7pm Monday Update:

A quick look at winter storm warnings, watches, blizzard warnings and the like setting up across the region. To emphasize – travel Wednesday afternoon and evening will be very difficult to hazardous if not, eventually impossible. Please begin to make your plans if this situation occurs. Prepare for power outages as well as winds increase and snow sticks to trees and branches.

Figure 1 update: watches and warnings issued by Monday PM for the Wednesday/Thursday storm.

End 7pm Update Monday.

Forecast Discussion: (with a watch issued, I’m posting early…)

First, from the National Weather Service:

Winter Storm Watch

Area included in alert: Central and South Weld County; Larimer County Below 6000 Feet/Northwest Weld County; North Douglas County Below 6000 Feet/Denver/West Adams and Arapahoe Counties/East Broomfield County

Alert brief: Winter Storm Watch issued March 11 at 3:24PM MDT expiring March 14 at 12:00AM MDT by NWS Denver CO

Heavy snow and very strong winds will develop over most of northeast Colorado Wednesday afternoon and Wednesday night. The most severe conditions are likely to develop just east of the I-25 Corridor where the heaviest snow is expected to fall. Power outages and wind damage will also be possible as winds gust to between 60 and 70 mph. Further west along the I-25 Urban Corridor, heavy snow and gusty winds will also be possible, with significant travel impacts. Rain is expected to develop Tuesday night, and then change to snow Wednesday morning and may become heavy at times. Mountain areas will also see more accumulating snowfall during this period. A powerful storm system will move into southeast Colorado or southwest Kansas by Wednesday morning, and then track slowly northeast into southwest Nebraska by Wednesday night. Meanwhile surface low pressure will intensify rapidly over southeast Colorado, producing the very strong winds and heavy snowfall. Stay tuned to the National Weather Service for the latest Watches and Warnings.

* WHAT…Heavy snow possible. Total snow accumulations of 5 to 9 inches possible. Winds could gust as high as 50 mph.
* WHERE…Greeley, Denver and Fort Collins.
* WHEN…From Wednesday afternoon through Wednesday evening.
* ADDITIONAL DETAILS…Travel could become very difficult by mid day Wednesday as rain changes to heavy snow. Strong winds could cause tree damage with scattered power outages.

Before going further – let’s look at the formal definition of a Blizzard: Officially, the National Weather Service defines a blizzard as a storm which contains large amounts of snow OR blowing snow, with winds in excess of 35 mph and visibilities of less than 1/4 mile for an extended period of time (at least 3 hours).

From the text of the watch above and the definition of a blizzard, places along I-25 (and east to Kansas and beyond) will meet blizzard conditions at times Wednesday afternoon and evening. Our storm is swirling over southern California, northern Baja (red L in Figure 1). It is pumping moisture up into the southwestern and south-central U.S.

The timing, seen in Figure 2, has rain showers increasing overnight Tuesday into Wednesday AM. Sometime Wednesday morning, before noon, it will change to snow. Snow rates and winds will pick up very rapidly.

Let’s page through the storm in 6 hour blocks. From midnight to 6am Wednesday (Figure 3) we have rain on the low lands and snow hitting the mountains (what’s new…?). Between 6am and noon Wednesday (Figure 4) the low beings to rapidly intensify in the SE corner or our state with very strong north – northeast winds driving moisture up slope to produce snow in the increasingly colder air. For the afternoon Wednesday (Figure 5) we are at about max storm impacts along I-25 and out on the plains with heavy snow falling just to the east of town. Winds Wednesday afternoon (Figure 6) are averaging over 25mph with areas in the 40 mph sustained in large regions to our east. Wow!

The storm begins to weaken after that and snow quits along I-25 as down slope flow ends the storm early for us (it will still be fairly windy and cold).

Model Snowfall Roundup:

The NWS gives the areas near us 5-9 inches of snow. (Models are showing a hint that the down slope hole is filling in some as the big day approaches. It might not be as strong as it has looked for a few days – meaning more snow for us).

The GFS (showing the down slope oval, in red) gives Longmont 3-4 inches (though there are foot totals east and west of us (Figure 8)). The total precipitation (water total – Figure 9) looks to be about 1/4th to 1/2 inch. Note an inch to 2 inches falls just to our east on the Plains.

The NAM gives us 3-5 inches (Figure 10).
The GEM gives us 2-4 inches (Figure 11).
The forecasters at weather5280.com give us 3-7 inches (the down slope oval is still present).

My personal take for Longmont is 3-6 inches with 1/2-3/4th inch of total water. I’ll be watching throughout Tuesday to see if the storm impacts inch any further west and will report it here, along with any watch or warning updates for areas near Longmont.

The longer range forecast:

We warm up and dry out after this monster storm slides to the east. We are back to about normal temperatures by Saturday (53F is normal). All the excitement is up front in this forecast discussion.

Figure 1: The water vapor satellite image from Monday afternoon from NOAA. Longmont is the pink dot. The green arrow shows the flow of subtropical moisture into the west.
Figure 2: the graphical forecast for the next 10 days for Longmont, CO from weatherunderground.com
Figure 3: the 6 hour weather conditions and surface isobar/pressure pattern Wednesday 12am -6am.
Figure 4: the 6 hour weather conditions and surface isobar/pressure pattern Wednesday 6am – 12pm.
Figure 5: the 6 hour weather conditions and surface isobar/pressure pattern Wednesday 12pm -6pm.
Figure 6: the 6 hour wind conditions and surface isobar/pressure pattern Wednesday 12pm -6pm.
Figure 7: the 6 hour weather conditions and surface isobar/pressure pattern Thursday 12am -6am.
Figure 8: the forecast accumulated snow map from the GFS and weather5280.com for Colorado, through late Thursday night.
Figure 9: the forecast accumulated precipitation (includes rain and snow water equivalent) map from the GFS and weather5280.com for Colorado, through late Thursday night.
Figure 10: the forecast accumulated snow map from the NAM and tropicaltidbits.com for Colorado, through Thursday night.
Figure 11: the forecast accumulated snow map from the GEM and tropicaltidbits.com for Colorado, through Thursday night.
Figure 12: the hand-drawn snowfall forecast for the Tuesday-Thursday snowstorm from 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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