Dry Spell to Break with Showers Tonight

The stretch of dry weather Winnipeg has been experiencing over the past month should come to an end tonight as a significant low crosses across the Central Prairies.

Analysis of 20:45Z Satellite Imagery for August 7, 2011

A system that brought all sorts of severe weather to Saskatchewan yesterday evening, including a potential tornado touchdown east of Regina, is pushing into Manitoba tonight and bringing with it the chance of severe weather for Southwestern Manitoba and much-needed rain to the Northern Red River Valley.

After battering residents of Saskatchewan from the Alberta border all the way to the southeast corner of the province, the main driving force for the entire system began to slowly move eastwards last night.  As it pushes through Saskatchewan, the favourable conditions that existed through much of Saskatchewan have been pushed eastwards to eastern SK and much of southwestern Manitoba.

Watches in effect for Manitoba at 4:50PM local time.

Weather watches are out for severe thunderstorms through much of southwest and Parkland Manitoba.  Thunderstorms will continue to develop into the evening and as the low pressure system and associated trough line move into the area, many have the potential to become severe with the main threats being large hail through Minnedosa and Dauphin, and the slight chance of a tornado in addition to heavy rains and large hail through southwest Manitoba.

6hr. QPF from the 12Z Aug 7 run of the GFS model valid for 09Z Aug 8 (~4AM local)

Winnipeg and the RRV will have a slight chance of some showers this evening (with the off chance of a lightning strike or two), however the extremely dry low-levels, with dew points only around 11°C, will make it unlikely that any cells that move into our area will actually be able produce rain that makes it to the surface.  However, by 9 or 10PM tonight, the low and trough begin to enter the RRV and a large area of rain with isolated embedded thunderstorms will develop, resulting in a large north-south band of rain that will pass over the RRV tonight.  Most areas will see 5-10mm of the wet stuff with a slight chance of 15-20mm in areas that see a thunderstorm tonight.  I think that the risk for severe weather will stay in the southwest portion of the province today; overnight we should just see rain and a slight chance of a light show.

Monday will be one of the most significantly different days we’ve had over the past month.  On the back side of this low, cooler air dives down from the northern Prairies.  Much of the Southern Prairies will see highs of only 18-22°C with brisk northwest winds gusting to 50km/h, which will make it feel downright cool relative to the last couple weeks.  Here in the Red River Valley, we also have a good chance of showers moving down from the interlake into the area by the early afternoon.

After our damp, windy, cool Monday, things improve through Tuesday and we’ll be back into the high 20’s by Wednesday.  More rain may be in store for the later half of the week, but more on that later.

I, for one, welcome a nice rainy night.  So does my lawn…

Winnipeg to Set Record for Driest July on Record

Sunshine and sweltering heat are the name of the game this summer in Winnipeg.  Barring any precipitation before midnight tonight, Winnipeg will set a new record for the driest July.

An official map showing a 30-day total departure from average precipitation valid from June 29, 2011 to July 28, 2011.  The dark brown areas through Winnipeg and the Whiteshell highlight the driest areas over the past month.  Areas north and south of Winnipeg have seen slightly more precipitation, with near-normal amounts through the interlake.  Central Alberta has been struggling with well above-average precipitation over this summer.

The previous record was set in July of 2006, when a mere 10.5mm of rain fell through the month.  So far this year, Winnipeg has seen but a scant 10.0mm of rain, although at least 0.5mm of that which has been recorded was a spurious observation of rain that didn’t actually fall.  In reality, the Winnipeg Int’l Airport has probably seen only 9 – 9.5mm of rainfall this month.

That being said, the story isn’t the same for the entire Red River Valley.  Winnipeg has been the victim of splitting rain systems this month, resulting in only slightly below-normal precipitation through the southern Red River Valley and near-normal precipitation north of Winnipeg into the Interlake.

As for the beginning of August, it looks like there’s a very slight chance of showers or thunderstorms Monday evening, but it looks very conditional and if anything gets going in North Dakota, I would say our chances drop to pretty much zero for getting any rain.  After that…more sun.  So if you like the sun and heat, enjoy!

Chance of Showers in Winnipeg Tuesday Evening

A developing low pressure system in Montana is set to move eastwards overnight bringing rain and thunderstorms to much of Saskatchewan and Western Manitoba.  Complex dynamics come into play as it moves into the Red River Valley, resulting in some uncertainty to exactly what Winnipeg and the Red River Valley will see come tomorrow evening.

A large area of rain and thunderstorms will develop north of a low pressure system tracking across the Northern Plains of the United States stretching northward to a parent low pressure system anchored to a strong upper feature moving across Central Alberta into Central Saskatchewan.  This area of precipitation will track into Southwestern Manitoba by Tuesday morning.

