In Brief:

Thursday started cloudy and cool with one last shot of relative warmth in the afternoon. An evening cold front slides down the state bringing a chance in the weather. Rain chances start low in the evening and increase through the the night into the morning Friday. Rain is likely late morning through early evening. Temperatures will slowly drop later Friday allowing rainfall to change to snow for a while later in the evening Friday. Longmont should see a coating to maybe 1 inch. Things clear out and warm for the weekend. Another chance of showers returns Monday followed by real spring like warmth.

Update 3/28 10am:

Snowfall happened, but not much in town (as expected). Figure 4 update. Boulder did better with 1-almost 3 inches. Enjoy the rapid warm up!

Figure 4 update : 24 hour snowfall reports through 7am Saturday in Boulder Co. from CoCoRaHS.

End 3/28 10am.

Update 3/27 3pm:

The forecast is largely still on track. Rainfall has been slow to develop along I-25 and has not yet begun in Longmont as of 3pm. The lingering showers may also last somewhat past sunrise Saturday. The timing of the best chances for precipitation stretch from 5pm to midnight Friday (Figure 1 update). The GFS gives us a coating to 2 inches now (a bit more snow than yesterday – Figure 2 update). The NAM backed off with only a coating to 1 inch building up (Figure 3 update). Enjoy a movie and a warm take-out meal this weekend!

End 3/27 3pm update.

Forecast Discussion:

A trough tilting off the West Coast of the U.S. (red line in Figure 1) is beginning to pump moisture into the middle of the country – right over Colorado (green arrow). We still have a ridge over our state on Thursday (Figure 2) but that trough is headed for us Friday (Figure 3). We are on the “uphill” side of the trough meaning that there will be lifting of the atmosphere. This matched with the moisture flow means rain.

Thursday noon, a cold front, pushed by this trough, is approaching the Four Corners region and pushing in from the northwest (Figure 4). By Friday morning, the front is past us with a deepening low in southeast Colorado (Figure 5). The deeper up-slope flow of moisture may hold off until Friday afternoon as is shown in this figure. The rain and mountain snow finally arrives Friday afternoon and evening with temperatures dropping low enough for some snow (Figure 6) in town. The better rain (later snow) chances run from 6am to 10pm Friday (Figure 8). We also cool to the 40’sF for a high temperature Friday and drop to the upper 20’sF by Saturday morning.

For rainfall amounts – the GFS is painting us with 0.25 to 0.5 inch of liquid. The weatherunderground (not in this run) has given us about 0.15 to 0.2 inches of water (Figure 10). For Friday nights snow, the GFS gives us a dusting to an inch (more on the west side of Longmont). Boulder might see 2-4 inches. Snow won’t be too far away (Figure 11). The NAM is a bit more excited about snow and gives us 1-1.5 inches (Figure 12). I’d like to err on the side of the trace to 1 inch region.

The Longer Range Forecast:

For the weekend, that trough is passing through the Midwestern states with a small ridge over our state warming us up quickly to the 50’sF (Figure 7). Another trough is right behind this ridge and will slide over the state Monday (Figure 7). The weatherunderground model gives us just a small chance of rain Monday afternoon (Figure 8) but the GFS ensemble is a bit more bullish on rain showers Sunday PM and Monday afternoon (Figure 9).

The long range, Tuesday and on, we get temperatures in the 60’sF with “mostly” dry conditions for a while. Enjoy when you go out!

Figure 1 update: a snippet of the graphical forecast for the next 2 days for Longmont, CO from weatherunderground.com
Figure 2 update: 10:1 (snow to liquid) snowfall totals through the next 48 hours from the GFS and weather5280com for Colorado made Thursday AM.
Figure 3 update: 10:1 (snow to liquid) snowfall totals through the next 48 hours from the NAM and weather5280com for Colorado made Thursday AM.
Figure 1: 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 NWS from Thursday AM.
Figure 2: The 500mb upper air analysis for early Thursday AM. Pink dot is Longmont. Red lines are troughs, blue lines are ridges.
Figure 3: The 500mb forecast upper air analysis for Monday AM. Pink dot is Longmont. Red lines are troughs, blue lines are ridges.
Figure 4: The forecast surface map for Thursday noon from NCEP.
Figure 5: The forecast surface map for Friday AM from NCEP.
Figure 6: The forecast surface map for Friday PM from NCEP.
Figure 7: The 500mb forecast upper air analysis for Saturday PM. Pink dot is Longmont. Red lines are troughs, blue lines are ridges.
Figure 8: the graphical forecast for the next 10 days for Longmont, CO from weatherunderground.com
Figure 9: The ensemble GFS 10 day weather graph from weather5280.com
Figure 10: precipitation totals through noon Saturday from the GFS and weather5280com for Colorado made Thursday AM.
Figure 11: 10:1 (snow to liquid) snowfall totals through Saturday noon from the GFS and weather5280com for Colorado made Thursday AM.
Figure 12: 10:1 (snow to liquid) snowfall totals through Saturday noon from the NAM and weather5280com for Colorado made Thursday AM.
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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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