Day 1 Convective Outlook for Atlantic Canada Valid for August 30, 2018

 

Convective discussion:

 

Risk of non-severe thundershowers over Atlantic coastal regions of Nova Scotia and central/Newfoundland this afternoon and evening.

 

Technical Discussion:

 

A cold front with an associated low pressure system is sweeping across Atlantic Canada today. Very moist tropical air (dew points 20 degrees plus) ahead of the front get replaced by much drier and stable air in its wake. Morning soundings are not showing much CAPE ahead of the front so just a risk of a thundershower is expected ahead of this fast moving feature. The thundershower risk is surface based for Nova Scotia and embedded for Newfoundland. Lots of clouds already in Newfoundland ahead of the approaching low pressure system.

 

Regional Impacts:

 

New Brunswick: Nil.

 

Nova Scotia: Risk of non-severe thundershowers.

 

Prince Edward Island: Nil

 

Newfoundland: Risk of non-severe thundershowers.

 

Labrador: Nil.

 

 

 

 

 

 

 

Day 1 Convective Outlook for Atlantic Canada Valid for August 29, 2018

 

Convective Discussion

 

Showers and possible thundershowers to push into western New Brunswick late this afternoon and evening as a shortwave trough moves into the area from the west.

 

Technical Discussion

 

This morning a low pressure system is producing some isolated showers over Newfoundland into the Labrador Sea. There remains a slight risk of a thundershower over Belle Isle Bank as the low tracks continues northeastward today. Most of the Maritimes are under fair weather this afternoon under a weak ridge of high pressure however the next shortwave trough will approach western New Brunswick by early this evening. A surface low pressure currently over central Quebec is expected to track along the St. Lawrence River eastward through northern New Brunswick overnight.

 

The environment within the warm sector and along the cold front will be conducive for the development of thunderstorms as low level moisture, surface heating and strong westerly shear increase this afternoon. Since the cold front is not expected to push through New Brunswick until the overnight hours, the main concern will be what may develop ahead of the cold front in the warm sector in the afternoon.

 

Looking at KCAR 12Z sounding, temperatures from 850mb upward are quite warm. However upper heights and colder air aloft will be pushing into Maine and New Brunswick will be falling though out the day, increasing the instability aloft. There is also not much cloud cover over New Brunswick currently so areas may be able to achieve their max daytime heating.  A westerly 40KT 850mb LLJ will be approaching western New Brunswick by the afternoon and with 0-6km shear near 50KTS, strong wind gusts (potentially near 40KTS) are possible especially where isolated cells develop into a squall lines. There is sufficient deep level shear so the possibility of supercells cannot be ruled out. Increasing precipitable water values of 40mm will also mean heavy rainfall rates are possible although cells should be moving quite quickly. At this time it appears the strongest threat for severe thunderstorms will be over southern Quebec and Maine where upwards of 2000J/kg may be reached. Areas of western New Brunswick mainly from Grand Falls and Victoria County to Fredericton and York County will be closely watched as cells could maintain long enough to advect into those areas before convection weakens in the evening.

 

Regional Impacts

 

New Brunswick: Showers and possible thundershowers with lightning strong wind gusts and locally heavy downpours this evening over western NB. Showers with a slight risk of a thundershower may extend into eastern NB overnight.

 

PEI and Nova Scotia: An isolated heavy showers is possible overnight as the cold front moves through.

 

Newfoundland and Labrador: Scattered TCU and associated showers as the trough moves out of Newfoundland this afternoon and the approaching low pressure system moves into southwestern Labrador.

 

 

 

Roberta McArthur

 

Day 1 Convective Outlook for Atlantic Canada Valid for August 28, 2018

 

Convective Discussion

 

Showers are expected ahead and along an upper trough extending from northern Labrador towards central Quebec that will push eastward today. There is the risk for thundershowers to develop over southern Quebec and Maine this afternoon that may continue into northwestern New Brunswick this evening. There is also the slight risk of a thundershower this afternoon and evening over central and southern Labrador.

 

Technical Discussion

 

A upper trough stretches from northern Labrador southwest toward the central United States with several short wave troughs lying within it.

 

One wave lies south of Hudson Bay and will track eastward towards southern Labrador by midnight.  Strong convection should remain west of our region today as the associated cold front and strong LLJ lie over southern Ontario. Looking at the KCAR sounding, there is currently a good capping inversion at 500mb but with the thermal ridge moving eastward today, there may be enough cooling aloft along with surface heating to breach that cap. There is however a 40KT 850mb LLJ currently over the St. Lawrence River which will help to provide uplift as it pushes into western New Brunswick early this evening. With increasing shear along with relatively high precipitable water values reaching 40mm, any showers or thundershowers that develop could give locally heavy rainfall rates and strong wind gusts. Given the timing of the trough passage being late evening and overnight, it is expected that much of the convection will dissipate as it tracks across northern New Brunswick overnight.  

 

For central and southern Labrador, they will be in a favourable left exit region of a 250mb 110KT jet as another short wave trough moves through northern Labrador with a cold front extending through central and southern Labrador. Deep convection is however limited by both 500mb and 700mb thermal ridges moving through the area so unless they can get some additional surface heating, they may not be able to break the cap and will only see TCU and showers with gusts potentially reaching 30 KTS along the cold front.

