In Brief:

Mild quiet weather moves in Friday and Saturday with temperatures quite a bit above normal. Then- much uncertainty awaits us on Sunday. There is tremendous disagreement on the path of the upper level and surface lows Sunday with some models giving Longmont a coating and other models giving us over 8 inches of snow. Some snow chances linger on through Tuesday then we warm up again for quite a while.

Update 2/21 2:30pm:

The models are coming into agreement a little bit. Everything is pointing towards more snow Sunday early morning through Sunday afternoon. The weatherunderground.com model is giving us 1-2 inches now. The GFS has increased Longmont’s snow total to 3-4 inches of snow (Figure 1 update). The NAM (Figure 2 update) gives us 4-6 inches while the GEM (Figure 3 update) gives us 4-5 inches of snow.

Notice that the NAM is predicting the opposite snow signal east and southeast of Denver from the GFS or GEM. Limon gets a near nothing or a lot of snow – still. Hopefully this will come into focus better in another 12 hours.

Figure 1 update: 10:1 (snow to liquid) snowfall totals through Monday noon from the GFS and weather5280com for Colorado made Friday afternoon.
Figure 2 update: 10:1 (snow to liquid) snowfall totals through Monday noon from the NAM and weather5280com for Colorado made Friday afternoon.
Figure 3 update: 10:1 (snow to liquid) snowfall totals through Monday noon from the GEM and weather5280com for Colorado made Friday afternoon.

End Update 2/21 2:30pm.

Forecast Discussion:

High pressure is here (Figure 1). Rejoice in the 50’sF again (both the decade, if you like, and the temperatures). Of course, change is on the way. You can see a low pressure center off in the Pacific ocean moving our way (a big red “L” in Figure 2). This low is also beginning to pump moisture up into the desert southwest. That system can be seen in Figure 1 highlighted with the pink arrow. By Saturday morning, it is coming on shore bring rain and thunderstorms to Southern California and Arizona (pink arrow again in Figure 3).

The Longer Range Forecast:

Our next storm (this same low) is a big mystery at this time. As the models go, we have a model fight underway! The extent of the storm seems to stretch from 2am Sunday to 2pm that same day. There are some lesser snow chances on to Tuesday, but we’ll not worry about those quite yet (Figure 4).

The problem with the next forecast involves the path of this southwestern U.S. storm. The upper level low is located right around Las Vegas on the GFS Saturday night (Figure 5). Then it’s right over the Oklahoma panhandle Sunday night (Figure 6). With cold air to the north, that circulation will pump moisture up slope into eastern Colorado. The exact path (it did look much further south of the current track a few days ago) will determine who gets how much snow.

Snowfall Roundup:

The weatherundeground model (Figure 4) gives us a coating to less than 1/2 inch (not worth printing out).
The GFS (Figure 7) gives us a coating to an inch. Notice the bullseye out near Burlington but only a dusting of snow at Limon.
The NAM (Figure 8) brings the low a bit further north (and a bit slower) and buries Burlington to Limon back to Denver with 1 to 2 feet of snow along the interstate (that will be closed Sunday IF this is occurs). Longmont receives an impressive 5-6 inches.
Hoping for a tie-breaker, the GEM (Figure 9) also puts a massive foot or two along I-70 and gives Longmont a still notable 3-5 inches.

I’ll issue updates through the weekend as this gets closer. Cut-off lows like this can ‘do their own thing’ since they are not connected directly to the jet stream flow. Anything is still possible as of Thursday PM.

Going further out into next week, our previous discussion had northwest flow setting up over the state making us quite cold and unsettled next week. IF the GFS is right, the trough next week will park further east putting us more under a warming ridge and out of the storm track. We might have warm dry conditions. To quote Yoda (not the baby version): “Always in motion is the future.”

Oh, and a shout out to Johnathan and his wife – readers here in Longmont! Hey!

Figure 1: The forecast surface map for Friday noon from NCEP.
Figure 2: the water vapor satellite image (browns/reds are dry air, whites and light grey is moist air, purple/blue is ice and high cloud tops). From the the weather channel from Thursday PM.
Figure 3: The forecast surface map for Saturday noon from NCEP.
Figure 4: the graphical forecast for the next 10 days for Longmont, CO from weatherunderground.com
Figure 5: The 500mb forecast upper air analysis for early Saturday PM. Pink dot is Longmont. Red lines are troughs, blue lines are ridges.
Figure 6: The 500mb forecast upper air analysis for early Sunday PM. Pink dot is Longmont. Red lines are troughs, blue lines are ridges.
Figure 7: 10:1 (snow to liquid) snowfall totals through Monday noon from the GFS and weather5280com for Colorado made Monday afternoon.
Figure 8: 10:1 (snow to liquid) snowfall totals through Monday noon from the NAM and weather5280com for Colorado made Monday afternoon.
Figure 9: 10:1 (snow to liquid) snowfall totals through Monday noon from the GEM and weather5280com for Colorado made Monday afternoon.
Figure 10: The 500mb forecast upper air analysis for early Thursday AM. Pink dot is Longmont. Red lines are troughs, blue lines are ridges.
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John Ensworth works from Longmont as the Principle Investigator for the NASA Science Mission Directorate Earth and space science education product review through the IGES (The Institute for Global Environmental Strategies – www.strategies.org) . He is in his 14th year running this review. He is an astronomer (from the 2nd grade onward) and became a meteorologist (in the 5th grade) when a thunderstorm in Arizona rained on his telescope when the weather service had only forecasted a 10% chance of rain. He has college degrees in physics and astronomy and climatology and a graduate degree in meteorology and earth science. He lectures at the Little Thompson Observatory in Berthoud, the Estes Park Memorial Observatory in Estes Park, and for a number of online universities. He built and runs a backyard observatory near Pace and 17th in northeast Longmont where he has lived for 8 years with his wife, daughter, son, and two cats. Invitations to open house nights at this observatory, LTO, and EPMO will be posted with future discussions when they are scheduled. Forecasting severe weather and snow amounts via text lead to this column. He began texting friends about the weather right after the September 2013 flood. The readers of this column will, hopefully, keep him honest in what he ‘thought’ he had forecasted for ‘the most recent’ storm.

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