While temperatures will soar to the 30°C mark today, much of the Red River Valley should be prepared for showers and thunderstorms to develop by mid-to-late afternoon as a low pressure system moves into the area and taps the available heat and moisture.
850mb Theta-E values at 00Z May 19th (this evening) from the NAM. A sharp warm front is evident jsut north of the US border in Southern Manitoba.
With a strong capping inversion in place over the Red River Valley today, heat and moisture being pumped northwards will be trapped at the surface, allowing our temperature to soar to the 30°C mark and pooling moisture that will push our dewpoints up to around 15°C. Much of the day will be a beautiful day, especially given that it’s only May 18th. Conditions will deteriorate somewhat by late afternoon, though, as a surface low lifts northwards into Southern Manitoba.
The surface low will travel along a pre-existing surface trough and be supported by a shortwave ejecting northeastwards from the main upper trough, still anchored through Montana and Wyoming. With the heat trapped near the surface, the Lifted Index is expected to drop to the -2 to -8 values, with the lowest values running along a line from Gretna, MB to Bisset, MB and increasing to the southeast of that line. The heat combined with ample moisture is also expected to produce high CAPE values generally from 2000 to 3000J/kg. Increasing mid-level lapse rates with the approach of the shortwave will provide enough destabilization to erode the capping inversion through the afternoon and allow showers and thunderstorms to develop along and ahead of the warm front, initially concentrated near the triple point of the system. The showers and thunderstorms will expand in coverage as the evening progresses and the low-level jet intensifies.
Severe Weather Threat
Today marks one of the first severe weather threats of the season. Things look promising, and have even caught the attention of some seasoned storm chasers from the United States:
On paper, things look fairly good that there’s a risk for severe weather. EHI values rise to between 1.5-2.0 by late afternoon, which when combined with the presence of 30-35kt of bulk shear certainly presents the chance for supercell storms to develop. A few things hamper my excitement, though:
- Bulk shear isn’t really all that impressive; the really good shear moves through Western Manitoba through the day today, lifting northwards into the Interlake. The shear diminishes to the SE and isn’t particularly strong where the greatest CAPE is. When it comes down to it, it’s likely that we’ll end up with marginally strong shear on top of marginally high CAPE this afternoon, with the most favorable conditions missing each other.
- CAPE values may be too high. The NAM & GFS have been forecasting dewpoints slightly higher than have materialized. The amount of energy for storms to work with is going to be heavily dependant on the dewpoints that develop this afternoon, and if they don’t make it as high as forecast then we’ll be seeing weaker storms.
- Too many storms. With so many marginally severe features, we may see quite a few showers and thunderstorms develop this afternoon as the cap erodes. If too many fire off, then it will be difficult for any one storm to oragnize itself into anything that poses a severe weather threat.
All that being said, a potential for severe weather does exist today across south-central and southeast Manitoba. The main threats should severe weather develop will be hail and/or the possibility of a tornado.
Thunderstorm Outlook for May 18th to May 19th. Please note: Analysis has only been done for Manitoba. Storms may develop elsewhere in the Canadian Prairies.
Residents in Southern Manitoba should stay up-to-date on the weather today to ensure that if severe storms do develop, the appropriate precautions can be taken.
