In Brief:

The high that is steering the airflow over the state jogs east today and gives us our best-yet flow of tropical moisture. Severe weather is possible today (Thursday) with hail and high winds being the biggest threat. After today, the high moves back to the west and we return to warm weather and daily/afternoon thunderstorm chances.

9am Update:

A Slight Risk (2 on a scale of 1-5) is now in place for the I-25 towns and areas eastward. A Marginal Risk (1 out of 5) is still in place for the Front Range foothills (about up to the Divide) . There is also a Flash Flood watch (conditions are right for locally flooding rains under heavier storms later today – this is not a warning, which means flooding is imminent or occurring) for the region from roughly around County Line Rt. 1 and areas eastward. Precipitatable water is deep (0.7 inch expected to approach 1 inch around Longmont later today) and the incoming shortwave is pretty strong for this time of year.

Storms should begin to form around 11am in the foothills and begin to impact the I-25 corridor around 1pm and continue until about 9pm. Keep an eye on your weather apps (my favorite on IOS are My Radar, RadarScope, Storm Shield, Dark Sky, RainAware currently).

Figure 1: The 9am screen capture of the radar and SPC thunderstorm risk map from MyRadar app from iOS.

End 9am Update.

Forecast Discussion:

The high on Wednesday was entering the Oklahoma and Texas panhandle with a very obvious moisture flow anti-cycling around it (Figure 1, bright green arrows). The GFS jogs this center over to central Oklahoma as a short wave moves in (the wiggle in the isoheight line at the tip of the left most green arrow – Figure 2). We’ll have cooler and very moist surface winds coming in from the east (upslope!) and moist air in the mid levels of the atmosphere flowing in from the southwest. With healthy daytime heating, severe weather is possible again.

The SPC has painted the northeast corner of Colorado with a Marginal Risk of severe weather – for hail and high winds – Thursday (Figure 3).

The GFS givs us 0.25-0.50 inch of rain (though thunderstorms can hit some areas with 1+ inch of water and miss other areas entirely – Figure 4). The weatherunderground.com model gives us 0.18 inch of water.

The Longer Range Forecast:

After this, the high wanders back to the Four Corners area and we get less moisture, but enough will be sitting around for daily afternoon storms. We warm a bit from Thursday’s low 80F temperature to hover around 90F for the next week.

With August beginning today – we can check on what the weather.com folks are thinking about the new month. Figure 6 is the temperature trend – they give us normal to maybe a bit below normal temperatures for the month as a whole. Figure 7 shows them expecting above normal precipitation for August. We’ll see!

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 Weather Channel from Wednesday.
Figure 2: The 500mb forecast upper air analysis for Thursday AM. Pink dot is Longmont. Red lines are troughs, blue lines are ridges.
Figure 2: The severe storm weather forecast for the U.S. from the Storm Predication Center in Norman, OK. Made Wednesday for Thursday.
Figure 4: the forecast accumulated precipitation map from the GFS and weather5280.com for Colorado, for the next 2 days.
Figure 5: the graphical forecast for the next 10 days for Longmont, CO from weatherunderground.com
Figure 6: The August temperature outlook for the U.S. from weather.com
Figure 7: The August precipitation outlook for the U.S. from weather.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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