In Brief:

A passing shortwave in the northern Rockies may spark isolated severe weather (hail and high winds) north of Denver today. We experience a cold front passage around 3-4pm today (Wednesday) and get to experience another taste of Fall Thursday. We warm to upper 80’sF for the weekend. There is a sign of a big cool down with showers coming to the state middle of next week.

12PM 9/12 Update:

We received 1/5th to 1/4th inch of water and repeated hits of hail on Wednesday (Figure 2 update). The largest hail stone in Longmont was reported at 1.5″ in size ( reported on Channel 7 news this morning -probably in north or northeast Longmont – Figure 3 update).

The Longmont storm reports are reproduced from the SPC here (Sorry about the strange table formatting – it won’t “fix.”):

Time Size LocationCountyStateLatLonComments
2237125 1 NW LONGMONT BOULDER CO401910512 . (BOU)
2234100 1 W
2230100 1 NNE
BOULDER CO401910511 . (BOU)
2228100 LONGMONT BOULDER CO401810511 . (BOU)
2227150 2 N LONGMONT BOULDER CO402110511 . (BOU)

Figure 2 update: 24 hour rainfall reports through 7am Thursday from CoCoRaHS.
Figure 3 update: the storm reports received by the SPC and the NWS through 7am Thursday.

End 12PM 9/12 Update.

8PM 9/11 Update:

There are still showers forming along a line that runs from Boulder to Longmont and off onto the Plains. Here, upper level support combined with the cold front to create a lot of pea sized hail (getting larger in the higher risk zones to the northeast – Figure 1 update). We drew the long straw on getting the phenomena of “training” set up, where storms form along a boundary and move like boxcars along with the upper air flow.

At least the hail (by the reports I’ve seen so far) remained small in town.

Figure 1 update: the RadarScope app from Wednesday afternoon.

End 8PM 9/11 Update.

Forecast Discussion:

One blob of instability is departing across the Great Lakes today as the next swings by northern Colorado today (Wednesday – Figure 1). The SPC has painted a tight gradient of risk across the state (Figure 2) that stretches from an Enhanced Risk (3 on a 1-5 scale) in the far northeast corner of the state to a Slight risk (2 out of 5) by Cheyenne, to a Marginal Risk (1 out of 5) around Longmont and north. There could be a few garden variety thunderstorms for northern parts of Denver but no convection near the Palmer Divide. Wow.

The primary risk Wednesday is for super cell formation. These strong rotating storms can create large hail and high winds. Tornado chances are confined mainly to the Slight and Enhanced risk zones.

Around Longmont, storm cells look to be quite isolated and zip by around 2-4pm (Figure 3). This is also the time the cold front, sliding down the Front Range, passes (Figure 4). Thursday will be the coolest day this week with mid 70’sF possible and temperatures about 5F below normal (Figure 5).

The Longer Range Forecast:

We dry out again Thursday through Sunday and sneak to almost 90F for highs each day (but not for many hours each day – Figure 4). There is a hint that next week, a series of cold fronts will cool us down and bring back rain chances. The coolest day is sitting way out at the 10 day mark with temperatures similar to this Thursday according to the model (Figure 4) but dramatically colder (20-25F below normal) according to the GFS (Figure 6). We’ll watch it here!

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 Wednesday AM.
Figure 2: The 8am screen capture of the radar and SPC thunderstorm risk map from MyRadar app from iOS.
Figure 3: The HRRR future radar for 3pm ET for Colorado from
Figure 4: the graphical forecast for the next 10 days for Longmont, CO from
Figure 5: the surface temperature anomaly (departure from normal) from the GFS for noon Thursday from
Figure 6: the surface temperature anomaly (departure from normal) from the GFS for noon Friday Sept 20 from
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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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