Convective Outlook Valid for Today , Tonight and Tomorrow

Regional Impacts

Today

Lab: Risk of thunderstorms near noon and later today over western sections.

Northern NB: Risk of a TCU or very brief isolated thundershower later today.

Offshore and over the Avalon: Embedded convection is ongoing and remains possible today.

Tonight

Thundershower activity is not forecast over land areas tonight.

Tomorrow

Lab: Thunderstorm risk expands further north and east along the shifting frontal boundary.

Coastline of Nova Scotia: Embedded thundershower risk circles around the offshore low and may approach the Atlantic coast late in the day (prog goes until midnight). Not forecasting anything severe at this time.

Convective Discussion

Atlantic Canada finds itself sandwiched between 2 elongated frontal boundaries this weekend. One is offshore and stretches from the Grand Banks to the Carolinas. The second is situated from Ungava Bay back to eastern Lake Superior. It is easy to see the forcings on the 500mb charts, but additionally the cloud field on satellite is quite pronounced this morning. These long wave troughs are the lifting mechanisms for any convection that occurs in the next 24 to 48-hrs. Moisture is ok, but shear is substantially lacking in NB. Labrador is a different story. Shear may actually be too great for the amount of lift available. That being said, if an organized cell can form tomorrow, gusty winds (of 60 to 70 km/h) nearby could be a factor at play. This will be monitored. **Also of note is the forest fire smoke that is still affecting some areas of central Quebec. This could act as CIN that the models are not accounting for.

Thunderstorm Outlook for Today

Thunderstorm Outlook for Tonight

Thunderstorm Outlook for Tomorrow

Forecaster: Tirone.

Convective Outlook Valid for Today , Tonight and Tomorrow

Regional Impacts

Today

Lab: Risk of non-severe thundershowers.

NL: Risk of embedded thundershowers over the Avalon, bringing moderate (to at times potentially heavy) rainfall.

Tonight

Thundershower activity pushes out over the eastern marine domain.

Tomorrow

Lab: Thunderstorms are possible over western sections.

NB: There is a risk of thunderstorms developing in the afternoon over central and northern areas.

Convective Discussion

An elongated frontal boundary continues to affect the eastern parts of Nfld today giving a chance of embedded thundershowers. The potential hazard here is moderate to heavy rainfall in convective burts. Pwats on St. John’s sounding this morning is 39, showing the airmasses’ tropical nature. This forcing will slump over the eastern marine domain tonight, marked in broad grey. There is a secondary forcing across Labrador and the Quebec Lower North Shore that will act as a TCU/CB trigger this afternoon. These storms are not expected to reach severe limits – CAPE is 500-750J/kg, effective shear 15-25kts. Isolated lightning is the main hazard. This trof is the forcing for Day 2 in NB.

Thunderstorm Outlook for Today

Thunderstorm Outlook for Tonight

Thunderstorm Outlook for Tomorrow

Forecaster: Tirone.

Convective Outlook Valid for Today , Tonight and Tomorrow

Regional Impacts

Today

Lab: Risk of a thundershower.

NS/NL: Risk of a thundershower near the Atlantic Coast.

NB/PEI: Nil

Tonight

NS/NB/PEI/NL/LAB : Nil

Tomorrow

NS/NB/PEI/NL/LAB : Nil

Convective Discussion

The frontal system from yesterday is currently over Northern Quebec. Based on a combination of Water Vapour imagery and observation analysis, the frontal system is beginning to decay, which is progressing it toward a closed-off low-pressure system. As a result, enough remnant forcing remains over Labrador to give rise to isolated thundershowers today, specifically along the Northeastern coast. For the remaining parts of the Maritimes and Newfoundland, the decaying frontal wave is allowing moisture to flow in from the US, likely due to spillover from TS Betha and a building ridge over the Northeast USA. Because of this setup, the plume of moisture and its convective properties can shift with the position of the ridge axis, as well as with how the synoptic setup progresses. Within this plume of moisture, models are indicating CAPE values over 1,500 J/Kg with effective shear values of 30-40KT. One thing to note, however, is that it was intentional to leave the grey region over the Atlantic predominantly off the coast; high-resolution guidance indicates a struggle to push convection inland, while the regional model indicates that convection could move inland. Based on the synoptic setup, it was a conscious effort to go with the high-resolution guidance. 

Thunderstorm Outlook for Today

Thunderstorm Outlook for Tonight

Thunderstorm Outlook for Tomorrow

Forecaster: Lambert

Convective Outlook Valid for Today , Tonight and Tomorrow

Regional Impacts

Today

NB: Risk of Severe thunderstorms with heavy downpours, small to moderate sized hail, strong wind gusts. Risk of  a tornado over Northwestern sections.

