Canadian Weather: Complex Montana Low/ Colorado Low Prairies - Canadian Weather

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Complex Montana Low/ Colorado Low Prairies Rate Topic: -----

#1 User offline   Frozen Tundra Icon

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Posted 18 March 2009 - 09:13 PM

Very active SW flow setting up for the prairies this weekend. Strong thermal gradient and copious moisture will set the stage for 'interesting' weather. In fact, the upcoming storm system looks very complex... GFS is having a hard time resolving the exact pressure and precip pattern.

The lead disturbance will eject thru Alberta and Saskatchewan on Sunday. In response to this upper forcing an inverted trough (previously depicted as a discrete Montana low) will set up at the surface over Saskatchewan. This will pump warm air over the eastern Prairies helping us melt more snow. Mid level forcing (frontogenesis) will generate a large area of deformation zone snow (cold 850 hPa temps) over Saskatchewan and eventually Central Manitoba.

Then the big player enters the stage... main upper wave generates Colorado type low which moves into central plains. Very impressive low level jet forms out ahead of it. Warm air and deep moisture (850 hPa dewpoints approaching 10 C!) surge north into SE Manitoba overnite Monday into Tuesday morning. Isentropic lifting of the unstable moist air, and mesoscale forcing (convergence from nose of LLJ) all target SE Manitoba. The result could well be severe elevated convection RRV/SE Manitoba.

Deformation snow band sets up over interlake and SW Manitoba in response to mid level frontogenesis etc. Looks like Winnipeg will be sitting with light drizzle and slightly above 0 C temps as main forcing stays off to NW...

In summary for Winnipeg/RRV... severe elevated storm potential overnite Monday with heavy rain and hail as main threats... followed by dry slot/light precip with significant snow band off to the northwest. Now watch that completely change as the next runs come in.

#2 User offline   Frozen Tundra Icon

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Posted 19 March 2009 - 07:36 PM

Details still remain sketchy but general idea is similar. Arctic front sneaks thru eastern prairies late Friday giving a cool, dry easterly flow on Saturday. Main moisture advection is diverted towards AB/SK. Montana low tracks over Saskatchewan with rain/snow band. This feature washes out as an inverted trough which drifts eastwards. Meanwhile, the main low ejects onto the central plains pulling deep moisture and instability northwards. Best threat for convection well be confined to the area of surface and 850 hPa warm fronts. On the other hand.. a steady snow band will be favored in areas NW of the low track. So what path will the low take? The GFS at least has shifted everything further east... so that Winnipeg is barely in the heavy snow band and convection remains well to the SE. Any other thoughts/comments at this still early stage?

#3 User offline   scottk Icon

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Posted 19 March 2009 - 07:44 PM

Personally I have absolutely no clue what will happen here. All I know is that a major storm system will emerge from the mountains early next week and nail someone on the Plains. Some model runs have shown 50 to 75mm of rain/snow in Manitoba and North Dakota, while other have shown a couple showers for the region. The most common solution shows 10 to 20mm of rain, followed by 10 to 20cm of snow. All the models seems to agree that the Friday-Sunday period will be warm, but after that it turns into chaos. This time of year it is extremely difficult to determine what will happen with these systems until they get much closer. When you embed strong convective activity within an already powerful cyclone it really screws things up.

#4 User offline   Frozen Tundra Icon

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Posted 20 March 2009 - 11:35 PM

Details starting to become a bit clearer. However there are two main sources of uncertainty and possible error as I see it.

First is the evolution of that Montana low/inverted trough... Models show very high QPF totals along MB/SK border. This is convection generated by low level jet (LLJ) advecting moisture and instability. Behind the low/trough, mid level forcing (700 hPa frontogenesis) could generate a heavy snow band somewhere over E Alberta overnite Saturday/ Sunday morning. The question becomes when and where this feature (Montana low/trough) washes out before the main Colorado low becomes dominant. This will have a big impact on those convective QPF totals especially in the Red River Valley (RRV).

The better, more defined LLJ will be aimed at Iowa and the central plains along the southern warm front (associated with main Colorado low before it ejects NE). There will likely be numerous MCCs forming in this region.

That brings us to the second problem.. the track of the main southern (Colorado) low. Latest run of GFS now more in line with GEMglo and ECMWF, supporting a warmer more NW track. The LLJ/instability associated with this feature will still target areas to the SE of Manitoba however. Of course if the low tracks a bit further east... SE Manitoba will be able to tap that elevated convection and QPF will be higher.

