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In Brief:
A gigantic trough is moving in to the western us for the next 10 days and beyond. Here comes a real shot of winter weather. It gets ushered in with a very potent storm Monday night into Tuesday midday that will dump up to a foot of snow on the Front Range and I-25 corridor and significant snow over much of northeast Colorado. Details are in the discussion. Another major storm will hit the western half of the state from before Thanksgiving on into the weekend. With temperatures remaining below normal for the next 10 days (after this storm hits) the snow will stick around across the state. Travel is going to be difficult both sides of Thanksgiving and Thanksgiving Day depending on which direction you are going.
Update 11/24 8:30pm:
All is on track for this major winter storm. Even the 'outlier' - the European model - has come in line with the rest of the models having the storm track further south than it previously thought - dumping serious snow amounts on the Front Range and NE Colorado Plains. I-25 really "gets it" this time. Figure 1 udpate is the weather5280 forecaster's take on the storm. Longmont is in the 8-14" area with 1 to 3 feet just to our west. Wowzie. Channel 7 is (in my opinion) overdoing it with 1-2 feet from Thornton to Ft. Collins. Channel 9 is more matching the weather5280 folks and I like their take. This will be a crippling storm for road and air travel later Monday evening and Tuesday. You won't get far very quickly. Period.
End 11/24 8:30pm update.
Forecast Discussion:
A lot of people are going to travel this week. The weather will impact a lot of that travel. Let's start with the words of the National Weather Service:
Winter Storm Warning 1:40 PM Nov. 24, 2019 – National Weather Service
...WINTER STORM WARNING IN EFFECT FROM 8 PM MONDAY TO 5 PM MST TUESDAY... * WHAT...Heavy snow expected. Total snow accumulations of 6 to 12 inches with higher amounts possible under the heaviest snow bands. Winds gusting as high as 35 mph. * WHERE...Portions of east central and northeast Colorado. * WHEN...From 8 PM Monday to 5 PM MST Tuesday. * IMPACTS...Travel could be very difficult to nearly impossible across the area, especially by Tuesday morning. The hazardous conditions will impact the Tuesday morning commute. PRECAUTIONARY/PREPAREDNESS ACTIONS... If you must travel, keep an extra flashlight, food, and water in your vehicle in case of an emergency. The latest road conditions for the state you are calling from can be obtained by calling 5 1 1. (See Figure 0)
That says it all really - travel will be difficult to impossible roughly Denver north and up into the mountains Tuesday. The start of the trough's evolution is shown in Figure 1. This keeps getting reinforcing shortwaves and more cold air pumped into it through next weekend. In town, the snow begins and starts to come down hard around or just after sunset Monday and pounds us into Tuesday morning (Figure 5).
Snowfall Roundup:
The GFS (Figure 2) gives us 10-14 inches of snow.
The NAM (Figure 3) gives us 10-13 inches of snow.
The GEM (Figure 4) gives us 12-15 inches of snow.
This really looks like The Big One (for this month at least).
Since many are traveling, and timing of the snow over the next week will vary from place to place, I've put the time series graph of weather up for directions in all directions around us in the next series of figures. Remember, with this much snow, road conditions will probably be poor for the 6-12 hours following the snowfall.
In Longmont (Figure 5), conditions will be poor for driving from Monday early evening to Wednesday morning. New snow will make travel slower than normal Thursday afternoon to Saturday.
In Estes Park (Figure 6), conditions will be poor Monday midday to Wednesday morning and again Thanksgiving afternoon to Sunday morning.
In the southwest mountains (Figure 7), conditions will be poor Monday evening to the Sunday morning (all week!!).
Toward Grand Junction (way out west Figure 8), conditions won't be that bad but roads will be slick Monday night to Tuesday and Wednesday evening on into the weekend.
Going north to Cheyenne (Figure 9), roads will be impassable Monday afternoon into Tuesday PM. There will be some additional snow build up on Friday.
Going south through Colorado Springs (Figure 10), they will be (oddly!) less impact from these storms on Monday evening to Tuesday.
If you travel across Kansas (Figure 11), there will be Tuesday and Thanksgiving Day snows only amounting to a couple of inches each time.
The Longer Range forecast:
Adding up the snow over the next 10 days in Longmont (Figure 12) the GFS latest run gives us 14-18 inches of snow. Because of the massive trough taking over the western U.S. temperatures won't go far above freezing after Monday night. The snow will largely stick around - literally. Updates will come as needed and as I return to internet.