Storm Forecast for Wed-Fri - Update Tue5/17

Update Tue May 17

Models coming into better agreement on upcoming system, so confidence is growing. Using the NAM/WRF as primary guidance, upper level low moves into the Great Basin on Wednesday, and slowly moves east into the UT/CO border area Thursday, then northeast into the WY/NE/CO border area on Friday. A resulting surface low forms over the TX panhandle region Wednesday, and presists in this area through Friday. All of this leads to some wild and diverse weather for CO for the next 72 hours. Mountains will receive 6-12 inches of snow, foothills will receive a mix of rain, thunderstorms and snow, and Plains will receive rain and thunderstorms with some reaching severe limits on Wednesday.

Precip will move into western CO Wednesday morning, with precip moving east over the foothills and Plains by Wednesday afternoon. Precip could be heavy at times Wednesday afternoon into Wednesday night, with thunderstorms, hail and snow all possible. Snow level looks to begin around 10,000 ft Wednesday afternoon, lowering to 7500-8500 ft by Thursday morning, so foothill areas above 8500 ft could see several inches of snow by Thursday morning. Precip continues Thursday with periods of showers and thunderstorms. Foothill areas could be shrouded in fog as well with upslope flow. Snow level looks to remain between 8500-9500 ft on Thursday, so more heavy wet snow possible in higher foothill areas. As upper low moves farther north on Friday, precip should become more showery and isolated in nature, but brief heavy showers and thunderstorms still possible through Friday night.

All total, foothill areas looking at 0.50 to 1.0 inches of liquid equivalent for the 3 days, with areas above 9000 ft seeing 2-5 inches of snow. Weekend then looks warmer but with isolated afternoon and evening showers and isolated thunderstorms each day. Yet another upper low is forecats to track across CO on Monday and Tuesday, bringing more precip.

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Update Mon May 16

Current upper level ridge will bring warm temps and southerly winds over CO today, with isolated afternoon showers and thunderstorms, mostly over central mountains. Upper ridge will give way to a very large upper level low that will envelope most of the western US. Upper low is forecast to move into the Great Basin by Wednesday and then track slowly to the northeast through Friday. Models are currently not in very good agreement over the track of this system, with the GFS moving the low into northwest NE by late Thursday, while the NAM/WRF keeps the low over western CO by late Thu, and the ECMWF is somewhere inbetween. Result is that confidence in forecast details not very high, but the broad forecast of unsettled weather from Wednesday through Friday looks good. Depending on the track of the upper low, we could see anywhere from showers and isolated thunderstorms to steady rain and snow. Given the time of year, I would tend to favor the slower NAM/WRF solution which would mean higher precip amounts for the foothills and Plains. Snow level will also depend on track of upper low. Favoring the NAM/WRF solution snow level would begin above 10,000 ft Wednesday, but lower to around 7500-8500 ft Thursday morning and remain between 8000-9000 ft through Friday. NAM solution would bring around 1.0-1.5 inches of liquid equivalent, whereas the GFS would bring 0.25-0.50 inches for this system. Either way, we're in for more cool and wet weather, which will continue to help with the fire danger. Snow amounts above 8000 ft look to be anywhere from an inch or two with the GFS solution, to 2-6 inches with the NAM/WRF solution. I suspect models will converge in the next day or so and forecast confidence will raise.

Weekend in general looks warmer and drier with only isolated afternoon showers and thunderstorms. Long range models continue to suggest anotehr upper low will track across CO early next week bringing more precip Monday through Tuesday.

Bump for 5/17 update