 

Regional Impacts

 

New Brunswick: Showers and possible thundershowers with locally heavy downpours and strong wind gusts late this afternoon or evening over northwestern NB. Elsewhere TCU and possible showers extending toward southern NB this evening.

 

Labrador: Showers this afternoon with a slight risk of a thundershower this afternoon.

 

PEI, NS, Newfoundland and Marine waters: No significant weather expected.

 

 

Roberta McArthur

 

Day 1 Convective Outlook for Atlantic Canada Valid for August 26, 2018

 

Convective Discussion

 

Slight chance of thundershowers across parts of NB tonight associated with a short wave trough. A slight chance of thundershowers near or along the Atlantic Coast of NS associated with a weak surface trough.

 

Technical Discussion

 

The dynamics and thermodynamics around ATL Can are relatively weak today so very little in the way of convection is expected. There is a 500/700 mb trough upstream over the GRTLKS region, but that is not expected to advect into the region later today. A shortwave trough at 500 mb will cross NB tonight which may trigger an isolated thunderstorm or 2. Also, a weak surface trough will cross over NS tonight giving a slight RSK of TSRA. The 12UTC sounding from YJT shows decent LLV moisture and 35 knots or so shear, but also 2 fairly strong capping inversions. Once daytime heating is maxed SCT TCU are likely with just the slightest RSK of a TSRA across CNTRL/ERN NL.

 

Regional Impacts

 

New Brunswick and Nova Scotia: slight risk tonight.

 

Newfoundland: very slight risk this afternoon and evening.

 

Labrador: nil.

 

Jeremy

 

Day 1 Convective Outlook for Atlantic Canada Valid for August 24, 2018

 

Convective Discussion

 

Upper troughs moving through the region do not seem to be appropriately timed with advecting fuel sources, this combined with upper level smoke over parts of Atlantic Canada should serve to suppress severe convection over all regions except over parts of the southern marine districts of Newfoundland and southeastern districts of the Maritimes, where thunderstorms will likely persist throughout the period.

 

 

 

Technical Discussion

 

A well developed upper trough will continue to move eastward through the region over the period. The most unstable regions are over NB in the wake of this upper trough where at 500 cold trough will move through today – this presents an interesting case that is discussed more below. The jets ahead of this primary upper trough (not the 500 hPa cold trough) do not line up well for any additional lift although CYYT may be in the right entrance region later this afternoon. Low to mid-level lapse rates ahead of this upper trough are too stable for the generation of severe convection, and the most unstable regions, are too try without sufficient mid-level moisture advection present.

 

Focusing in on New Brunswick, the 12Z Caribou sounding demonstrates that insitu air has unstable lapse rates with possibly sufficient moisture to generate some convection, albeit unlikely to be severe, the problem is a trigger source. The primary deeper-layer moisture axis is expected to remain north, with little change expected in the dewpoint with the air advecting into the region. As previously mentioned the primary upper trough has moved well east, so only a weakening surface cold front –may- be able to help CU/TCU growth this afternoon. The larger inhibiter of severe convection is widespread smoke aloft over the province which will decrease insolation, and thus the amount of available energy to use for severe convective development. In the absence of a strong dynamical trigger, the auto-convect temperature as calculated off the 12Z Caribou sounding is 29 degrees, and with the smoke aloft affecting maximum insolation and thus the maximum daytime temperature, severe convection is not expected.

 

The well established trough over the southern Maritime, and eventually Newfoundland marine, districts will likely remain active today. Primary threats with these potential thunderstorms would be wind gusts capable of tapping into the 850 and 700 40 kt jet that was observed at CYYT (little change in magnitude is expected until well south of the marine districts), so potentially between 30 and 40 knots in the strongest thunderstorms over the warmest water. Cloud top cooling may allow thunderstorm activity to continue over the Southeastern Grand Banks through the latter hours of this outlook.

 

Photo of the smoke aloft – many thousands of feet up - (Purple-Blue Haze well established over New Brunswick, parts of NS and PEI, and near the front moving through Labrador).

 

Regional Impacts

 

Nova Scotia, New Brunswick, Newfoundland, Labrador, & Labrador Marine: No severe convection is expected.

 

Nova Scotia and Newfoundland Marine: Isolated to occasional thunderstorms likely over the warmest waters of Laurentian Fan, with a risk extending  to the Southern Grand banks as the trough moves east. Primary threat in thunderstorms are gusts to 30 to 40 knots.

 

Marshall Hawkins

 

Operational Meteorologist – Atlantic Storm Prediction Centre, Meteorological Service of Canada

Environment and Climate Change Canada / Government of Canada

Marshall.Hawkins@canada.ca  / Tel: 902-426-7779

 

Météorologue opérationnel – Centre de prévision des intempéries - région Atlantique, Service météorologique du Canada

Environnement et Changement climatique Canada / Gouvernement du Canada

Marshall.Hawkins@canada.ca / Tél. : 902-426-7779