Convective Outlook Valid for August 20th, 2019

Convective Discussion.

 

A few areas of convective activity to note mainly over Newfoundland and Labrador today as a quasi-stationary low over northern Quebec with a trough extending through Labrador, Newfoundland, and eastern Nova Scotia today.

 

The air mass within the trough extending from Newfoundland towards eastern Nova Scotia is quite moist in the low levels with dew points remaining near 20C. Surface temperatures are already near the daytime highs expected over western areas of Newfoundland despite the current cloud cover. The question will be how much additional surface based heating there will be should the cloud break up more. The inhibitor for this afternoon is a slight warming expected in the mid levels which could cut the potential energy of any convection that develops. Despite this, any showers already on the go and what could develop this afternoon will produce locally heavy downpours given the high pecipitable water values in the area as well as strong wind gusts. There should only be a slight risk of a thundershower over eastern Nova Scotia early this afternoon as the trough is currently moving over the area and some coastal convergence could provide further lift. Dew points will still remain high for a few hours after this trough moves through as the more pronounced change in humidity is still further west over New Brunswick.

 

For central Labrador, the area remains under an unstable westerly flow.  A 500mb cold pool lies over central Quebec to central Labrador providing sufficient lift given daytime temperatures in the upper teens. Air mass type thunderstorms are expected but some further organization is possible given convergence near the trough and low pressure centre that is expected to form over eastern Labrador this evening and push off into the Labrador Sea tomorrow morning.  Precipitable water remains moderately high so convection this afternoon will likely produce some heavy downpours. Hail is also a risk with freezing levels dropping below 10,000ft.

 

Regional Impacts.

 

Newfoundland: Locally heavy downpours and strong wind gusts.

 

Labrador: Locally heavy downpours, strong wind gusts, and small hail.

 

Nova Scotia: An isolated thundershower over eastern sections this afternoon.

 

New Brunswick and Prince Edward Island: nil

 

 

Roberta McArthur

 

Convective Outlook Valid for August 19th, 2019

Convective Discussion.

 

It should be active afternoon and evening for much of western New Brunswick and into the rest of the Maritimes and Labrador overnight as a short wave trough extending from a low pressure system over Hudson Bay tracks through the region.

A warm front has been producing some heavy showers and occasional embedded thundershowers this morning over Maine which will continue into New Brunswick and the Bay of Fundy but will also continue to fizzle into the afternoon. The cold front is not far behind over southern Quebec and will begin to track through New Brunswick late this afternoon and into tomorrow morning over Nova Scotia and Newfoundland.

 

For New Brunswick behind the warm front this morning, there should be some clearing in the warm sector ahead of the approaching cold front this afternoon. Low-level moisture is already quite high in the region with dew points 18-20C and precipitable water values reaching 40 mm. Surface based convection is expected to begin this afternoon as temperatures reach the mid 20s given sufficient clearing. Upper divergence and further cooling aloft will provide further instability into the afternoon with CAPE values nearing 1000 J/kg over northwestern New Brunswick. Low to mid-level shear is also expected to increase to 30-40KT by the evening. These ingredients will all help to initiate and potentially sustain severe thunderstorms into the evening. Multicells may organize into bow lines or supercells ahead of the cold front. The main threat will be heavy downpours, strong wind gusts, and small hail. The potential for a tornado cannot be ruled out given the amount of low-level shear mainly over northwestern New Brunswick as well.  

 

Over Labrador, there risk of non-severe thunderstorms over central and western areas as late this afternoon and evening as the trough from the low pressure centre over Hudson Bay progresses northeastward. Locally heavy downpours are possible.

 

Regional Impacts.

 

New Brunswick: Strong wind gusts which may approach 90 km/h, locally heavy downpours(25+mm), and small hail (1-2cm.

 

Nova Scotia and Prince Edward Island: Locally heavy downpours and strong wind gusts.

 

Labrador: Locally heavy downpours

 

Newfoundland: Nil

 

 

Roberta McArthur

 

Convective Outlook Valid for August 18, 2019

Convective Discussion.

Early morning convection across southwestern NB has weakened and will continue to weaken this morning. There were a couple embedded TSRA, but increasing stability will negate any more CBs. The only area where cells are possible are across ERN LAB. Adjusting the surface temp to around 26 from YYR gives close to 500 J/kg. A surface trough will act as a trigger this afternoon. Sat pix shows some decent clearing across the region as well. Precipitable water is fairly low, but freezing levels are low and the shear is modest at around 35 knots or so. Expect some gusty winds and possible small hail in a couple of the cells. Elsewhere no convection is expected in Atlantic Canada.

Regional Impacts.

Labrador: scattered TS this afternoon giving gusty winds and possible small hail for ERN regions.

Nova Scotia/New Brunswick/PEI/Newfoundland: nil.

Jeremy

Convective Outlook Valid for Augut 17 2019

Convective Discussion

No severe impacts expected. Highest chance for convection for Labrador from the west to the  Mid Labrador coast. A slight chance for Newfoundland this afternoon, and a chance for a thundershower to move into westernmost New Brunswick.

For Labrador a trough extending from Hudson Bay to the Mid Labrador Coast, combined with an upper jet over northern Labrador, with a right entrance near the trough axis, will give some support for lift. As the upper jet moves east this afternoon, so will the instability . Moisture isn’t great, with precipitable water in the 15-25 mm range. CAPEs are on the low, with the highest expected less than 500 J/kg,  with the highest amounts at the southern edge of the threat zone. Also shear is marginal at about 30 kts. Expect a few thundershowers with some moderate wind gusts. The freezing level is near 9000ft, so small hail is possible.

For Newfoundland there is little dynamic forcing. The there is a weak upper trough moving through, followed by weak ridging from mid-levels up, and an associated weak upper jet. The main forcing mechanism will be daytime heating, and some moisture. The 12Z tephigram from Stephenville was less stable than model guidance, and with a forecast high near 25 C and a dewpoint near 13 C, a parcel lift gave a CAPE around  300-400 J/kg. Based on this there is a chance for isolated thundershowers this afternoon, with some gusty winds.

Regional Impacts

Labrador: Occasional non severe thundershowers with some gusty winds and a chance for small hail.

Newfoundland: a possibility for isolated  thundershowers this afternoon.

New Brunswick: a slight chance for thundershowers this afternoon in the extreme northwest, but only if the cloud breaks up today.

Doug Mercer

Convective Outlook Valid for August 16th, 2019

Convective Discussion

 

A weakening short wave trough with an area of light to moderate rain over Maine this morning is expected to break up into scattered showers this afternoon as it pushes into western New Brunswick. The risk for thundershower activity this afternoon should remain over Maine into southern Quebec where dynamics are more favourable.

 

Meanwhile, there is a low pressure system over Ungava Bay slowly tracking eastward with a cold front extending south and westward toward northern Labrador and Quebec. Ahead of this cold front, there may be enough upslope lift over the Torngat Mountains combined with an approaching 30KT 850mb jet to initiate some elevated convection this afternoon. Central Labrador remains in an unstable southwesterly flow and temperatures may be close to reaching their convective temperature needed to initiate thunderstorms this afternoon but there is a lack of a real trigger and with moisture dropping this afternoon and a warm layer at 700mb, there is less of a risk other than scattered TCUs in showers.

 

Regional Impacts

 

Labrador

Possibility of isolated thundershowers over the Torngat Mountains ahead of the cold front this afternoon. TCU in showers for central Labrador.

 

New Brunswick

Showers with possible local heavy downpours this afternoon over western sections.

 

Newfoundland, Nova Scotia and Prince Edward Island

None expected

 

 

 

Roberta McArthur