An usually cold air mass and a plume of Pacific moisture combined to bring heavy snow to the mountains of northwestern Wyoming. The highest amounts were in Yellowstone Park where close to 2 feet fell in some of the higher elevations.
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The episode as NWS RIW wrote it up.
An usually cold air mass and a plume of Pacific moisture combined to bring heavy snow to the mountains of northwestern Wyoming. The highest amounts were in Yellowstone Park where close to 2 feet fell in some of the higher elevations. Over a foot of snow also fell in portions of the Tetons and Absarokas. At lower elevations, temperatures remained warm enough for much of the precipitation to fall as rain or wet snow that did not accumulate.
The local office's own words before the first event, as it sent them, from the Iowa Environmental Mesonet's archive of National Weather Service text.
NWS Riverton, WY wrote its last area forecast discussion before the first event at Sep 20, 2:06 PM MST, 7 hours 54 minutes earlier.
Area Forecast Discussion National Weather Service Riverton WY 306 PM MDT Wed Sep 20 2017 .SHORT TERM...(This evening through Thursday) Issued at 303 PM MDT Wed Sep 20 2017 Next storm system is developing to our west with the main circulation center over srn WA/far nrn OR early this afternoon with developing baroclinic stretching from nrn NV into swrn ID. Another cold and area of good cold air advection is also noted just off the Pacific Northwest coast. These two combined with the building upstream ridge will help with the development of a significant upper low (closed off) over swrn ID by late Thursday afternoon. As the ridge builds and folds over, the upper low moves very little into Friday before beginning to shear later Friday. Part of the energy shears off to the ne into MT while the srn portion lingers to our sw through the end of this period (and likely longer!). Baroclinic boundary begins impacting the west this evening and as the trough strengthens Thursday to our west, strong QG forcing will develop across the entire west with widespread rain and snow. The nrn piece of the jet may also provide some right entrance region support tonight in the nw. Snow levels are tricky but it looks like they have a good chance of changing to snow for a while in the Jackson Valley towards 12z with the potential for a couple inches of snow Thursday morning. Further south, it may just stay rain or a mixture in the Star Valley and Upper Green River Basin (maybe the higher spots could see some brief changeover). Next crew can continue to evaluate and see what the snow potential looks like, especially around the Jackson Valley. It's not out of the question, that a quick 1 to 4 inches falls between Jackson and the Moose/Moran area tomorrow morning. H7 temps only fall to -2 to -3C at h7 but significant pcpn could be falling at sunrise and that may be just enough to cool the lower levels enough for a changeover. Otherwise, snow levels will stay rather high in the Salt/Wyoming Range over to the south half of the Wind Rivers where snow levels may be up around 10K feet yet but lower to around pass level or a little lower towards Togwotee Pass. Another area which could see significant pcpn developing Thursday will be in our far nrn zones behind strengthening cold front along with increasing QG forcing Thursday into Thursday night and some jet streak support at times. It looks a little too warm yet for COD to changeover to snow Thursday morning but colder air filters in Thursday night. Strong QG forcing shifts into our nwrn quadrant Thursday night with the best pcpn likely occurring from YNP se to the northern Wind Rivers. A strong jet couplet may also develop in the nwrn quadrant. Heavy wet snow is likely during this period. The baroclinic boundary will also bring a swath of showers into the swrn and western part of Central Wyoming Thursday night. The ern sections will remain dry. As the low slowly edges ewd and the cold front becomes better established east of the divide Friday, the pcpn chances start to increase from west to east across areas east of the divide (srn sections as pcpn continues up north). It gets more tricky Friday night into Saturday as the trough shears apart. How much energy will remain in the southern part of the trough. The GFS and even moreso on the Euro, has a decent closed circulation with another bout of precipitation that develops Friday night into Saturday. The NAM only tries to wrap some moisture into our far ern zones on Saturday. I would think with the upstream ridge development there will be a decent srn low with a much better chance of pcpn east of the divide Friday night and Saturday. The GEM also supports this idea. Most of the precipitation in our central sections between Riverton/Lander and Casper are generally too warm for snow but areas above 6,000 to 6,500 feet could certainly seem some snow Friday night into Saturday. .LONG TERM...(Thursday night through Wednesday) Issued at 303 PM MDT Wed Sep 20 2017 By the middle of the weekend, the mean trough over the western US looks to be slightly positively tilted and just to the west of Wyoming, though the primary areas of lift and precipitation are to the east of the trough. The characteristics of the trough are still a bit sketchy this afternoon with all models splitting off a southern area, each with varying intensities. The Canadian Global is the most bullish with a southern stream cutoff, and the GFS and ECMWF are less so. As a result, Sunday should continue to have cool air over the area and a chance to slight chance of precipitation southeast of a line from Buffalo to Kemmerer. Rain is expected in the lower elevations with snow above about 8000 feet. The trough axis should move eastward right over Wyoming by late Sunday night shifting the focus of the precipitation into SE WY. Monday though should still see some mountain showers, particularly over the Bighorns, Wind Rivers, and Absarokas. Temperatures won't warm much on Monday with 700mb temps still around -3 to -5C. The split flow on Tuesday should result in clearing skies and slightly warmer temperatures with a few high elevations showers. 