Forecast Discussion:

Update 9am:

The severe weather threat today has changed just a bit. Longmont is now under a Marginal Risk (1 on a scale of 1-5) with large hail being the most likely threat (Figure 1 update). *Just* on the east side of I-25, there is a Slight Risk with a 2% chance of tornado formation within 25 miles of a given spot (Figure 2 update). There is a 5% chance of large hail falling within 25 miles of most of Longmont (Figure 3 update). This jumps to a 15% (including a 10% chance of hail larger than 2 inches in diameter) just on the other side of I-25.  Storms should initiate around 1-2pm in the foothills and spread across I-25 in the early to mid-afternoon (earlier than recent storm events).

Figure 1 update: the day 1 SPC thunderstorm and severe weather forecast made Friday (valid Friday).
Figure 2 update: the day 1 SPC tornado forecast made Friday (valid Friday).
Figure 3 update: the day 1 SPC large hail forecast made Friday (valid Friday).

End 9am update.

Pretty simple today, with a trough to our west (the ridge is passing), we’ll have more lift over Colorado today – BUT- the convergence zone (or dry line) is expected to set up from Ft. Collins then down to the south southeast out on the Plains to our east (about 30 miles or so away to the east).  Figure 1 draws this as a yellow line.   Even if thunderstorms start in the foothills, they won’t really take off and get severe until they hit the deeper moisture to the east of that line.

Figure 2 shows the resultant marginal (1 on a scale of 1-5) and enhanced (2 on a scale of 1-5) risk of severe weather (hail and strong winds) just to our east and beyond.  IF things change, and the convergence line sets up further west, I’ll update this post in the morning.

Figure 1: The forecast surface map for this afternoon. From NCEP.
Figure 2 update: the day 2 SPC thunderstorm and severe weather forecast made Thursday (valid Friday).

The longer range forecast:

The fair chances of rain showers today are shown on the left edge of the bottom graph. That is followed by a dry Saturday and Sunday AM.  Sunday afternoon into Monday, we’ll have another cool trough effect the state (cool down in the blue circle – Figure 3) and multiple chances of storms/showers. Will severe weather come back again? We’ll look at this more tomorrow!

Figure 3: The 10 day temperature trend and precipitation (ensemble) from the GFS and weather5280.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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