Convective Outlook for Atlantic Canada valid for July 3rd, 2019

Convective Discussion

 

Risk of thundershowers over western New Brunswick this afternoon and early this evening with no convective activity expected elsewhere.

 

Clearing skies over western New Brunswick will increase solar insolation which may be enough to trigger a few non severe thundershowers.  An approaching ridge  from the west brings subsidence to the area, so any convection that could get triggered should be  moderated due to the mid level stability.  Anywhere from 100 to 500 J/Kg can be expected over the area but, again, positive buoyancy should be limited to mid levels which should limit CB development.

 

Regional Impacts

 

New Brunswick

 

Scattered thundershowers possible over western NB but severe thunderstorms not expected.

 

Nova Scotia

None

 

PEI

None

 

Newfoundland & Labrador

None

 

 

Barrie MacKinnon

 

Convective Outlook for Atlantic Canada Valid for July 2nd, 2019

Convective Discussion

 

The main area of focus will be over Maine with surface based convection having already begun, this convection will spill into New Brunswick this afternoon. Overall dynamics for severe thunderstorms are not overly favorable however there weak surface trough with moderate amount of CAPE > 1000J/kg and high moisture. The main concern will be heavy downpours possibly exceeding 25mm hour and with the low freezing levels 2-3 cm hail is also possible. It is unlikely but some of this convection could move across weather Nova Scotia this evening.

 

Over Newfoundland some embedded CB’s ahead of the surface low pressure system will generate locally heave rainfall amounts.

 

Regional Impacts

 

Nova Scotia

Locally heavy downpours, 15 - 20 mm possible if thunderstorms move into Southwestern parts of the province this evening.

 

New Brunswick

Main concern will be over western regions with heavy downpours 20-25mm and moderate sized hail possible.

 

Newfoundland

Embedded thunderstorms ahead of the surface low could give locally heavier rainfall of 15 – 20mm. Over western Labrador very low topped thunderstorms could develop with some rain and small hail.

 

PEI

None

 

 

Convective Outlook Valid for July 1st, 2019

 

Convective Discussion

 

There are a few areas of convective potential for today into tomorrow morning. A slow moving upper low currently near Sable Island will continue to track eastward and stall just east of Cape Breton by this evening. Meanwhile, a trough currently over Hudson Bay will track eastward towards central Quebec and western Labrador this evening and reach northwestern New Brunswick by tomorrow morning.

 

Under the upper level feature south of Nova Scotia, most of the convection should remain over eastern coastal regions of Nova Scotia, Eastern Shore, Sable and marine waters farther east this afternoon and evening near the vicinity of the low centre. Due to the slow nature of this system, some areas are expected to get locally heavy downpours. A rainfall warning is in effect for Victoria county due to upslope enhancement in easterly winds. Precipitable water values remain in the 30-35mm range looking at the Yarmouth and Sable Island morning soundings. Winds should remain quite stable over the marine waters but any convection that makes it onshore may see brief wind gusts reaching 30kts. Rain could be heavy at times over southern Newfoundland today as the low continues to drift closer but the threat of convection is low.

 

Areas of western Labrador will see an increasingly unstable air mass this afternoon as weak troughing moves into the area and convective temperatures are expected to be reached to initiate surface based convection, especially over areas that have cleared out of the stratus early this morning. CAPE values could reach the 500-700 J/kg range but with weak shear this morning, generally pulse type storms are expected. Into late afternoon, however, the 0-6KM shear is forecast to increase from 20 to 30KT which may allow some thunderstorms could be maintained or possibly organize into small squall lines. This along with fairy low freezing levels may produce small hail, brief heavy downpours and strong gusts. Conditions are not expected to reach severe warning criteria.  The upper flow may hinder initially some of the surface based convection as well due to warming aloft as a warm front approaches the region from central Quebec. An extension of this trough will also make its way farther south toward Maine and northwestern New Brunswick by early tomorrow morning.

