Day 1 Convective Outlook for Atlantic Canada Valid for July 12, 2018

Convective Discussion

Other than the embedded convection associated with  Post-Tropical Storm Chris, no severe convection is expected across Atlantic Canada today. For details regarding the impacts of Chris, please refer to official forecasts from the Canadian Hurricane Centre, Newfoundland and Labrador Weather Office, and the Atlantic Storm Prediction Centre.

Technical Discussion

Primary threat for severe convection is in the developing frontal regions of Post-Tropical Storm Chris encompassed by the grey area above. The effects generated by these thunderstorms would likely not be a severe departure from the already predicted synoptic conditions. As the warm front develops toward the end of the period, cloud top cooling may begin to play a roll over the Northern Grand banks for the generation of nocturnal thunderstorms.  Otherwise, no significant weather is expected over the remaining regions in Atlantic Canada.

Regional Impacts

Newfoundland:  Heavy embedded showers and isolated thunderstorms associated with the rain bands of Chris.  The impacts driven from synoptic scale processes are more likely to be an issue than an individual embedded thunderstorm cell. Primary threats with these thunderstorms would be wind gusts and downpours, both factors are not likely to be a significant departure from the prevailing weather conditions produced by Chris.

Newfoundland Marine: Heavy embedded showers and isolated thunderstorms expected with the passage of Chris. During the latter hours of this outlook, once the warm front is more developed, the risk of thunderstorms extends northward to the Northern Grand Banks. Primary threat with these thunderstorms would be wind gusts and these gusts are not likely to be a significant departure from the weather synoptic conditions produced by Chris.

Maritime Marine: Heavy embedded showers and isolated CBs over eastern regions associated with the passage of the developing fronts. Primary threat with these thunderstorms would be wind gusts and these gusts are not likely to be a significant departure from the prevailing weather conditions produced by Chris.

Nova Scotia: Moderate to at times heavy embedded showers expected over eastern Cape Breton associated with the passage of Chris otherwise no significant convection expected.

Labrador: Isolated TCU possible over southeastern regions this afternoon. No significant convection expected.

New Brunswick, PEI: No significant convection is expected.

Marshall Hawkins

Day 1 Convective Outlook for Atlantic Canada Valid for July 11, 2018

Convective Discussion

A trough swings through the region and then begins to interact at the mid levels with Hurricane Chris towards the end of the period. Shower or thundershower activity is possible for parts of the Nova Scotia Spine and possibly further up to Cape Breton. Less of a threat of thundershowers moving into central Newfoundland. The most active region, or potential threat, for severe convection is over the southern Nova Scotian Marine district which will be more impacted by the northeastward track currently forecast by the Canadian Hurricane Center.

Technical Discussion

A well structured upper trough moving over the region will pass over Nova Scotia and western Newfoundland during the outlook period. CAPE Values remain low across all the region and can only be modified to produce CAPE values less than 500 J/kg through significant  mechanical lift (via upper level trigger) or unphysical autoconvection temperature for the regions in the slight risk zones. Freezing levels remain above 10k feet for the maritimes with a decrease expected post trough passage, significant hail is not expected and shear values are marginal ahead of the trough.

There is uncertainty as to how this trough will interact with Hurricane Chris toward the end of the outlook period. It will likely begin to undergo deformation during the later half of the period which increases the uncertainty involved with the northward transport of moisture content. This primarily effects the likelihood of stronger convection over the southern Nova Scotia marine district.

Regional Impacts

Newfoundland: Slight chance of a thundershower ahead of the trough later this afternoon mainly confined to the interior. Increased mid level moisture transport ahead of the trough may supply sufficient fuel; however it depends on how much drying occurs as the air mass with more moisture crosses the land. Mid level lapse rates (NWP) are only favourable should moisture increase prior to the trough passage. No severe convection is expected.

Labrador: Southeast of the trough occasional TCU activing with dry gusts possible. Risk of a thundershower is low, isolated CB possible. No severe convection is expected. Northwest of the trough although vorticity remains near the upper low, only TCU generation is expected( and maybe an isolated CB) as there is insufficient moisture for severe convection. Slight risk of a thundershower here. No severe convection is expected.

Nova Scotia: Showers and/or thundershowers possible from the interior of southwestern Nova Scotia northeastward along the spine to and including the highlands of Cape Breton. The chance of severe convection is low; however, thundershowers are possible which would include moderate to heavy downpours. Rates and downpour accumulations are currently expected to remain below warning thresholds. Freezing levels are near 12,000 ft and are expected to drop in the wake of the feature, significant hail is not expected.

New Brunswick & PEI: No significant severe weather is expected. Isolated TCU activity over eastern PEI possible later this afternoon.

