Category: Forecasts

Slight Chance of Thunderstorms Tonight, Then Cooler

Brad Leave a comment

So, once again, the possibility of severe thunderstorms exists through the RRV this afternoon and into the overnight period; but how likely are they?

The various models are hinting at it:

The Canadian GEM-REG Model Output

The US GFS Model Output

The US NAM Model Output

Both the GEM-REG and GFS are showing convection over the RRV tonight, however the NAM keeps the convection almost exclusively in the states.

For us, I would lean towards a few showers tonight with maybe a thunderstorm, however I won’t completely rule out the current forecast of showers & thunderstorms.  The models have traditionally pushed the 850mb jet too far N pretty much every day, so colour me suspicious of real convection moving into the RRV (remember Tuesday night?).

The most likely scenario will be convection developing near the triple point of the low & fronts in ND this afternoon, then moving NE and just clipping SE MB.  If, however, the warm front manages to push into extreme southern Manitoba, there could be a higher likelihood of thunderstorms tonight.  Some convection may initiate in or just to the west of Winnipeg by late afternoon due to daytime heating that could become severe, however that would most likely die off as the sun goes down.

All in all, for Winnipeg, I’d say there was a 60% chance of showers or thunderstorms early this evening, and a 40% chance of thunderstorms overnight.  Ultimately, we won’t know what it will look like until things start happening in 4 or 5 hours from now.

After today, a cold front sweeps through Southern Manitoba on Friday and will bring us into a cooler regime with dewpoints in the high single digits to low double digits, and daytime highs in the low twenties.  In my opinion, a refreshing change from the sticky conditions we’ve had the past few days.

Severe Thunderstorms Tonight

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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.

Extreme Heat in Store for Monday; Will be Short-lived

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Monday will bring Winnipeg fully into the tropical airmass that has been building to our south for the past several days.  As the upper ridge over the central United States continues to push north, Winnipeg and the rest of the RRV will want to break out the air conditioners.

A building upper ridge will continue to push north out of the United States into Southern Manitoba overnight and tomorrow.  After a very slight chance of rain or a thunderstorm in the morning, the temperature will soar into the low 30’s through much of Southern MB.  Dewpoints close to the SK border will be low enough to not warrant too much of an issue, but as you move east towards the RRV, positively tropical dewpoints of 22C to 25C will persist resulting in Humidex values reaching into the low 40’s.

Fortunately, unlike as was forecast in the long range models last week, there is a fairly consistent agreement that the hot weather will not last.  A large MCS is forecast to develop on Tuesday night in northeastern Montana, which will likely head west of Winnipeg, however for most of the day we will be clouded over with debris cloud from the system and have a chance of showers or thunderstorm.  Tuesday will still be muggy, however, with dewpoints still in the low 20’s.  With all the cloud, though, the temperature will only climb into the mid 20’s instead of the low 30’s.

After that a powerful impulse is forecast to sweep across the northern Prairies on Wed/Thur/Fri, which will bring Winnipeg into a drier air mass next week.

Thunderstorms on Tap for Southwestern Manitoba and The RRV

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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.