 

 

Regional Impacts

 

Nova Scotia…

Locally heavy downpours over eastern mainland Nova Scotia and Cape Breton

 

New Brunswick…

Isolated thunderstorms may affect northwestern New Brunswick early tomorrow morning.

 

Labrador…

Small hail, strong winds and locally heavy downpours are possible for western and central Labrador late this afternoon and overnight.

 

 

HAPPY CANADA DAY!

 

Roberta McArthur

 

Convective Outlook Valid for June 30th, 2019

 

Convective Discussion

 

Today we are looking at a few different areas of convective activity, an area of low pressure slowly moving south of Nova Scotia and surface based convection for this afternoon over interior areas of Newfoundland and Labrador.

 

An upper level low over southern Quebec will be slowly drifting southeastward today and align with a surface low pressure system which is developing southwest of Nova Scotia and will drift eastward this afternoon. Elevated thunderstorms have already been firing up over southern Maine and have slowly drifted towards the western marine areas and western Nova Scotia. There has also been quite a bit of convection over the extreme southern slope waters of the Maritime marine district associated with the low’s warm front. That general area of convection is expected to round up the eastern and northern side of the low affecting parts of eastern Nova Scotia this evening and overnight. The main threat with this convection today and overnight will be heavy downpours. There is not much wind shear aloft to drive convection along that develops and precipitable water values are ranging from 30-35mm. There could be some localized areas of 30-50 mm mainly over extreme western Nova Scotia and parts of Cape Breton. There is also some upper level support being on the left exit of a strong 250 mb jet and a cold pool at 500mb over central Quebec tracking to the southeast. Some late afternoon convection is also possible over northwestern New Brunswick associated with the upper level low, especially if they get some clearing this afternoon.

 

For Newfoundland and Labrador, the main trigger for thunderstorms this afternoon will be daytime heating as well as slight cooling in the upper levels enhancing instability. Goose Bay sounding required a convective temperature of 20C with 300 J/kg of CAPE. There is very little shear however so only non-severe pulse type storms are expected west of Goose Bay giving brief gusts and locally heavy downpours. Similarly over Newfoundland, the main risk for any convection will be over interior areas where maximum daytime heating is likely with temperatures expected to reach 23C, combined with slight troughing in the upper levels.

 

Regional Impacts

 

Maritimes…

Locally heavy downpours over western Nova Scotia and Cape Breton. Isolated downpours and brief gusts over northwestern New Brunswick.

 

Newfoundland and Labrador…

Risk of locally heavy downpours and brief gusts over central Newfoundland and central/western Labrador.

 

 

 

Roberta McArthur

 

Convective Outlook Valid for June 29th, 2019

Convective Discussion

Modifying the 12Z Caribou sounding to the convective temperature of 24 deg gives 830 J/kg of CAPE. There is 35 knots of 0-6 km shear. The shear is unidirectional and is all above 3 km. At Caribou, the shear from 0-3 km is very low so based on this sounding, squall lines are unlikely. There is a strong 80 knot jet in the upper levels over the southern Maritimes as indicated by the 12Z sounding at Yarmouth. However, over NS and PEI, CAPEs are low so severity is not expected. The prog sounding for 18Z over western Labrador indicates that with daytime heating to the forecast high of 19 deg, convective initiation would give up to 800 J/kg of CAPE. Shear over western Labrador is low, so only pulse storms are expected if they form.


 As is often the case, the challenge today is the amount of sunshine this afternoon to trigger the convection. At 11 am there is still a lot of cloud cover over the areas outlined as potential for convection. For NB where there is a slight risk of conditions approaching or meeting severity, the most likely scenario is for strong storms to form over srn QC and then advecting late this afternoon or this evening over the province.

Regional Impacts

 

Maritimes…

Risk of thundershowers over most of NB, mainland NS, slight risk PEI. Possible wind gusts and small hail over NB

Newfoundland and Labrador…

Risk western Labrador.

 

Stephen Hatt
Forecaster
ASPC