Severe Thunderstorms Tonight

Environment Canada has issued a severe thunderstorm watch for most of southern Manitoba for this evening. A few thoughts on the coming hours…

For Southwestern MB:

The main threat for severe storms should end this evening. High temperatures and dewpoints combined with daytime heating have contributed to a favourable environment for severe thunderstorms. These ones should primarily be driven by the sun, so once the sun starts going down, so too should the intensity of the storms.

For the Red River Valley:

The main threat for severe storms will continue through the evening as an 850mb jet pushes up out of North Dakota into the RRV. The main threat with these storms will primarily be rain. The storms are developing on the nose of the 850mb jet, just downwind of an area of moisture convergence, so as the evening continues, the storms will have more and more moisture to work with. This, combined with the fact that storms will most likely be training (multiple storms moving in rapid succession over the same area), will result in some areas in the RRV getting large quantities of rain. My initial estimate would be that some areas will receive total amounts of 70-90mm locally, however it could potentially be higher than that. Most areas will probably receive on average 15-25mm of rain.

Other threats from these storms will primarily exist during the evening hours when they still could be surface based. If any storms can develop by themselves this evening, there’s a slight chance they could become supercells given the SSE flow through much of the RRV. If that is able to happen, then there would be a chance of large hail (my estimates would be quarter to loonie sized) and strong winds (90+ km/h). Give the backed surface flow, there would also be a very slight chance of a supercell becoming tornadic, but the overall setup doesn’t look to favourable for that; you never know what storms will do when working with 3000+ joules of CAPE, though.

This MCS will stick around for a good portion of the night, resulting in a widespread wet and noisy evening for many residents in Southern Manitoba.

Thunderstorms on Tap for Southwestern Manitoba and The RRV

A trough line anchored to an upper low in central Saskatchewan will move across the Red River Valley tonight, bringing with it strong to severe thunderstorms.

As an upper low tracks eastward through Saskatchewan, an associated surface trough will slowly progress with it towards the SK/MB border. By late this afternoon, the trough line will be positioned from about Nipawin in central SK south towards Estevan, SK. Along with this feature, an upper wave will be moving into the area as well, with a 850mb cold front stacked on top of the surface trough by 21Z.

With the surface trough/upper cold front triggers in place, any storms that develop will have ample instability (SBCAPES of 1500 J/kg to 2000 J/kg alongside LIs of -6 to -9) as well as favorable wind profiles, with south to southeasterly surface winds veering to westerlies aloft. As well, a 40kt west-southwest jet will be poking into southern SK by late this afternoon.

Thunderstorm risk through 02Z August 7th

Thunderstorm Outlook for August 7th, 2010 valid until 00Z

What does this all add up to? Likely several strong storms with the outside chance of a severe storm in the Kamsack, Yorkton, Moosomin, Carlyle, Swan River, Dauphin, Minnedosa, Virden and Melita areas. These areas will likely see a series of strong thunderstorms develop this afternoon, with a slight risk of severe thunderstorms. The biggest risk from these will probably be large hail (maybe loonie to golf ball sized) and heavy rain. Should a storm develop supercell characteristics with a good southeasterly inflow, there is a slight chance that a tornado may develop.

As the sun sets, a 25kt 850mb jet will override the warm front in North Dakota and help develop a MCS that will move eastwards into the RRV late this evening into the overnight period. The biggest risk from these storms will be heavy rain.

After this noisy evening, Winnipeg and the RRV should be in for a nice day for the next couple of days with highs approaching or reaching the low 30’s before the weather cools off a bit mid-week with the upper ridge flattening out as a strong impulse enters the northern Prairies bringing some cloud and showers/thunderstorms into the southern Praries. There is a slight chance of an isolated thundershower on Sunday along the remnant trough line in the RRV.