In Brief:

Thursday was one more quite warm day. A trough (in the upper atmosphere) kicked down a weak front in the afternoon cooling us a couple of degrees for Friday. Another front cools us about 8-10F Friday going into Saturday. There are small rain chances/thunder shower chances Thursday, Friday, and Saturday afternoon’s. Next week we hover around 80F (still above normal for these dates) with a better rain chance mid-late next week.

Forecast Discussion:

Taking a look at the at the current (Thursday) upper air map, a trough is moving into the western states (Figure 1). Since the wavelength is shorter than the nation is wide (about 3000 miles) – the dip in the jet stream flow will move eastward (it will be progressive). In this figure, you can see the low for tropical depression Imelda (lighter winds than a tropical storm) dumping rain in SE Texas under a big ridge (it is strange to see a ridge sticking out of a Low!).

By Friday morning, the next front is oozing in from the northwest. Thursday to Friday we’ll have pretty strong, mostly dry, southwest flow over the state. Imelda has drifted mainly northward in this time.

There are afternoon rain chances with the approaching trough (cool air aloft) and “some” moisture in the lower atmosphere Thursday, Friday and Saturday (with cooler moist post-frontal flow in place over NE Colorado – Figure 3 on Saturday).

The Friday/Saturday front will be the main cool down – it will be driven by the passage of the west coast trough (Figure 4).

The Longer Range Forecast:

The ridge to the west moves in Sunday/Monday warming us to the low 80’sF (Figure 4). The models are disagreeing on what will happen next week (Figure 3). The weatherunderground model has backed way off on mid-week rainfall, but the GFS has a cut off low headed for the four corners area (maybe a bit south) which could give southern portions of the state some nice rains (Figure 5). Over the next 10 days, the GFS (having removed the giant blob of rain near the Four Corners area, then the blob of rain along the Front Range foothills that we saw in recent discussions) now puts a swath of 1″+ rainfall across the southern counties (Figure 6). The GFS now puts down snow in the central and southern mountains over the next 10 days (Figure 7) and removes the foot+ we saw up north. It is a tough time for the models!

Note, the date for the average first freeze for Longmont is 9/27. On Figure 3, it is clear that is not going to happen this year.

Note also, the first day of astronomical Fall is almost here. It will occur Monday, September 23, 2019 at 1:50 am MDT. From Figure 3 – it will feel somewhat like Fall – as it should.

Figure 1: The 500mb current upper air analysis for Thursday AM. Pink dot is Longmont. Red lines are troughs, blue lines are ridges.
Figure 2: The forecast surface map for Friday AM from NCEP.
Figure 3: the graphical forecast for the next 10 days for Longmont, CO from weatherunderground.com
Figure 4: The 500mb forecast upper air analysis for Saturday PM. Pink dot is Longmont. Red lines are troughs, blue lines are ridges.
Figure 5: The 500mb forecast upper air analysis for Tuesday PM. Pink dot is Longmont. Red lines are troughs, blue lines are ridges.
Figure 6: The forecast rainfall totals for the next 10 days from the GFS and weather5280.com
Figure 7: The forecast snowfall totals for the next 10 days from the GFS and weather5280.com
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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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