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

 

Day 1 Convective Outlook for Atlantic Canada Valid for August 23, 2018

 

Convective Discussion

 

The Maritimes are expected to remain quiet on the severe convection front, while over Labrador in the wake of a frontal wave, conditions may produce thunderstorms this afternoon.

 

Technical Discussion

 

Nil sig convection is expected over the Maritimes, post frontal, capped soundings suggest that although there is a well stacked upper trough moving through, dew points in the low teens, and jets aloft, that there will be insufficient thermodynamics/dynamics to create significant convection during the valid period of this product.

 

Over Newfoundland, the fuel is present ahead of the cold front, and the upper level jets are further to the west than the surface cold front – only a 35 knots jet extends ahead of this feature. CYJT and CYYT profiles exhibit some potential instability; contrasting between CYJT, CYYT, and CYZV profiles for warm sector to post-cold frontal, mid level lapse rates do not favour deep convection.

 

For Labrador, diminishing risk ahead of the trowal/warm front as it moves northward, cloud tops will warm with insolation so this threat will diminish as upper features move northward and the day goes on. In the wake of the trowal, there is an unstable region extending south of the Churchill Valley to the provincial boundary. This region will not likely be triggered by surface based convection given the current cloud cover (however cloud cover likely to continue to diminish), and is showing the hallmarks of a region able to produce convection through upper level forcing.  There is an approaching upper low and associated region of positive vorticity advection,  “decent” divergence present at the 250hPa level across all of central Labrador, and “weaker” divergence signatures as we descend through to 850 hPa.  CAPE values shown on NWP may be artificially low because of the Skinny Cape vs Virtual temperature issue, and I believe that this region may be more unstable than NWP suggests. The GFS has 1000 J/kg of CAPE over CYYR by 18Z, but overall the NWP cape envelope from the 06Z runs of the RDPS and GFS are similar (stretching from CZUM to the western portion of Eagle River). Because there are multiple Jets with left exit regions possibly reaching the area discussed above (peak Jet at 250 ~ 110kt over the middle-Gulf) , should thunderstorms form they would be able to produce strong wind gusts, potentially reaching 80 (or at worst possibly 90 km/h), or small hail as freezing levels  are near 12,000ft AGL (cross-front average).  0-6km Bulk shear this afternoon over the Labrador target region should be near 40 knots.

 

For the Maritime and Newfoundland Marine district:  Upper level triggered (via low level jet proximity) thunderstorm risk continues over the warmer waters of the Nova Scotia Marine district and that risk may extend into the west and southern regions of the Southwestern Grand Banks. Primary threat with these thunderstorms include wind gusts of 35 -40 knots associated with the passage of a similarly robust 850 jet.

 

Regional Impacts

 

Newfoundland: There is a weak risk of a thundershower with the passage of the cold front this afternoon. At this time the risk is not deemed significant enough to merit any additional action other than what is forecast currently by the NLWO. Severe convection is not expected.

 

Labrador:  Diminishing risk ahead of the trowal/warm front as it moves northward, cloud tops will warm with insolation. Thundershowers expected to develop this afternoon south of the Labrador public forecast regions, with a moderate risk for the development of severe weather mainly south of the Churchill Valley. Should thunderstorms/showers form they would be capable of producing strong wind gusts, and potentially small hail.

 

Maritime and Newfoundland marine districts: Isolated thunderstorms primarily over warm water regions. Wind gusts reaching 35-40 knots are the primary threat.

 

Labrador marine: Severe convection is not expected.

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

 

Day 1 Convective Outlook for Atlantic Canada Valid for August 22nd, 2018

 

Technical Discussion

Low risk of non severe elevated thundershowers this afternoon and tonight over parts of NB and NS.  These thundershowers are associated with a trough that will move over the area.  If any thundershowers develop, they will be scattered.  The highest likelihood of thundershowers is over southwest NS.

 

Regional Impacts

 

Nova Scotia – Showers with embedded heavier amounts up to 10 mm in thundershowers.  Most likely over southwestern NS.

 

NB – Showers with embedded heavier amounts up to 10mm in thundershowers.

 

PEI – none.

 

Nfld & Lab – none.

 

 

Barrie MacKinnon