Convective Outlook Valid for Today , Tonight and Tomorrow

Regional Impacts

Today:

NS/NB/PEI: Risk of thunderstorms giving moderate wind and heavy downpours.

Lab: Risk of thundershowers.

NL: Nil.

Tonight:

NS/ PEI: Risk of thunderstorms giving heavy downpours.

Lab: Risk of thundershowers, mainly over higher terrain.

NB/NL: Nil

Tomorrow:

Lab: Risk of thunderstorms giving moderate wind, heavy downpours and a risk of small hail.

NL: Risk of thunderstorms along the southwest coast.

NS: Risk of a thundershower.

NB/PEI: Nil

Convective Discussion

A Low off the southwest coast of Nova Scotia is having its frontal wave sheared apart, which is allowing for a complex setup for today and into tomorrow. Looking at water vapour imagery, along with upper-air soundings, we can see that the main cold front is back towards the US Eastern Seaboard; meanwhile, the upper jets have sheared off from the low centre as the trowel wraps through Nova Scotia and New Brunswick, creating a dry slot. Along with the synoptic setup, there is vorticity advection affecting Northwest New Brunswick, and a departing ridge over Labrador, which is allowing for topography-induced troughs to develop. With this setup, we are seeing Precipitable Water values (PWAT) nearing 50mm, along with CAPE values hovering around the 500 J/kg mark. As such, going forward with this setup, it is most reasonable to use the Most Unstable CAPE (MUCAPE) values, to which we are seeing a swath of MUCAPE values between 500-1000 J/Kg around the Maritimes and Labrador.

Tonight, the disconnected frontal wave from today will progress eastward, allowing for some isolated thunderstorms to persist in the overnight hours for the Maritimes. For Labrador, there is a risk of thundershowers overnight; This drier air could allow for some thunderstorms overnight, though there is more uncertainty with this.

Tomorrow, remnants from today’s frontal wave will likely stay just to the southwest of Newfoundland, and model guidance indicates that convection will struggle to move inland. For Labrador, there is a stronger low-pressure system moving through Northern Quebec and Southern Baffin Island, which model guidance suggests a cold front could extend from Ungava Bay to Southern Quebec. The warm sector will enter Labrador with this frontal system, bringing a slight risk of hail, moderate winds, and rainfall. As this system progresses, the risks could change, as there are some timing uncertainties.

Thunderstorm Outlook for Today

Thunderstorm Outlook for Tonight

Thunderstorm Outlook for Tomorrow

Forecaster: Lambert

Convective Outlook Valid for Today , Tonight and Tomorrow

Regional Impacts

Labrador… Risk of thunderstorms today across western and central areas – may approach severe limits briefly in the Churchill Falls area today with gusty winds and heavy downpours.

Nova Scotia…  Chance of thundershowers in the overnight tonight with shower activity arriving from the SW. This risk extends eastwards in the day on Thursday. Moderate rainfall is the only hazard being forecast over land areas.

New Brunswick… Risk of an isolated thunderstorm near the Quebec border in the afternoon on Thursday.

Convective Discussion

The synoptic pattern is slowly advancing to the east in the next 24 to 48hrs. This brings a change in the weather across the Maritimes.

Ample surface moisture is in place today(dew points near 19), and effective shear(30kts) in the vicinity of the 2 forcings is good. The delay in anything getting going is simply the pace of the trigger itself. In Labrador, the trof will spark up this afternoon and may bring gusty winds and decent downpours. Small hail is also possible but shouldn’t exceed 1.5cm. Offshore from NS over the southwestern marine district, severe storms are possible and may form a decent line bringing gusts to near 90km/h. This will be monitored in case Squall watches or Warnings are required (depicted in yellow). As the low continues to wrap further and slowly advance east tonight, the thundershower risk moves into Nova Scotia. But it is forecast to be embedded in the overall greater shower activity (depicted in grey). It’s very warm at 700mb and quite dry in the upper levels this morning(Yarmouth sounding). Going to take some time to moisten the entire column to support any lightning activity. By tomorrow afternoon, the 100kt southerly jet at 250mb is just east of Halifax and situated as such that severe weather is not being forecast over any land areas. The orientation is not conducive this time.

Thunderstorm Outlook for Today

Thunderstorm Outlook for Tonight

Thunderstorm Outlook for Tomorrow

Forecaster: Tirone.

Convective Outlook Valid for Today , Tonight and Tomorrow

Regional Impacts

Labrador… Risk of thunderstorms today and tomorrow, with heavy downpours and some gusty winds possible.

Newfoundland… Risk of thunderstorms across eastern Newfoundland today, with some heavier downpours and frequent lightning possible.

PEI… Slight risk of thunderstorms today.

