Forecast Discussion:

Even though we had a marginal risk (on the eastern side of Longmont, at least) – nothing happened around her for a second day.  Denver and south Denver got hit by hail and thunderstorms again.  For yesterday, though, Figure 1 shows that Longmont picked up just a few hundredths of an inch of rain on Monday.  Boulder didn’t do much better except down on their south side where 1/10th of an inch fell.

Figure 2 shows parts of Ft. Collins picked up 1/3rd to 1/2 inch of rain while other parts of town didn’t see any rain. Estes Park still only got a few hundredths.  Berthoud saw a sprinkle.

Figure 3 has Denver getting up to 1 and 4/10ths inch of rain in one spot, wide spread 1/2 inch amounts, and other places getting nearly nothing.  Oh the joys of thunderstorm rain coverage.

Today and tomorrow will see dry weather with temperatures near 80F. (There is always a chance of a lonely storm wandering off the mountains to get us…. but a small chance.)

Then things change for the weekend… see below…

Figure 1: the CoCoRaHS precipitation reports for Boulder county by 7am Tuesday (the prior 24 hours of accumulation).
Figure 2: the CoCoRaHS precipitation reports for Larimer county by 7am Tuesday (the prior 24 hours of accumulation).
Figure 3: the CoCoRaHS precipitation reports for the Denver Metro by 7am Tuesday (the prior 24 hours of accumulation).

The longer range forecast:

Our next system is a cutoff low that one can see in the water vapor satellite image in Figure 5 (red L off the California coast).  As that approaches, thunderstorm chances begin to return.  Figure 4 has us painted with a chance of thunderstorms on Thursday, but severe weather is just a few 10’s of miles off to our east and northeast. There is guidance that severe weather chances will be much higher around Longmont on Friday.

Figure 6 is a complex (messy) upper air forecast map for Thursday AM.  Our thunderstorm chances are increasing as that low (in figure 5) moves towards us (in figure 6).

Messy patterns are harder to forecast, so we’ll see a lot of variability in the forecast until the weekend gets closer. Figure 7 is the GFS precipitation total by Monday next week.  The super bullseye of rainfall that was near Longmont yesterday is now up around Ft. Collins and further up into Wyoming. We’ll see that hop around a bit each day until it gets a hold on reality. Still, Figure 7 puts Longmont on the edge of the 1 inch rainfall region.

More later!

Figure 4: the day 3 SPC thunderstorm and severe weather forecast made Tuesday (valid Thursday).


Figure 5: the water vapor satellite image live Tuesday PM (reds/browns are dry air, whites and grey (and pink) are moist air.
Figure 6: The 500mb upper air analysis forecast from the GFS for Thursday AM.
Figure 7: The total precipitation estimate between Tuesday PM and Monday May 21st at noon from the GFS and
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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 – . 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.


  1. New to the observer, fed up with low quality of t.c… Very surprised and pleased to see this level of wx reporting. Thank you!

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