In Brief:

With peak heat peaking Friday afternoon, we cool a bit for Saturday then cool a lot (20F or so from Friday’s temperatures) to the 60’sF for a high on Sunday. A strong cold front Saturday afternoon will kick off rain and thundershowers Saturday afternoon and evening tapering off later Sunday. We remain very seasonable next week.

7am Update:

The forecast seems on track today with frontal passage (FROPA) around 3-4pm. Because of this slightly later passage, temperatures should make it to the lower 80’sF today. The high temperature Sunday looks to be 20F lower in the lower 60’sF. This will feel like a real chill.

With those high temperatures, the approaching trough and front, and the moisture flowing in- there is a chance of thunderstorms around frontal passage and immediately after it that might put down larger hail. The SPC has painted us with a Marginal Risk (1 on a scale of 1-5) of severe weather. The local forecasters are not mentioning severe weather – concerned that the atmosphere will stabilize too quickly (cool air moving in below warm). They don’t think much thunderstorm activity will occur.

Figure 1 update: The severe storm weather forecast for the U.S. from the Storm Predication Center in Norman, OK. Made Saturday for Saturday.

End 7am Update:

Forecast Discussion:

The first wind shift/cool front is coming through as this post goes up Friday afternoon. Some storms are beginning to slide across the I-25 line. It has hit 84F at my site in NE Longmont. The big front coming tomorrow (Saturday) sweeps through around 3pm Saturday (Figure 1 and Figure 2, blue line). There will be a chance of severe weather nearby (east of Denver and Longmont). That ‘may’ get painted further to the west if the front slows down on Saturday.

The Longer Range Forecast:

By Sunday morning, rainfall will continue (Figure 4) as the front pushes into Texas and central Kansas. This is being powered by a big trough sitting overhead Sunday AM (Figure 5). The placement of the depressed heights also supports the mid 60’sF temperatures that day.

Over the next 3 days, the GFS still gives us minimal rain totals, a tenth to a quarter inch (Figure 5) while the weatherunderground model gives us about a quarter inch as well.

We warm back to the 70’sF Monday – Wednesday then see low 80’sF with a chance of a wandering afternoon thunderstorm every day.

Figure 1: The forecast surface map for Saturday night from NCEP.
Figure 2: The severe storm weather forecast for the U.S. from the Storm Predication Center in Norman, OK. Made Friday for Saturday.
Figure 3: the graphical forecast for the next 10 days for Longmont, CO from weatherunderground.com
Figure 4: The forecast surface map for Sunday morning from NCEP.
Figure 5: The 500mb forecast upper air analysis for Friday AM. Pink dot is Longmont. Red lines are troughs, blue lines are ridges.
Figure 6: 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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