123 FXUS65 KPIH 210834 AFDPIH Area Forecast Discussion National Weather Service Pocatello ID 234 AM MDT Wed Oct 21 2020 .SHORT TERM...Today through Friday. A strong northwest flow aloft was evident in satellite imagery this morning as an embedded disturbance sheared southeast over the Canadian Rockies. This disturbance is expected to enhance the winds across the region today and thus a Wind Advisory remains in effect for a large portion of the Snake River Plain and Eastern Magic Valley. In addition to the wind, the disturbance is expected to bring a chance of valley rain and snow and mountain snow to the Montana and Wyoming border regions this afternoon and tonight. The disturbance exits east through Montana and northern Wyoming Thursday with a dry and colder northerly flow overspreading southeast Idaho in its wake. The next upstream storm system drops south out of Canada late Friday with winter-like impacts anticipated for the weekend (more on that in the LONG TERM section below). Huston .LONG TERM...Friday night through next Wednesday. An early season winter storm and invasion of VERY cold air remain on tap for SE Idaho this weekend, with potential travel impacts. A shield of precip, likely all snow, will begin to work into the Central Mntns/MT border area late Fri eve, intruding further into the nrn reaches of the Snake Plain, Upper Snake Highlands, Teton Valley, and Caribou Highlands by sunrise Sat as a trough of low pressure sinks south into the nrn Rockies. Widespread snow shower activity will overspread the entire CWA NW to SE Sat AM, likely mixing with rain at lower elevations near Hailey and from the Snake Plain southward as AM temps creep into the upper 30s/low 40s. We continue to undercut NBM temps by a couple degrees for highs as AM dew points will be below freezing, and the early start time to precip combined with melting-induced cooling in the column may work to keep temps a bit more stagnant than the models are anticipating. In any case, expect daytime highs to occur during the AM as a sharp cold front surges into the Central Mntns and over the MT divide before noon, with temps falling everywhere for the rest of the day. A late AM/early afternoon "handoff" should occur where the front pushes the heaviest precip out of the Central Mntns and focuses the activity across the Upper Snake Highlands and locales along and east of I-15, with everything continuing to transition SE and diminishing Sat night into Sun AM. The front looks to cross all of the Snake Plain by mid-afternoon, and the rest of the srn/ern highlands by eve. Strong winds are likely ahead of, along, and immediately behind the front, and a Wind Advisory may be needed if forecast trends hold. A previously-noted trend in the long-term guidance toward an earlier precip start time/earlier frontal passage seems to have stabilized. We continue to see notable differences in placement of heaviest QPF between the GFS/ECMWF, and poor consensus to ensemble clustering, keeping our confidence in the QPF forecast a bit low. With the NBM living in a wildly unreasonable dimension of existence, leaned on WPC QPF guidance again for the forecast with some slight nudges where GFS/EC consensus is highest and to trend toward ensemble means. All in all, this has resulted in a SLIGHT decrease in the forecast, now ranging from 0.10" in the Snake Plain/srn highlands, to 0.10-0.25" for the Wood River Valley/Upper Snake Plain NE of Idaho Falls, to 0.30-0.60" in the Central Mntns west of Challis and Mackay and across the Island Park region, Big Holes, and Caribou Highlands. Taking into account sfc temps/snow ratios, this yields snow totals of up to 1 inch in the Snake Plain/srn highlands, 1-2 inches in the Wood River Valley/Upper Snake Plain, and 3-7 inches in the Central Mntns west of Challis/Mackay, Island Park region, Big Holes, and ern/Caribou Highlands by Sun AM, again a very slight decrease over the previous forecast. While snow totals themselves aren't terribly high, we remain concerned about travel impacts for 2 reasons. 1) Pattern recognition and signals in the courser, long-range guidance do suggest potential for a wind convergence-induced band of snow to develop in the Snake Plain, which COULD increase snowfall totals, especially in bench locales east of Idaho Falls and around Pocatello. 2) More importantly, only slightly modified arctic air pouring into the region behind the front has to potential to cause a "flash freeze" on any wet road surfaces as temps plummet into the teens and single digits Sat night/Sun AM. This may cause ice to form. More high-res guidance will become available over the next few days as this event approaches to further evaluate these hazards, but we encourage everyone to remain cognizant that notable travel impacts remain possible. Winds chills may fall well below zero Sun AM. Snow showers should taper off Sun in the east as highs struggle to approach freezing, and fall again into the low teens/single digits Sun night. Prepare for this deep freeze by winterizing your homes/lawns, and make sure to keep pets warm. Slow/gradual warming is expected Mon-Wed as high pressure nudges back in. - KSmith && .AVIATION...Strong W/SW winds remain the main aviation hazard today (likely really ramping up after 16z or 10 AM), and a Wind Advisory remains in effect including KBYI/KPIH/KIDA. These winds will align fairly well with the major runways at most terminals, but likely will go SW at KSUN by this afternoon setting up a crosswind there. We still plan to hold any precip at VCSH with the 12z TAFs as the high-res HREF members continue to keep precip very limited, with otherwise VFR cigs/vsbys. Winds should gradually relax tonight with much calmer and dry conditions for Thurs/Fri. - KSmith && .FIRE WEATHER...A passing disturbance will bring strong gusty winds to the Twin Falls and Idaho Falls BLM today and a chance of valley rain and snow and mountain snow to the Salmon-Challis and Targhee NF today and tonight. Colder temperatures will begin to filter into the region Thursday in the wake of the exiting system. A second winter- like storm system follows Friday night through Saturday night with total snow accumulations of 3 to 7 inches possible for the Salmon- Challis, Targhee, and Caribou NF regions and perhaps a dusting in the Snake River Plain Saturday as the cold front progresses through the region. Much colder temperatures will linger over the region early next week as a mostly dry north flow remains positioned over the region. Huston && .PIH WATCHES/WARNINGS/ADVISORIES... Wind Advisory from 10 AM to 9 PM MDT today for IDZ051>055. && $$