Forecast Discussion:

We climbed into the upper 80’sF on Wednesday and will do about the same on Thursday/today.  Figure 1 is the surface map for this afternoon.  You can see a low pressure trough east of the mountains that may help trigger thunderstorms in the returning moisture (it will provide lift and a convergence zone at the surface).  Our weekend storm is visible out in Utah and Arizona with a front up in Montana.  They will come together in northern Colorado, but more on that below.

Figure 2 is the next 10 days forecast from We do see an afternoon elevation of rain chances today, but the real action doesn’t pick up until late morning or the lunch time hour Friday.

Figure 3 is the SPC severe storm outlook for this afternoon.  Longmont is right on the edge between dryness and a chance of a thunderstorm.  Just a handful of miles east, the severe risk is Marginal (1 on a scale of 1-5) and almost immediately further east: Slight (2 on a scale of 1-5).  The primary risk will be hail and high winds in a few supercells (thunderstorms with rotation and exceptional strength).


Figure 1: forecast surface analysis for Thursday PM from NCEP.
Figure 2: The 10 day graphical forecast for Longmont from Green boxes show patterns of better rain chances.
Figure 3: the day 2 SPC thunderstorm and severe weather forecast made Wednesday (valid Thursday).

The longer range forecast:

Returning to Figure 2, after an initial push of thunderstorms on Friday, a front sweeps south across the state and significant up-slope flow begins from roughly 6pm Friday to 6pm Saturday. Also note the big 20F drop in temperatures again as we pass into Saturday.

Figure 4 shows that with this stronger system moving in, we have a marginal (and are on the edge of a slight) risk of severe weather – hail and strong winds will be the main threat again. Could a weak tornado form though?  Always be alert.

The really interesting feature, that we’ve been watching for a few days, is the expected rainfall. The GFS had a bullseye of a LOT of rain around Longmont two days ago.  Yesterday, it moved it up into southern Wyoming, primarily.  But today, Figure 5 shows that it is back around Longmont, Boulder and just east of I-25 from Cheyenne down to the Palmer Divide.  Longmont would get over 3.5 inches of rainfall (by Monday noon) if this verifies.  Figure 2 shows just under an inch of rainfall.

Figure 6 is another model, the NAM, that only goes out to midnight Saturday into Sunday. It gives Longmont 2 to 2.5 inches of rain.  Right now, you get to pick 1, 2 or 3 inches as our next storm total.   Beyond that, the boxes in Figure 2 again show the afternoon thunderstorm chances that follow this storm for the next full week.

Figure 4: the day 3 SPC thunderstorm and severe weather forecast made Wednesday (valid Friday).
Figure 5: The total precipitation estimate between Wednesday PM and Monday May 21st at noon from the GFS and
Figure 6: The total precipitation estimate between Wednesday PM and Sat PM/Sun AM midnight from the NAM 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.

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