700mb temps should warm then on Wednesday to 1 to 3C as fold over ridging impacts the northern Rockies. Northern WY may see some slight showers due to a weak shortwave trough in NW flow, but the Canadian and ECMWF have this feature and the GFS does not. && .AVIATION...For the 00Z TAFS through 00Z Friday Issued at 303 PM MDT Wed Sep 20 2017 East of the Divide...KCOD/KCPR/KLND/KRIW/KWRL Routes Conditions will be VFR through the forecast period over the lower elevations. Areas of mvfr ceilings and visibility in snow showers will develop over the Absaroka and Wind River Mountains from 03z -06z. West of the Divide...KBPI/KJAC/KPNA/KRKS Routes Conditions will be VFR through 04z over the lower elevations. Areas of mvfr to ifr ceilings and visibility in snow showers will develop over the mountains around 04z becoming widespread through 12z. && .FIRE WEATHER... Issued AT 303 PM MDT Wed Sep 20 2017 Fuels at critical levels for most of the low elevation forecast zones east of the Divide and across southern Wyoming. (Please see the Fire Weather Forecast product for more information). A major storm system will begin to impact the region tonight with the beginning of a rather prolonged period of precipitation across western Wyoming with from 1 to 2 inches of liquid expected across the western mountains through Friday. Heavy areas of snow also expected at times in the mountains. Otherwise, winds are expected to remain quite gusty out of the west to southwest today as strong winds aloft transfer energy down near the SFC...gusting 30 to 55 mph at times over much of the forecast area...especially across the mountains. Minimum relative humidity today will generally range from near 20 across the low elevations of central Wyoming to as much as 70 percent in the mountains. Min RH's will be much higher Friday and Saturday. Smoke dispersion in the afternoons will be fair to excellent on Thursday...and fair to good Friday. && .RIW WATCHES/WARNINGS/ADVISORIES... Winter Weather Advisory from 9 PM this evening to noon MDT Friday for WYZ001-002-024. Winter Storm Warning from 9 PM this evening to noon MDT Friday for WYZ012-014-015. && SHORT TERM...Skrbac LONG TERM...McDonald AVIATION...Hulme FIRE WEATHER...Skrbac
Hazardous Weather Outlook National Weather Service Riverton WY 151 AM MDT Wed Sep 20 2017 WYZ001>020-022>030-211200- Yellowstone National Park-Absaroka Mountains-Cody Foothills- North Big Horn Basin-Southwest Big Horn Basin- Southeast Big Horn Basin-Owl Creek and Bridger Mountains- Bighorn Mountains West-Bighorn Mountains Southeast- Northeast Johnson County-Southeast Johnson County- Teton and Gros Ventre Mountains-Jackson Hole- Wind River Mountains West-Wind River Mountains East- Upper Wind River Basin-Wind River Basin-Lander Foothills- Green Mountains and Rattlesnake Range- Natrona County Lower Elevations-Casper Mountain-Star Valley- Salt River and Wyoming Ranges-Upper Green River Basin Foothills- Upper Green River Basin-South Lincoln County- Rock Springs and Green River-Flaming Gorge-East Sweetwater County- 151 AM MDT Wed Sep 20 2017 ...WINTER STORM WARNING FOR THE WIND RIVER, TETON AND GROS VENTRE MOUNTAINS TONIGHT THROUGH FRIDAY MORNING... ...WINTER WEATHER ADVISORY TONIGHT THROUGH FRIDAY MORNING FOR THE NORTHERN AND WESTERN MOUNTAINS... This Hazardous Weather Outlook is for Western and Central Wyoming. .DAY ONE...Today and Tonight Unsettled with a good chance of increasing mountain snowfall tonight down to 6500 feet. Significant mountain snowfall expected across the western, northwestern mountains. Please see the winter weather statement for more details. Windy across most of the forecast area with gusts as high as 45 mph over the lower elevations...55 or more mph across the higher ridges and passes in the central and northern mountains. .DAYS TWO THROUGH SEVEN...Thursday through Tuesday Thursday through Friday...Significant mountain snowfall continues over the western central and northern mountains. Rain may change to snow in the Cody Foothills early Friday. Friday night through Sunday night...Widespread mountain snowfall and valley rain at times especially along and east of the Divide. Snow could lower to valley floors at a few locations east of the Divide. .SPOTTER INFORMATION STATEMENT... Spotters are encouraged to report snowfall amounts. Thank you!
Special Weather Statement National Weather Service Riverton WY 941 PM MDT Wed Sep 20 2017 WYZ001-002-012-014-015-024-210800- Yellowstone National Park-Absaroka Mountains- Teton and Gros Ventre Mountains-Wind River Mountains West- Wind River Mountains East-Salt River and Wyoming Ranges- Including the cities of Lake, Mammoth, and Old Faithful 941 PM MDT Wed Sep 20 2017 ...Cool and very wet Pacific storm system headed into western Wyoming tonight... The strong Pacific storm headed towards western Wyoming will still bring copious amounts of precipitation with it. However, temperatures with this Pacific air mass will be much warmer than originally thought, keeping snow accumulation levels very high...at or above 10000 feet through Thursday night...with only slight chances for snow mixing with rain below this level. That said, anywhere from a half an inch and two inches of liquid precipitation can be expected to fall between tonight and Friday morning across the western mountains and valleys. Heaviest accumulating snow over the same time frame will range from 4 to 8 inches above 10000 feet...with locally higher amounts possible.
The national radar mosaic for the moment the first event began.
3 winter storm. Local standard time, as filed.
Heavy snow fell in the mountains of Yellowstone Park. Some of the highest amounts were 23 inches at Two Oceans Plateau and 21 inches at the Sylvan Lake SNOTEL.
The SNOTEL at Grand Targhee reported 15 inches of snow.
Heavy snow fell in the higher elevations of the Absarokas. The highest amount was 17 inches at the Blackwater SNOTEL site.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in Wyoming in 2017Every state in 2017This day in weather historyWarnings in Wyoming, September 2017
NOAA National Centers for Environmental Information, Storm Events Database, compiled from the National Weather Service’s monthly Storm Data. Complete through 2026-06; NCEI adds each month about 75 days after it ends and revises past years a few times a year. Rebuilt 2026-09-24. Each event’s entry at NCEI: 718550, 718552, 718551.