Nova Scotia…  Slight risk of thunderstorms today, mainly across eastern areas.

New Brunswick… Risk of isolated thunderstorms, mainly over northwestern sections late in the day.

 

Convective Discussion

The region currently remains under a somewhat anomalous pattern characterized by weak blocking. Most of the Maritimes lie between a trough extending from the Great Lakes northward into Labrador and a slow-moving low-pressure system approaching the Avalon Peninsula. This pattern is resulting in generally weak steering flow and a continued focus for convective development across portions of the region.

Across Labrador, conditions are expected to remain much the same as they were yesterday, although vertical motion may be slightly weaker. CAPE values in the 500 to 1000 J/kg range, combined with moderate low-level shear, will continue to support fairly widespread convection through the afternoon and evening. Rainfall rates in the strongest storms will likely reach 15 to 20 mm/h.

The low-pressure system over the marine area south of Newfoundland has transported more moisture into eastern portions of the Maritimes than previously anticipated. This increase in moisture has raised the potential for showers and thunderstorms in areas where low-level convergence is sufficient to overcome existing convective inhibition. However, with the exception of eastern Newfoundland, the shear profile across the region is generally not supportive of severe thunderstorms. As a result, most convection is expected to be relatively weak and disorganized, with only a low risk of severe weather. Nevertheless, locally heavy downpours remain possible where thunderstorms are able to develop.

Over eastern Newfoundland, the combination of stronger forcing associated with the nearby low-pressure system and somewhat more favourable wind profiles will support more organized convection. The most likely scenario remains clusters of thunderstorms capable of producing frequent lightning and locally heavy rainfall, with rainfall rates of 15 to 25 mm/h possible in the strongest cells. Strong downdrafts and hail are not expected to be significant concerns at this time.

Thunderstorm Outlook for Today

Thunderstorm Outlook for Tonight

Thunderstorm Outlook for Tomorrow

 

Mel Lemmon

Convective Outlook Valid for Today , Tonight and Tomorrow

Regional Impacts

Labrador… Risk of thunderstorms with downpours and small hail possible, today and tomorrow.

Newfoundland… Risk of elevated thunderstorms tomorrow with some heavier downpours.

PEI, NS… No Risk of thunderstorms.

New Brunswick… Risk of thunderstorms northwest tomorrow.

Convective Discussion

A surface trough will continue to move across Labrador today shifting slightly farther east tomorrow, with moderate instability developing during the afternoon. CAPE values are forecast to be in the 500 to 1000 J/kg range, while strong low-level winds will provide support for organized convection where thunderstorms are able to develop.

Scattered thunderstorms are expected along and ahead of the trough this afternoon and this evening. Rainfall rates could reach 15 to 20 mm/h, although the progressive nature of the storms should limit the duration of heavy rainfall at any one location.

While severe weather is not anticipated, some storms may become organized into short line segments or a convective complex. Should this occur, locally heavier rain and some gustier winds are possible.

Across eastern Newfoundland, a low-pressure system tracking near the Avalon Peninsula will provide additional lift and support for elevated convection. While surface-based instability is expected to remain limited, elevated thunderstorms may develop within the broader area of precipitation associated with the system. The strongest thunderstorms over eastern Newfoundland will be capable of producing brief periods of heavy rain, with rainfall rates of 15 to 25 mm/h possible.

Thunderstorm Outlook for Today

Thunderstorm Outlook for Tonight

Thunderstorm Outlook for Tomorrow

Mel Lemmon

Convective Outlook Valid for Today , Tonight and Tomorrow

Regional Impacts

Labrador… Risk of thunderstorms with downpours and small hail possible, today and tomorrow.

Newfoundland, PEI, NS… No Risk of thunderstorms.

New Brunswick… Risk of thunderstorms just north of province.

Convective Discussion

A ridge of high pressure over eastern Atlantic Canada will suppress convection across much of the region. Over the next couple of days, this ridge will gradually shift east, allowing an area of low pressure to move slowly across the remainder of the region. There is some potential for deeper convection just ahead of the low and its associated surface trough over Labrador, both today and tomorrow.

For Labrador, surface-based CAPE values between 500 and 1000 J/kg today and tomorrow will provide sufficient instability for thunderstorm development. Today, however, the triggering mechanism will be subtle. By tomorrow, a surface trough will provide a more focused trigger along with slightly higher low-level moisture. As a result, locally heavy downpours and small hail will become somewhat more likely ahead of this feature. Deep-layer shear is moderate, while low-level shear of around 20 knots may be sufficient to organize convection into a line or possibly support training thunderstorms.

Thunderstorm Outlook for Today

Thunderstorm Outlook for Tonight

Thunderstorm Outlook for Tomorrow

 

Mel Lemmon