So all in all... several different scenarios could emerge. For what its worth, GEM has been consistently advertizing between 50 -75 mm of QPF for portions of the RRV... More comments?

#5 User offline   Frozen Tundra Icon

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Posted 21 March 2009 - 07:35 PM

Winter storm warnings out for northern and central AB/SK with a watch for N Manitoba. This is for northern Montana low as it treks out into the prairies. There will be a strong 750 hPa baroclinic zone (thermal boundary) supporting a heavy snow band from E central Alberta to N Manitoba.

Models going crazy with QPF and convection for Sunday nite/Monday morning for S Manitoba in warm advection part of the system. Although the LLJ will be a very impressive 65 kts... nose will be aimed at central/northern Manitoba. Will have to rely on embedded waves and broad isentropic lift to generate convection. I still suspect QPF may be a bit too high.

Details still to be worked out for second, southern part of the system...

#6 User offline   scottk Icon

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Posted 22 March 2009 - 04:41 AM

This storm has certainly become more clear over the past couple days. The models have been fairly adamant in their prediction of 50mm of total QPF. I think this number is a good baseline, however with embedded convection a model estimate can end up meaning next to nothing.

Since the system will be so dynamic the models may really be struggling with it behind the scenes. The GEM continues to boast what I would call a completely unrealistic estimate of 50 to 100mm of QPF. The GEM often suffers from big-time exaggeration of weather systems, I think this is a prime example.

My prediction is for a general 30 to 50mm of precipitation, with more or less depending on the exact target of convective bands and cells (some areas will probably get close to 75mm, but most will not).

#7 User offline   Frozen Tundra Icon

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Posted 22 March 2009 - 05:49 PM

Things unfolding pretty much as discussed... deformation snow/freezing rain band sitting in an arch from Calgary thru Meadow Lake SK. Warm advection Tstorms over SW Manitoba.

Infact, the convection over central N Dakota and SW Manitoba is fairly impressive. Whole complex looks fairly progressive.. however the potential for cells building back into the LLJ will have to be monitored. This would lead to training of cells over the same areas (propagation vector and advection vector canceling each other out).

Nice clear slot forming over SE Manitoba... this could lead to further destabilization and will have to be watched.

LLJ shifts east tonite and RRV valley will be under the gun. 70 kts screaming right off the gulf.It should be able to tap 850 hPa dewpoints between 10 to 12 C over the central plains.. amazing. Surface warm front looks to stay south.. but with all that instability and lift from upper wave... another round of convection is likely. Precip amounts will be variable owing to the convective nature of the precip.

Still looks like a round of steady rain/snow/freezing precip in deformation zone of Colorado low late Monday perhaps thru Thursday. The further west one goes ... the more likely they will see significant snow and sooner it will change over from rain. Western RRV like Portage, Carmen, Morden may see large accumulations...

#8 User offline   Frozen Tundra Icon

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Posted 22 March 2009 - 06:45 PM

Minnedosa - Neepawa - Russell - Riding Mountain National Park
1:05 PM CDT Sunday 22 March 2009
Severe thunderstorm watch for
Minnedosa - Neepawa - Russell - Riding Mountain National Park issued

Risk of thunderstorms this afternoon giving wind gusts to 90 km/h.

This is an alert to the potential development of severe thunderstorms with large hail and damaging winds.

Monitor weather conditions..Listen for updated statements. If threatening weather approaches take immediate safety precautions.



A complex low pressure system centred in Montana will continue to push showers and thunderstorms through southwestern Manitoba this afternoon. There have been reports of pea size hail and gusts to 80 km/h this afternoon. These storms have the potential to give local gusts to 90 km/h.

*************************************************************************

heavy rain potential also significant threat... unofficial estimates of up to 35 mm already in that region. With frozen ground and snow cover present.. threat for overland (sheet water) flooding is high...

#9 User offline   Frozen Tundra Icon

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Posted 23 March 2009 - 08:49 PM

Areas of SW Manitoba received between 40 - 55 mm of rain courtesy of Tstorms forming ahead of the Montana low yesterday. The warm, moist advection from the LLJ spawned another bullseye of convective precip over SE N Dakota and WC Minnesota. Twenty-five to fifty mm of rain were recorded in those regions.

