In Brief:

Our warmest day of the week happens today (Friday) before a cool front later today, a cold front on Saturday, and a cool front on Sunday knock us down to temperatures in the 60’sF Sunday. We have a chance of severe thunderstorms later today (Friday) with large hail and damaging winds.

There is another good chance of rain Saturday night into Sunday morning then showers possible the rest of of that chilly Sunday. We return to normal temperatures for next week.

11am Update:

The SPC has removed the chance of tornadoes in Colorado today. Hail and damaging winds (and local flooding) – along with lightning are all still a threat where the stronger cells form and move.

End 11am Update.

9:30am Update:

With a lee trough (a downwind dip in atmospheric pressure caused by air flow over the Rockies – similar to water going over a rock in a stream and dipping below stream level on the downstream side), some easterly air flow at the surface bringing in moisture, and the hottest temperatures of the year (upper 80’sF in most places) will create some severe weather just west of I-25 and eastward (Marginal Risk (1 on a scale of 1-5) in Figure 1 update).

Figure 2 udpate states that there is a 2% chance of a tornado within 25 miles of any given point in the green shaded area. Figure 3 update states there is a 5% chance of large hail (1 inch in diameter or greater) within 25 miles of any given point in the brown shaded area.

Precipitable water will rise to almost 0.9″ so locally heavy rain and local flooding is possible. Storms should start to fire (non-severe) up in the mountains around 12pm then move into the I-25 corridor around 2-3pm. The upside is that storms should be fairly widely separated. Not everyone will get hit – many will see nothing.

Figure 1 update: The severe storm weather forecast for the U.S. from the Storm Predication Center in Norman, OK. Made Friday for Friday.
Figure 2 update: The tornado risk forecast for the U.S. from the Storm Predication Center in Norman, OK. Made Friday for Friday.
Figure 3 update: The large hail risk forecast for the U.S. from the Storm Predication Center in Norman, OK. Made Friday for Friday.

End 9:30am Update.

Forecast Discussion:

Let’s start looking back to yesterday and see what those isolated storms could do – and what they did, they did over Longmont (to bad HRRR). Figure 1 shows 3/4th of an inch up to 1.5 inches of rain in town with the electrically active thunderstorms that formed on a boundary along I-25 Wednesday evening. I lost an Ethernet switch to the zapping (it had been plugged into the wall next to the power strip with surge protection. Doh!)

Today we get some good heat with a ridge right overhead as the deep trough in the West approaches (Figure 2). Friday AM we will be about 5-8 degrees above normal (Figure 3). Mid to upper 80’sF will feel toasty out in the sun.

Our first wind shift and the start of the cooling arrives early afternoon today (Friday – first blue line in Figure 4). There is a fairly good chance of showers in the afternoon AND some of the storms that form along I-25 and eastward will become severe (Figure 6). The primary risk will be for large hail and damaging winds.

The Longer Range Forecast:

The main cold front arrives around 3pm Saturday (2nd blue line in Figure 4) and kick off more showers and storms into the night. Chances of rain remain somewhat high through Sunday with very cool temperatures that get another reinforcing shot of cool air in the afternoon (third blue line in Figure 4). The main rain will fall on the northeast plain early morning Sunday and late night Saturday (Figure 5).

The GFS gives us between 1/4th inch and 1/2 inch of water (Figure 7). The weatherunderground model (Figure 4) gives us about 1/4th inch of water.

Next week looks very normal, as early June weather goes.

Figure 1: total new rainfall totals for Wednesday/Thursday up to 7am for Boulder county from CoCoRaHS.
Figure 2: The 500mb forecast upper air analysis for Friday AM. Pink dot is Longmont. Red lines are troughs, blue lines are ridges.
Figure 3: The forecast temperature departure from normal map from the GFS and weather5280.com for Colorado, Friday AM.
Figure 4: the graphical forecast for the next 10 days for Longmont, CO from weatherunderground.com
Figure 5: A 6 hour snapshot of precipitation and type and surface pressure (black lines) ending Monday sunrise from the GFS and weather5280.com
Figure 6: The severe storm weather forecast for the U.S. from the Storm Predication Center in Norman, OK. Made Thursday for Friday.
Figure 7: the forecast accumulated precipitation map from the GFS and weather5280.com for Colorado,over the next 3 days.
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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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