Marshall Hawkins

Day 1 Convective Outlook for Atlantic Canada Valid for July 10, 2018

Convective Discussion:

A front east of an occluded low west of Labrador will move east across the Big Land. Ove the Maritimes the same front will drop south over New Brunswick this afternoon, spreading thundershowers south across the province and into Prince Edward Island. There is a possibility of significant weather, mainly downdrafts, for New Brunswick and parts of Southern Labrador, and small hail for Southern Labrador

Technical Discussion:

Currently the cold front is over the St. Lawrence River heading south, and there is another weak trough over Central New Brunswick that has triggered a few cells along and east west line.  Most of the tephis over NB show a weak cap that should break sometime this afternoon with temperatures into the low 30’s, and enhancement with both the trough and front should organise things. Shear values near 50 knots over NB diminishing to about 35 over NS, should stay steady or increase. CAPEs should be healthy, in the 500 to 1000 J/kg range. Moisture is more marginal, with dewpoint  values near 14-17 for NB and PEI. Also precipitable water amounts are in the 20-30 mm range, so not huge.  The big risk here is for downdrafts, probably from mid-afternoon until near sunset. Midlevel cooling as the main front approaches should enhance thing sin the middle to late afternoon as well.

Labrador is similar, with CAPEs in to 500-1000 J/kg range, and shear a little lower. Over the mountains south of Lake Melville the relatively low freezing levels give a risk for hail. This may affect the Trans-Labrador Highway.

The Avalon may see isolated showers into the overnight periods

Regional Impacts:

New Brunswick: Downdrafts possibly to 90 km/h. Thundershowers.

Labrador south: Downdrafts possibly to 90 km/h. Small hail, especially over higher terrain.

Avalon Peninsula: Chance of thundershowers into the overnight period.

Elsewhere: nothing significant

Doug Mercer

Day 1 Convective Outlook for Atlantic Canada Valid for July 9, 2018

Convective discussion

A weak low/frontal feature west of Labrador will move across tonight to lie east of Labrador Tuesday morning.  This system has a cold front associated with it that will drop down over the Norther Gulf  tonight. This will give some thundershowers, and over western Labrador may give significant wind gusts.

Technical discussion

A weak low near the surface is just west of Labrador, with some support from a weak trough extending to mid-levels just west of a thermal ridge. This is moving through western Labrador and cooling aloft is occurring behind it. Moisture is currently marginal, but there are precipitable water amounts of 25-35 mm near the front and low.  Also, an axis of 18 degree surface dew points are approaching from the southwest (south of the cold front), and should start moving into Western Labrador by late this afternoon.

Shear is strong, and there are two midlevel jets approaching western Labrador, one in the extreme southwest and the other further north. Shear values are between 45 to 70 kts.  Based on the soundings and forecast afternoon temperatures, CAPEs in the 500-1000 J/kg range are expected for western Labrador, dropping to 250 over eastern Labrador.

The main threat is downdrafts, possibly reaching 90 km/h for Churchill Falls and west, followed by short intense downpours. Freezing levels of near 10,000 feet allow the possibility of small hail over the western higher terrain and the mountains, but again, moisture levels are limited.

Regional impacts

Labrador:  possibly severe downdrafts Churchill Falls and west . Elsewhere isolated to scattered thundershowers.

New Brunswick/ Northern Gulf of St. Lawrence/Western Labrador Coast/ Atlantic slope waters: isolated thundershowers.

Elsewhere: none.

Doug Mercer

Day 1 Convective Outlook for Atlantic Canada Valid for July 8th, 2018

 

Convective Discussion

In general it will be a quiet day across most of the region with a showers expected along the warm front that will push its way through Labrador, Eastern Quebec and into Western Newfoundland later today. In the short term convection will be somewhat suppressed do to the thermal ridge aloft and fairly low dew point temperatures. Later this afternoon however the dynamics and thermodynamics will become more favourable over western Labrador.  Aloft  the 250 mb jet and 500 mb jets will begin to strengthen with strong diffluence at the upper levels. As well the thermal ridge will continue to track east and we will get into some cooling aloft later today in the order of 2- 4 degrees at 500 mb, this along with the low level moisture advection will aid to destabilize the atmosphere. With the strength of the low level winds >40 kts and the impending thunderstorms, it is not unlikely that wind gusts will exceed warning criteria west of Labrador. Close attention should be paid to the speed and timing of thunderstorms that may move into western Labrador early this afternoon or evening. Should thunderstorms be very active wrt lightning watches may be required.

 

 

Regional Impacts

 

For Labrador

 

Strong winds and heavy rain possible should thunderstorms approach Western regions later today.

 

For PEI, NB, NS and Newfoundland

 

No Impacts expected.

 

Mel Lemmon