In Brief:

Warm dry weather continues Saturday and most of Sunday. Some afternoon thundershowers are possible coming off the mountains. A front approaches Monday just after noon and thunderstorm chances increase. Temperatures drop about 20F into Tuesday with rain showers through Wednesday AM.

10:30am Update:

If you noticed this morning, there were low clouds and a lot of moisture around the area. A “Denver cyclone” – a counterclockwise rotation of the air caused by the wind directions on the Plains and the mountain and land forms around the Denver area, has tapped into some rich Spring-time moisture. With an approaching low and short wave troughs, the chance of severe weather has greatly elevated along I-25 (Figure 1 update).

The Storm Prediction Center in Norman, OK has placed Longmont and the I-25 corridor under a slight risk of severe weather (2 on a scale of 1-5). The strip of marginal risk (1 on a scale of 1-5) is really thin and only up a bit into the foothills to the west.

Storms should initiate around 2pm, give or take an hour or so (quoting the NWS). The risk is higher closer to Denver and to the southeast where the risk jumps up to Enhanced (3 on a scale of 1-5) and Moderate (4 on a scale of 1-5). The primary risk will include large hail and a few tornadoes. Watch out also for all the nasty tricks thunderstorms can put out (strong winds, lightning etc.). Watch your apps for severe thunderstorm watches and warnings as they are issued later today.

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

End 10:30am Update.

Forecast Discussion:

We are sitting under (relative) ridging aloft with a big trough out on the West coast (rain and mountain snows continue over the western states (Figure 1)). We have high pressure at the surface. Really nice weather rules Saturday through Monday AM. We’ll see afternoon clouds Saturday (Figure 2) and afternoon thundershowers Sunday afternoon (Figure 2 as well).

A trough will keep forming downstream of the Rockies out east of us initiating severe weather along a dry line out on the Colorado Plains. You can see the rain chances over the next two days in Figure 3. The excitement is confined to the most distant counties. The storm prediction center has a marginal and a slight risk out there with an enhanced risk just over our border with Kansas (Figure 4). If you are traveling east, for the long weekend, keep your eyes open.

The Longer Range (and very longer range) Forecast:

For Memorial Day and the BolderBoulder, we climb to 70F around noon after which the front arrives. Rain chances jump to 50/50 around 2pm and continue through early Wednesday AM. For near start time – 8am in Boulder (red line in Figure 2), temperatures will be in the mid 50’sF with increasing clouds and a very low chance of a shower.

The West Coast trough moves eastward to park over our state even into Wednesday evening (Figure 5). I don’t think we’ll see snow in Longmont, but the mountains will pick up more.

For fun, let’s look way out into June to see what the longer range indications are. Figure 6 are the sea surface temperature anomalies along the equator in the Pacific. The El Nino signal is strong with the reds and oranges (warmer than average water temperatures) up to the west coast of South America. Remember, the recent model runs (Figure 7) show that this should continue through the rest of the year.

We’ve seen a long trend of cooler than normal temperatures in the center of the the nation and the mountains of the West. That is predicted to continue next month (Figure 8). Longmont will see temperatures that are normal to a bit below normal on this forecast. The state is 99.99% drought free (the drought index statistics (https://droughtmonitor.unl.edu/ ) state that 0 people in Colorado are experiencing drought conditions). The long range forecast for June is for wetter than average conditions to continue (Figure 9).

Figure 1: The forecast surface map for Sunday night from NCEP.
Figure 2: the graphical forecast for the next 10 days for Longmont, CO from weatherunderground.com
Figure 3: The severe storm weather forecast for the U.S. from the Storm Predication Center in Norman, OK. Made Saturday for Sunday.
Figure 4: The 500mb forecast upper air analysis for Wednesday PM. Pink dot is Longmont. Red lines are troughs, blue lines are ridges.
Figure 5: The sea surface temperature (SST) anomalies (departures from normal) for the start of May from the NWS Climate Prediction Center.
Figure 6: The El Nino model ensemble showing most models agreeing that El Nino conditions will remain in effect through December/January/February (DJF) at the end of the year from the IRI/CPC.
Figure 7: the temperature forecast from NOAA and the Weather Channel for June 2019.
Figure 8: the precipitation forecast from NOAA and the Weather Channel for June 2019.
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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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