In Brief:

We start the week very mild (above average about 10-12F) until fronts hit Tuesday night and Wednesday morning. Temperatures drop and snow will fall from Wednesday afternoon through most of Thursday into Friday early morning. Still, only a few inches if snow is expected to build up. Another storm is showing up on the horizon middle of next week.

11/18 8am update:

This is a quick check in with the GFS concerning the Wednesday/Thursday storm. The trend is away from as much snow in northern Colorado and the NE Plains with the best snow building up in the SW mountains of the state (Figure 1 update). We are still sitting in the, roughly 1-2, locally 3 inch area in our snowfall expectation. It will start as rain before temperatures drop low enough for snow. Thursday will be quite chilly but lows at night will only be in the 20’sF – this is pretty normal for this time of year.

Figure 1 update: The forecast snowfall totals for the next 5 days from the GFS and weather5280com for Colorado made Monday morning.

End 11/18 8am update.

Forecast Discussion:

We are high and dry except for some showers in the northern mountains (Figure 1). With a healthy ridge overhead (blue line in Figure 3) we are sitting about 10-12F above average Monday and Tuesday (Figure 2). It is a boring-weather start to the week.

The Longer Range Forecast:

But fun is coming. There is a west coast trough approaching in Figure 3 with a cut-off low pulling moisture into the desert southwest visible to our distant southwest. By Wednesday and Thursday that cutoff merges with the trough and the whole thing slows down giving us cold air, up slope flow, and moisture (Figure 4). Weather underground shows about 2 inches of snow falling from Wednesday evening (after some rain) through early Friday morning. The GFS gives us about 2-3 inches over this storm (Figure 5) while the NAM gives us about 2-4 inches (Figure 6).

Figure 1: The forecast surface map for Monday evening from NCEP.
Figure 2: the graphical forecast for the next 10 days for Longmont, CO from weatherunderground.com
Figure 3: The 500mb forecast upper air analysis for Tuesday morning. Pink dot is Longmont. Red lines are troughs, blue lines are ridges.
Figure 4: The 500mb forecast upper air analysis for Tuesday morning. Pink dot is Longmont. Red lines are troughs, blue lines are ridges.
Figure 5: The forecast snowfall totals for the next 5 days from the GFS and weather5280com for Colorado made Thursday morning.
Figure 6: The forecast snowfall totals through Thursday early morning from the NAM and tropicaltidbits.com for Colorado.
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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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