Convective Discussion
Two “waves” of thundershowers or thunderstorms are possible on Monday for the Maritimes. No significant convection expected for Newfoundland or Labrador.
Technical Discussion
A two event scenario is looking more likely as the 12Z guidance from both the RDPS and the GFS are keying in on a warm-sector trough moving through the region prior to the passage of the primary cold front.
< (1) Morning warm sector low-to-mid level trough…
Confidence in the location of convection with the passage of this warm-sector trough is low. The new foreign and domestic NWP timing of the warm-sector trough that is calculated to arrive near 12Z over western New Brunswick is consistent with the previous 06Z adjustment (see previous convective outlook); however, the parameterized thunderstorm activity is not in agreement. The 12Z GFS maintains a solution with convection traveling eastward through northern New Brunswick while the 12Z regional, valid at the same time, has its “thunderstorms” over Fredericton and Southern York County. 12Z 1.5 PVU analysis for the upstream conditions around the upper trough associated with the surface cold front is acceptable; however, no extra confidence was gained on trying to pint-point the leading warm-sector trough, so confidence remains low on that feature. Should thunderstorms form with this feature there is a risk of severe convection associated with this trough. A low level jet near 50 knots at 850 hPa combined with NWP dewpoints near 12 degrees over western and northwestern New Brunswick. Main threats with this feature – should one form – would be significant rainfall eclipsing 25 mm/h rainfall rates, and peak wind gusts near 50 knots. Givin the increasing shear scenario, the chance of a supercell forming along this trough line is increasing but remains minimal as modified NWP soundings are generating only 500 J/kg with NWP 0 to 6 km shear near 45 knots. To reitorate, confidence with the dynamics of this feature is low.
(2) Late afternoon / evening primary cold front passage…
The primary cold front is expected to begin its descent over the region late Monday afternoon or early Monday evening – there remains some discrepancy with the timing across foreign and domestic solutions. Ahead of this feature projected afternoon dewpoints for the majority of New Brunswick are near 16 degrees, and similar for the Cobequids for NS and Prince County, PEI. Dew points in excess of 20 degrees are calculated by some NWP for western New Brunswick. Lifted indecies for Monday afternoon over New Brunswick range from 0 to -2 over western New Brunswick, with the odd -4 over Maine. CAPE values relative to the 12Z RDPS run are suggesting sub 500 J/kg for New Brunswick by 18Z Monday afternoon, with between 500 to 1000 J/kg of CAPE for central and western Maine. The largest uncertainty with the formation of late afternoon convection ahead of / associated with this descending cold front is the cloud coverage after the passage of the morning warm-sector trough and the timing of the cold front. Naturally,should an increased cloud coverage scenario be incorrect and more insolation is available then the thunderstorms will be more likely to form, CAPEs will be larger (slight modifications to some NWP soundings extend the 500-1000J/Kg swath further eastward into New Brunswick and produce near 1200-2000 J/kg westward), and the probability of more significant severe convective weather will increase. Zero to 6 km shear remains near 50 knots…therefore their remains a risk of supercell formation ahead of/associated with this feature. Furthermore, should the leading edge of the cold front be faster, as the 12Z GFS is currently suggesting, a more significant afternoon scenario could emerge for New Brunswick, but again, it all depends on the cloud coverage in the wake of the initial trough.
Final notes: Freezing levels are expected to remaind near 13,000 feet for the majority of NB so the threat of hail is not as signficiant as torrential downpours. NWP generated helicity ahead of this cold front fluctuates from sever to non-severe. Confidence is low in the RDPS NWP helicity; that being said, super cell composite parameters from the 00Z RDPS run are near 30 for northwestern New Brunswick Monday afternoon. If the cloud cover is sufficiently low, and all other energetics remain the scame, there will like be an active cold front scenario producing longer lived, multicelled line of thunderstorms with strong wind gusts, and torrential down pours. If the cloud cover remains too high, conditionally-stable to stable low levels would inhibit surface based convection ahead of the cold front. Along the cold front, heavy showers and scattered thundershowers are still likely, but the more severe threat would like remain over Maine, where on average, cloud cover is expected to be lower, and convective indecies are more significant.
The graphical outlook above is assuming a cloud cover scenario that is more condusive to thunderstorm development.
Regional Impacts
For Newfoundland: Nil significant convective weather.
For Labrador: Slight risk around the upper sfc/upr low Monday aft/evening over south western labrador (primarily south of Labrador city and Churchill falls). Severe convection is not expected.
For PEI and Nova Scotia: Possible thundershowers (perhaps a thunderstorm) associated with the passage of that mid level trough. Pimary impacts should convection materialise would be high rainfall rates, potentially exceeding 25 mm/h, and gusty winds potentially exceeding 40 knots. Thundershowers and Thunderstorms possible Monday evening into Tuesday morning with the passage of the cold front. Primary threats with these features would be high rainfall rates, and gusty winds.
For New Brunswick: Thundershowers or thunderstorms possible Monday morning depending on the vigor of the warm-sector trough. Primary threats with this feature would be strong wind gusts near 50 knots , and downpours generating rainfall rates in excess of 25 mm/h. Similar conditions for the passage of the primary cold front, should the cloud cover regime be conclusive to thunderstorm formation.
Marine Impacts: Risk of thunderstorms/thundershowers associated with the warm-sector trough, as well as the passage of the cold front. Primarily for western and southern gulf waters.
Marshall Hawkins