In Brief:

We have near normal to slightly above normal temperatures through the weekend with daily chances of afternoon thunderstorms moving off the mountains (yes I’m sounding like a broken record). There is a cool down slated for Sunday.

8:30am Update:

The SPC has expanded the Marginal Risk (1 on a scale of 1-5 in severity) of severe weather to just include Longmont (Figure 2 update). Figure 3 update is a close-up that just happens to exclude Airport Road but does include Main St. in the Marginal Risk. That map is not intended to be scrutinized THAT closely – it is an artifact of how the larger graphic boundary is drawn, but I though it was funny.

The primary risk for the widely separated cells (less severe than yesterday in Colorado) will be for hail and strong gusty winds. Very few will experience these along I-25, but be aware.

Figure 2 update: The severe storm weather forecast for the U.S. from the Storm Predication Center in Norman, OK. Made Tuesday for Tuesday.
Figure 3 update: a closeup of Longmont in the IOS MyRadar app showing the same SPC severe weather outlook graphic overlay.

End 8:30am Update.

8pm Update:

The low chance of severe weather around Longmont Monday PM has diminished to zero. Enjoy your evening.

It is really interesting to see the line of thunderstorms stretching north south ahead of the west coast trough from central Mexico up to Canada ( purple blobs in Figure 1 update).

Figure 1 update: 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 Weather Channel.

End 8pm Update.

Forecast Discussion:

Every day will start clear, then cloud up in the afternoon as thundershowers move off the higher elevations to hit a few people with lightning and heavier rain (and small hail) – Figure 1. The risk that a thunderstorm or two gets stronger on Monday will be due to that west coast trough, that we’ve watched for a week or more, as it finally drifts by us down in New Mexico (Figure 2).

Longmont is just on the edge of a Marginal Risk of severe weather Monday afternoon (this is posted early – Figure 3). For Tuesday, that risk should remain further out east as the trough migrates a bit further east as well.

The Longer Range Forecast:

The trough cools us a bit Wednesday and Thursday (but not much, just to the upper 70’sF) then it is back to the 80’sF for the end of the week and weekend. Another trough approaches Sunday and we cool to the 60’s with a bit better chance of rain. Next week will be a bit cooler than average with those daily chances of afternoon showers.

Figure 1: The forecast surface map for Tuesday night from NCEP.
Figure 2: The 500mb forecast upper air analysis for Tuesday PM. Pink dot is Longmont. Red lines are troughs, blue lines are ridges.
Figure 3: The severe storm weather forecast for the U.S. from the Storm Predication Center in Norman, OK. Made Monday for Monday.
Figure 4: The severe storm weather forecast for the U.S. from the Storm Predication Center in Norman, OK. Made Monday for Tuesday.
Figure 5: the graphical forecast for the next 10 days for Longmont, CO from weatherunderground.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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