In Brief:

A strong short wave and in-place moisture will interact to create a lot of thunderstorm activity today (Sunday) in the mountains that will merge into a couple of lines of occasionally severe storms this afternoon. After that, we have seasonable weather this week with mostly dry conditions.

9/9 11am update:

The storms performed as advertised dumping 1/10th to a quarter inch across our fair city (Figure 1 update). Storm reports so far (Figure 2 update) show most of the damaging wind reports came from the Western Slopes but there seem to have been few tornadoes out to our east. Wild times!

Figure 1 update: 24 hour rainfall reports through 7am Monday from CoCoRaHS.
Figure 2 update: the storm reports received by the SPC and the NWS through 7am Monday.

End 9/9 11am update.

Forecast Discussion:

There is a blob of notable instability approaching the state today (green circle over Utah and neighboring states – Figure 1). This is being created by a strong short wave coming in from the west (Figure 2). These ingredients have led the SPC to paint a bullseye of Marginal (1 on a scale of 1-5) and Slight (2 on a scale of 1-5) severe weather risk over the Front Range and NE portions of our state (Figure 3). The primary risk from the (forming) lines of nearly continuous thunderstorms will be strong winds, but some hail is possible as well.

Storms will fire around 12p-1pm in the mountains and begin to fill in hitting the urban corridor between 4-7pm (Figure 4) but a few storms will be possible all afternoon and in spots later in the evening.

The Longer Range Forecast:

We resume our gentle glide into Fall on Monday with some comfortable weather by mid-week. There still is a 90F in the distance (Figure 5).

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 Sunnday AM.
Figure 2: The 500mb current upper air analysis for Sunday AM. 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 Sunday for Sunday.
Figure 4: the forecast radar map from the GFS and weather5280.com for Colorado, for 5pm Sunday.
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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