Update 5/20 9:30am: The Forecast Discussion from the Cherrywood Observatory – May 18-20, 2020

Figure 3 update: the Wednesday convection and severe weather chances made Wednesday from the SPC in Norman, OK.

In Brief:

Hot dry weather (and elevated fire danger) will be the rule through Wednesday. There will be a slight chance of an isolated severe storm Tuesday and Wednesday afternoon (really isolated). A pattern change brings a return to seasonable weather Thursday into the weekend. Then things remain cool for the weekend with more rain (just in time for Memorial Day).

Update 5/20 9:30am:

A dry line (a convergence of dry/hot air from the west and moist cooler air from the east) will set up near I-25 today. It looks like this line will settle to the east of I-25 by the time an upper level trough arrives this afternoon to kick off the bigger storms (Figure 3 update). Some thunder may be heard west of I-25 but severe conditions are not expected here. It will be windy today with an elevated fire danger. A cold front will enter the state later today further enhancing storms out in the northeast corner of Colorado.

Note, we did break 90F yesterday (DIA hit a record 92F). The thunderstorms that did form yesterday kicked up a lot of wind in the evening with lowered visibilities in blowing dust. We might see strong winds near thunderstorm cells today as well.

Figure 3 update: the Wednesday convection and severe weather chances made Wednesday from the SPC in Norman, OK.

End 5/20 9:30am update.

Update 5/19 11am:

From the NWS in Boulder via Twitter

NWS Boulder@NWSBoulder Heads up! Confidence is increasing that dry microbursts will create very strong winds up to 60 mph late this afternoon and evening. Secure loose objects around your house and drive safely especially if you are in a high profile vehicle. #COwx


End 5/19 11am update.

Update 5/19 9am:

First, an apology, I just discovered yesterdays forecast discussion sat in the Draft state and didn’t go live until just now. Yikes! And there is already an update to it.

There is a Marginal chance (1 on a scale of 1-5) of severe weather this afternoon along and east of I-25. Storms should begin to fire in the mountains around 1-2pm. From the foothills to near I-25, gusty winds, lightning, locally heavy rain and pea sized hail will be seen in a few widely scattered thunderstorms from around 1-3pm. Near I-25 and out onto the eastern plains, stronger winds and larger hail is possible with the (still widely scattered) stronger storms around 3-5pm – Figure 1 update and Figure 2 update. Storms will hit more stable air further out on the plains and will die out this evening (after sunset). Most folks will only see a thunderstorm in the distance as they pass.

Figure 1 update: the Tuesday convection and severe weather chances made Tuesday from the SPC in Norman, OK.
Figure 2 update: the Tuesday damaging hail (and high winds in this case) chances made Tuesday from the SPC in Norman, OK.

End 5/19 9am update.

Forecast Discussion:

A very large ridge is parked over the state Monday-Wednesday taking temperatures very close to 90F (but will it actually break that heat ceiling? – Figure 1). You can see the airflow coming up out of the trough in the west and flowing up and over the ridge then back down under the low in Illinois (Figure 2). This pattern has rain and showers in the western states and in the Ohio Valley and southeastern U.S. but dry weather and mostly clear skies in the Mountain West (Figure 3).

I didn’t put the maps here, but the SPC (Storm Prediction Center) paints a Marginal and Slight Risk of severe weather just to our northeast Tuesday and Wednesday afternoon east of Ft. Collins and up into Wyoming. We might see a very isolated storm get strong somewhere around the northern I-25 corridor. There is a very low chance of one of those storms hitting Longmont (Figure 4). Watch for updates (if needed) over the next couple of days.

The Longer Range Forecast:

By Wednesday PM – that the western trough is finally moving eastward. We are on the uphill side of this trough for a while as cooler air moves in (Figure 5). By the weekend, a mean trough is dominating the western half of the U.S. (Figure 6). Because of this deepening trough and its ability to tap some subtropical motion; rain and thunderstorm chances really pick up starting Saturday night. Some thunderstorms (severe in spots?) will be in the area Sunday PM.

Memorial Day forecast:

We will have near normal temperatures Monday with a small chance of rain now and then throughout the day (Figure 4).

Figure 1: The 500mb upper air analysis for Monday AM. Pink dot is Longmont. Red lines are troughs, blue lines are ridges.
Figure 2: 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 NWS from Thursday PM. Blue lines are ridges, red lines are troughs (like the 500mb maps).
Figure 3: The forecast surface map for Tuesday AM from NCEP.
Figure 4: the graphical forecast for the next 10 days for Longmont, CO from weatherunderground.com
Figure 5: The 500mb forecast upper air analysis for Wednesday PM. Pink dot is Longmont. Red lines are troughs, blue lines are ridges.
Figure 6: The 500mb forecast upper air analysis for Saturday PM. 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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