In Brief:

The afternoon storm pattern rolls on. Tomorrow (Wednesday), more moisture will flow into the state. The thunderstorms that do form will be capable of dropping a lot of rainfall in a small area (over an inch, maybe even 2 inches in a few neighborhoods). Things calm down the next day with temperatures in the lower 80’s F (with a few afternoon thunderstorms) until a cool front moves in Saturday evening. Rain chances increase Saturday night and we cool to the 60’sF Sunday. Next week is more of the same with 70’s F for highs each day.

11:30am Update:

The better moisture seems to have only made it up into southern Colorado. The precipitable water values went from 0.47 inch to 0.64 inch this morning at Denver. Storm motion IS going to be slow, coming out of the northwest (280 degrees) at only about 2-4 mph. If you have a cell move over you, you will see local flooding possible. The first storms to form this morning are up near the Continental Divide and are not moving hardly at all.

The HRRR is not excited about giving anyone precipitation along I-25 north of central Denver (Figure 1 update). There is a Flash Flood Watch over many counties in southern Colorado including Pueblo, Trinidad, over to San Louis Peak and up to Cotopaxi, to name a few spots.

Figure 1 update: the HRRR model total accumulated percipitation forecast through 1am Thursday (15 hour forecast) from weather5280.com

End 11:30am Update.

Forecast Discussion:

The position of the upper level and surface lows tonight (Tuesday) and Wednesday will pump more moisture into the atmosphere over Colorado. A value I’ve not brought up since last year is precipitable water. It is a measure of moisture in the atmosphere that can be wrung out by rain making processes. Tuesday afternoon, it has climbed to 0.5″. It may be as high as 0.8 to 1.0″ of precipitable water Wednesday – which means that thunderstorms that form might drop locally flooding rain (an inch or two, but just over a few neighborhoods) Figure 1. Still, overall rain chances will be fairly low Wednesday – most won’t see much water at all (Figure 2).

The 48 hour GFS shows local spots up to 2 inches (Figure 3). Don’t put your money on that exact pattern, just the percentage cover and peak amounts.

The Longer Range Forecast:

Things stay as they are with warm 80’sF each day and afternoon thunderstorms in spots. A cool front moves in Saturday evening (blue line Figure 2). It is driven by a northern branch jet stream trough (red line north of the state in Figure 4). This will put a chill in the air Sunday and keep us very seasonable all next week.

Figure 1: The forecast surface map for Wednesday night from NCEP.
Figure 2: the graphical forecast for the next 10 days for Longmont, CO from weatherunderground.com
Figure 3: the forecast accumulated precipitation map from the GFS and weather5280.com for Colorado,over the next 2 days.
Figure 4: The 500mb forecast upper air analysis for Monday AM. Pink dot is Longmont. Red lines are troughs, blue lines are ridges.
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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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