For the Canadian RRV... that warm moist air at 850 hPa was undercut by a cool, dry flow from the ESE lowering precip totals to 5 -10 mm. Now we have to focus on the Colorado Low as it moves up from the central plains...

Interesting precip pattern from GFS... not sure I am buying it entirely. Think best bet for heavy snow band to set up will western Dakotas SW Manitoba and possibly the interlake.

20 - 30 cm is not out of the question for upslope regions of riding mtn etc. Good 700 hPa baroclinicity present and below zero 850 hPa temps in these areas.

For RRV things get murky... at least as far as GFS is concerned. At first we seem to be in a 'no mans land' between the well defined deformation zone and warm advection/LLJ part of the storm.

Initially warm nose at 700 hPa is progged over Winnipeg with aforementioned deformation off to the NW. As band works east... a back flow of warm advection at 850 hPa occurs.

Consequently, the rain/snow band for would stay just west of Winnipeg until at least mid afternoon. I have a feeling it could even bisect the city... what a forecasting nightmare.

This is only from GFS. I am disappointed that EC does not release more parameters from GEM to the public. If NCEP/NOAA can do it.. it seems odd that the powers that be at MSC or EC would not follow that lead.

#10 User offline   Frozen Tundra Icon

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Posted 24 March 2009 - 03:41 AM

Nice rain band between Fargo and Grand Forks. Moving north at approx 30 kmh. I suspect it will reach Winnipeg by 3 am weakening to drizzle/ light rain.

RUC wants to bring thru 0 C isotherm at 850 hPa by 7 am just as lift and precip band re-intensify... If it verifies rain snow line may get hung up between Winnipeg and Steinbach...

Wildcard is the warmish/moist ground... snow will not stick at first, this complicates any guess of accumulation.

#11 User offline   Frozen Tundra Icon

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Posted 24 March 2009 - 05:03 PM

We got rain, ice pellets, and then snow here in Winnipeg with about 1-2 cm of sticky accumulation. For now deformation zone has backed off the NW. Models hinted all along that Winnipeg could end up in this stratus filled dry slot...

From looking at RUC, warmer 850 hPa temps start to back in from the east by late afternoon... surface temps likely will stay below zero with the northerly flow. I think potential is there for light freezing drizzle by rush hour.

Gem and GFS then hit us with another blob of precip overnite.. this would be snow. It seems to be associated with the 700 hPa low and what looks like decent frontogenetic forcing.

But here I am guessing... looking at the 700hPa thermal gradient is only a crude way of estimating forcing. If we had access to quantitative forecasting tools such as Q vector analysis... mid level forcing would be a lot easier to pick out.

Therefore forecasts (including EC) are necessarily more qualitative.

#12 User offline   Max Icon

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Posted 26 March 2009 - 02:35 AM

Sounds great, looking at the surface obs for Winnipeg looks like there was quite a bit of Snow over the past 24 hours.
Max

Montreal, QC

#13 User offline   Frozen Tundra Icon

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Posted 27 March 2009 - 01:28 AM

Yeah.. Winnipeg ended up with quite a bit of snow. Approximately 25 cm fell in total over the last two days... this comes after about 10-15 mm rain and 5 mm melted from ice pellets. The low became vertically stacked and stalled in NW Ontario with SE Manitoba in a persistent deformation band.

Today the clouds became very thin and the late March sun worked its magic. Model soundings for Winnipeg revealed a near surface adiabatic layer below a thin saturated zone to about 850 hPa. This instability was released in the form of convectively enhanced snow bands... heavy snow rates and gusting winds would briefly mix down.. it was an interesting day to say the least with quite a few nice sunny breaks.

#14 User offline   Frozen Tundra Icon

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Posted 27 March 2009 - 01:36 AM

Yeah.. Winnipeg ended up with quite a bit of snow. Approximately 25 cm fell in total over the last two days... this comes after about 10-15 mm rain and 5 mm melted from ice pellets. The low became vertically stacked and stalled in NW Ontario with SE Manitoba in a persistent deformation band.

Today the clouds became very thin and the late March sun worked its magic. Model soundings for Winnipeg revealed a near surface adiabatic layer below a thin saturated zone to about 850 hPa. This instability was released in the form of convectively enhanced snow bands... heavy snow rates and gusting winds would briefly mix down.. it was an interesting day to say the least with quite a few nice sunny breaks.

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