12Z 26 July GEMREG 12-hr QPF valid 12Z 27 July

By tomorrow morning, these two discrete but connected systems will begin to change paths.  The low moving across the Northern Plains will continue to move due east, skirting along the International Border as it moves towards the Great Lakes.  The low moving into Saskatchewan will become captured by the upper feature and will quickly halt it’s eastward progression and slowly move northwards near the AB/SK border.  Connecting these two features is a large trough line that will disconnect from the US low and swing northwards into the Interlake region of Manitoba.

12Z 26 July GEMREG 12-hr QPF valid 00Z 27 July

The above image shows the total precipitation by 00Z Tuesday evening.  The disconnect of the two features is somewhat evident on this image; the bullseye of higher precipitation amounts on the AB/SK border is the enhanced precipitation associated with the slow-moving captured low pressure system.  The nose of precipitation pushing into Northern Minnesota with the arm extending south through the Dakotas is associated with the quicker-moving US low.  And lastly, the higher amounts through the Dauphin/Minnedosa regions is due to the trough line rotating northwards.

The challenge in this situation is how much precipitation will move through the Red River Valley.  As these features move apart, the lift that generates precipitation will dramatically weaken, and we could see a wall of rain to our west quickly fall apart before it reaches Winnipeg or the Eastern RRV.  If we’re able to get some sunshine, we may get enough heating to help produce convection to enhance the rainfall as it moves our way.  Lastly, slight variations in the track of either low could also dramatically alter where the precipitation falls.

Unfortunately, for an area on pace to set a record for it’s driest July on record, it isn’t inconceivable that a solid line of rain entering the Red River Valley won’t quickly split, and have part of the rain head northeast into the interlake and another part skirt along the International Border, leaving Winnipeg and many other areas in the RRV cloudy with nary a drop of rain.

It’s a difficult situation to forecast, and ultimately, it’s too early to have any certainty with where these systems will track as none of them actually exist quite yet.  One thing for certain: this is the kind of system that we’ll be able to forecast what will happen quite a bit better by tomorrow morning!  If the rain does move through the whole RRV, we may see a much-needed 5-10mm of rain, with a slight chance of local amounts near an inch if some embedded thunderstorms are able to get going.

That’s what I’m hoping for.  Although the water table remains quite high in Southern Manitoba, surface soil moisture is now extremely low.  This, coupled with  the late start to the growing season, has resulted in significant stress on crops that haven’t had the time to develop deeper roots yet.  For many farmers in S. MB, a half-inch or inch of rain would be quite welcome!

Rain Returns to the Red River Valley

While July has been a wonderful summer month (perhaps a little too wonderful at times), my lawn and my wallet are both ready for some cooler weather and some rain.  The cooler weather certainly has arrived; was anyone outside yesterday?  Even with the wind it was wonderful!  And, fortunately for my tomato plants, it looks like some rain is on the way as well.  Read on to find out the details!

16:15Z Daytime Multispectral Composite Satellite Image

The upper ridge that brought near-record Humidex values to Southern Manitoba earlier this week has finally moved off towards the east, which has allowed the upper trough situated along the B.C. Coast to begin moving inland.  As it migrates eastwards, it will spawn two low pressure centres: one over Central AB and a second one that will move through Montana and along the North Dakota/South Dakota border.  This will result in a rather complicated setup come Saturday morning with a surface low pressure system anchored to the upper low over Central AB with a secondary surface low pressure system anchored to a shortwave rounding the upper trough across the ND/SD border, with an elongated surface trough connecting the two.

GEM-REG Fri. July 22 12Z Model Run valid Sat. July 23 @ 12Z; 3hr. QPF + MSLP

In short, this setup will bring with it two bouts of precipitation.  The first will arrive through the morning hours on Saturday.  It’s exact placement is still quite certain, as it will be produced by nocturnal convection along a 30kt southeasterly low-level jet.  As the evening progresses today, it will become quite a bit clearer as to where exactly the thunderstorms will track.  That being said, it’s quite likely that areas near the international border will see 10-15mm of rain tomorrow morning with a few embedded thunderstorms as this system passes by.  Currently, it looks as if it’s up in the air as to whether or not it will reach as far north as Winnipeg, however I suspect we’ll see a sprinkle or two tomorrow morning.  I certainly won’t dismiss the possibility of a good shower or thunderstorm here tomorrow morning with local amounts around an inch, though!

After that, the upper trough will begin to swing in during the afternoon, and that will bring with it a band of rain that will track through in the late afternoon and evening, bringing an additional 10-15mm to what has fallen already.

Total Accumulated Precip. from the 12Z GEMREG Fri. July 22 valid 12Z Sun. July 24 (Sunday Morning)

All in all, most of the RRV should see 0.75-1.25 inches of precipitation tomorrow distributed between two distinct rain events: relatively early in the morning and late in the afternoon.  Cloudy skies will likely dominate between the two events, limiting our daytime high to only about 20°C.

After this system clears out tomorrow evening, we should have a few nice sunny days with highs in the mid-twenties.  The next chance for rain looks like Tuesday evening, when a weak system moving through brings the slight chance of some late-day showers or thunderstorms.