PEI/NS: Risk of a thundershower.

NL: Risk of a thundershower over southeastern sections of the island.

LAB: Nil.

Tonight

Lab: Risk of a thundershower over central sections.

NL: Risk of a thundershower over Southern and Southeastern sections of the island.

NB/PEI/NS : Nil

Tomorrow 

NS/PEI/NL /Lab : Risk of a thundershower.

NB: Nil.

Convective Discussion

A frontal system near the Hudson Bay is progressing east and is the initiation forcing for the convection expected today. Currently, we have significant moisture across the Maritimes, along with curved jets moving through the US Eastern Seaboard. The frontal system, paired with these jets and ample moisture, with a cooler/drying airmass behind the cold front, is allowing for there to be a risk for severe thunderstorms in Northwestern NB, with the risk of a tornado today. This setup is giving Mixed Layer CAPE (MLCAPE) values near 1500 J/kg with Most Unstable CAPE (MUCAPE) exceeding 2,000 J/kg along the NB/Maine boarder. Additionally with this amount of instability, and the jet structure previously mentioned, we have a significant amount of Shear and Helicity. Looking at Convective model soundings we are seeing a Cloud Layer shear near 70 Knots, with Storm Relative Helicity (SRH) within the Surface to 3km nearing 140 m2s2. All of these properties do give rise to the possibility of supercells in Northwestern New Brunswick. As such when considering supercells the bunkers right movement seems the most reasonable, which is also fairly aligned with the mean wind direction (generally a Northeast direction).

As the frontal system progresses through Atlantic Canada tonight, we are expected to see a slight weakening in the jet structure and frontal boundaries. As such, there remains a risk for thunderstorms through central Labrador and near the Atlantic Coast. Model guidance does want to push convection onshore but the problem becomes that with the weakening forcings, making the land-sea transition during the overnight hours will be a struggle.

Tomorrow, the synoptic setup becomes more difficult for model guidance to capture the thunderstorm risk; as such, the regions in grey are likely to change as we move closer to the valid timeframe. The synoptic setup is that the low progressing through Atlantic Canada today has the possibility of getting stalled over Labrador with a building ridge over the continental US, and the Azores high. This setup has models indicating a potential Atmospheric river coming from the Gulf of Mexico into Atlantic Canada. As this setup requires several conditions, some of which there is low to moderate confidence in occurring, this is the justification for the grey region not having more details.

Thunderstorm Outlook for Today

Thunderstorm Outlook for Tonight

Thunderstorm Outlook for Tomorrow

 

Forecaster: Lambert

Convective Outlook Valid for Today , Tonight and Tomorrow

Regional Impacts

PEI.. Risk of thunderstorms tomorrow.

NB.. Risk of thunderstorms overnight, risk of severe thunderstorms for tomorrow.

NS.. Risk of thunderstorms overnight and tomorrow, heavy rainfall rates possible.

Newfoundland.. Risk of thunderstorms later tomorrow.

Labrador.. No thunderstorms expected.

Convective Discussion

Moisture will gradually increase across the region over the next 24 hours as a low-pressure system and associated deep upper trough, extending well into the eastern United States, slowly approach the Maritimes.

A plume of exceptionally high precipitable water is forecast to skim the Atlantic coast of Nova Scotia tomorrow morning before moving across eastern Nova Scotia early in the day and potentially affecting eastern Newfoundland later in the afternoon. Very heavy rainfall rates are possible within this plume, with rates likely exceeding 30 mm/h at times. While the plume may linger near the coast for much of the day, it is currently expected to only briefly affect any given location, limiting storm-total rainfall amounts.

Across New Brunswick, moisture levels will be somewhat lower than over eastern Nova Scotia. However, stronger instability is expected to develop ahead of the surface trough and approaching upper-level disturbance. Surface-based CAPE values are forecast to exceed 1000 J/kg, while deep-layer shear is expected to exceed 40 kt, supporting a risk of severe thunderstorms.

The shear profile appears relatively straight, favouring fast-moving thunderstorms and supercells. Storm motion is expected to be generally toward the northeast at approximately 50 km/h, allowing for both left- and right-moving supercells. These storms may produce small hail, strong wind gusts, and locally intense rainfall. Although rainfall rates could be high, the rapid storm motion should limit most rainfall totals to around 25 mm.

Thunderstorm Outlook for Today

Thunderstorm Outlook for Tonight

Thunderstorm Outlook for Tomorrow

Mel Lemmon