Afternoon Showers & Cool Weather (and Frost!)

Afternoon showers in the RRV with the slight chance of an isolated thunderstorm or two will mark the arrival of a much cooler arctic airmass in Southern Manitoba.

Under cloudy skies most of the day, the RRV will see increasing southerly winds today as a low pressure system approaches from Southern Saskatchewan.  As the low passes by this afternoon into the early evening, the RRV will see widespread showers with the slight chance of a thunderstorm.

Friday 21Z 3hr QPF w/MSLP Panel from GEM-REG 00Z Run

Current indications are that most of the RRV will see 5-10mm of rain.  There may be accumulations up to 15mm in a few isolated localities due to enhanced convection.  The rain will end by midnight with only a slight chance of a few hang-back showers in the cooler air.

The main story after that is that Winnipeg will be under a much cooler airmass.

Sat 00Z 850mb Temperature Panel from GEM-REG 00Z Run

With current 850mb temperatures hovering around 6°C, expect a chill in the air in the mornings over the weekend as the 850mb temperatures get down to the -2°C range.  This should translate to overnight lows of -3 to 0 through much of southern Manitoba over the weekend. 

Pick your tomatoes if you haven’t!  This weekend certainly holds the potential for widespread frost with the cooler air moving in over the Province. The areas most likely to see frost would be areas west of the RRV, and locales in the RRV north of Winnipeg, with patchy frost possible to the south and southeast of Winnipeg.

Showers With A Chance of Thunderstorms Tonight & Friday

Showers and isolated thunderstorms will push into the Red River Valley tonight, signalling the start of the next major low pressure system to move across the prairies.

A low moving NE out of Eastern Montana has spread rain into much of Southern Saskatchewan through the day today, and is posed to bring more rain into Southern Manitoba.  Thunderstorms have initiated along a warm front draped W-E across North Dakota and will move northwards, supported by a 40-50kt southerly 850 jet and large scale ascent with a strong southerly flow aloft overriding the warm front.

Winnipeg should see rain beginning late this evening into the overnight period and through much of the day tomorrow as the precipitation will continue to blossom coming out of the United States.  All in all, most of the Red River valley should see 20-30mm of rain by the time the rain lets up on Friday evening, with local amounts possibly reaching 50mm underneath a few of those isolated thunderstorms, should they manage to develop.

This is, unfortunately, not good news to many of the Farmers in the RRV, who have had to battle saturated grounds through much of the latter half of the summer.  This is also bad news for the RRV in general, as we continue to have the soil re-saturated as we approach the winter freeze.  Should a long stretch of dry weather not occur, this wet fall could lead right back into a wet spring due to overland flooding.

Unsettled Weather in Store Next Week

Having broken the 30° barrier another time in the 2010 summer, summertime sun will give way to more unsettled weather next week.

A weak cold front will pass across Southern Manitoba tonight, bringing with it a very slight chance of showers (most likely nothing) and moving us into a cooler airmass for tomorrow, which should be a mostly cloud day.

By Monday morning, a warm front (shown above in the 850mb temperatures) will be positioned SW-NE across Southern Manitoba, extending from a surface low positioned in central North Dakota.  For several days, as the low moves northeast along the baroclinic zone, scattered showers will ride along the warm front.  Each night, there exists a potential for non-severe nocturnal thunderstorms in the RRV and east near the intersection of the warm front and a weak 850mb jet extending up into Southern Manitoba.  Most of the energy associated with the 850mb jet will remain in the states, so I don’t think that any severe weather will be on tap.

Get out and enjoy tomorrow, Sunday through Wednesday will be cooler (think low 20’s for highs) and rather unsettled, with mainly cloudy skies with occasional sunny breaks and showers lurking around during the days, with areas of organized precipitation occurring overnight. on Sunday night and Monday night.

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.