

Thunderstorm Outlook for Today

Thunderstorm Outlook for Tonight

Thunderstorm Outlook for Tomorrow

 

Mel Lemmon