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