A Pacific storm system brought heavy snow to the mountains of southeast Idaho along with the Sun Valley area. One foot fell in Ketchum with over 10 inch amounts in many mountain sites.
Every event with a place to put it; 3 filed by county or forecast zone are drawn at its center, in a lighter ring. Tap one for its entry below.
Deaths and injuries are direct and indirect together, as the offices filed them. Damage is property and crops in the dollars of the year, not adjusted for inflation, and only where an office estimated it.
The episode as NWS PIH wrote it up.
A Pacific storm system brought heavy snow to the mountains of southeast Idaho along with the Sun Valley area. One foot fell in Ketchum with over 10 inch amounts in many mountain sites.
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 Pocatello, ID wrote its last area forecast discussion before the first event at Jan 4, 3:21 AM MST, 4 hours 39 minutes earlier.
Area Forecast Discussion National Weather Service Pocatello ID 321 AM MST Mon Jan 4 2021 .SHORT TERM...Through Wednesday night. A busy period of weather is in store across central and eastern Idaho through Tuesday morning as a strong cold front moves through the region tonight. The lead surge of precipitation will arrive via strong moisture transport and warm advection, to name a couple of forcing mechanisms. The overall timing of the system has slowed 6-12 hours overall compared to 24 hours ago, with precipitation expected to reach the Sawtooths, Wood River Valley and Island Park region just before Noon late this morning. This wave of precipitation will then spread north and east across the remainder of the area through early afternoon. Mainly rain is expected across much of the eastern Magic Valley south of Richfield as well as the southern Snake Plain, with mainly rain across the south hills below 6,000 feet elevation or so. In the Idaho Falls area, a mixture of rain and snow is expected, possibly changing to all rain late this afternoon. Otherwise mainly all snow is expected across Craters and the INL and points northward, with an outside chance for rain to mix in with the snow in Rexburg. Rainfall amounts have increased slightly across the eastern Magic Valley and Snake Plain, and some minor ponding of water is possible as this rain will be falling mainly on frozen soil. A hydrology section will expand on this more below. This evening, the main trough axis and surface cold front will enter the central Idaho mountains between sunset and midnight, then continue sweeping eastward through the early overnight hours, exiting east into Wyoming early Tuesday morning. Mid-level instability release, strong upward vertical motion and enhanced frontogenesis will lead to locally heavy snow showers as these features pass through, along with convectively-induced wind gusts. As the trough and attendant surface front passes through, strong northerly winds with gusts to 50 mph will emerge out of the continental Divide, meeting strong, gusty up-valley (SW) winds across the Snake Plain, and this will set the stage for a convergence event across the Upper Snake Plain early Tuesday morning. At the same time, snow will be gradually diminishing across the central Idaho mountains and Sawtooths. As we progress through the post- dawn hours Tuesday morning, a combination of cyclonic flow and favorable orographics will promote continued snow shower activity through the morning across the eastern highlands/WY border region. Snow and wind will wind down during the afternoon. But wait! There's more. We've issued a slew of new headlines, and a discussion/justification of those follows. We've upgraded the southern Wood River Valley (including Hailey and Bellevue) to a Winter Storm Warning as generally 5 to 8 inches of storm total snowfall is expected here. From Stanley south through the Wood River Valley, snow may be heavy at times this afternoon. We've upgraded the Beaverhead/Lemhi Highlands zone which includes I-15 from Monida to Dubois because of significant blowing snow potential Monday night, mainly after midnight, in association with strong northerly winds that are expected to develop after the passage of the mid-level trough. Farther south, the combination of snow and blowing snow will lead to difficult travel conditions across the south hills. Most of the snow/blowing snow impacts will be felt above 6,000 feet elevation, so we've made this the elevation delineator for the headlines, although some snow is possible below this revelation. Snow and blowing snow expectations on top of a cold existing snowpack in the Craters of the Moon and INL area necessitated adding that zone to a Winter Weather Advisory. The most complicated headline issuance decision was centered on zone 53, the zone including Idaho Falls and Rexburg. We could have gone either way on a Wind Advisory vs a Winter Weather Advisory here, as little to no snow accumulation is expected in the Idaho Falls area through Monday evening, with several inches of snow and blowing snow possible towards the Ashton Hill north of Rexburg. Given the potential for light accumulations with the convergence zone Tuesday morning in Idaho Falls, we opted to lean towards a Winter Weather Advisory here. Finally, strong, gusty winds will accompany the cold frontal passage across the eastern Magic Valley and southern Snake Plain Monday night and early Tuesday morning. The strong Isallobaric component and rapid sfc pressure rises behind the front will promote widespread SW-W winds of 25-35 with gusts of up to 50 mph, locally higher in the Yale/Idahome areas. That said, locally heavy snow is possible along the frontal boundary here as well Monday evening, so winter impacts are indeed possible beyond wind impacts. The extreme northern portion of the Magic Valley from Richfield north to Carey will see accumulating snow and blowing snow, but opted for the Wind Advisory in place of a Winter Weather Advisory. Winds and snow will gradually diminish Tuesday morning. The combination of rain, snow, wind and blowing snow will make for an impactful period Monday afternoon into Tuesday morning across central and eastern Idaho. A ridge of high pressure will briefly nudge into the area Wednesday, promoting cold and dry conditions. Late Wednesday night into early Thursday morning, snow showers are expected to develop, mainly across the central Idaho mountains and Montana border region as a weakening area of low pressure approaches. Some timing adjustments to the forecast may be forthcoming, as models have been somewhat inconsistent with the timing and strength of this system. AD .LONG TERM...Thursday through next Monday. A couple weather disturbances will impact the region late this week. Deterministic models push the first disturbance through Idaho Thursday evening with minor differences in timing. It moves through rather quickly, so snowfall accumulation and impacts should be minimal. Details on the second system vary among models, and have a greater uncertainty on timing and impacts. We should see this second wave sometime Friday evening into Saturday morning. Ensemble spreads offer little additional confidence, though none are showing any signs that the second wave will bring significant impacts at this point. One scenario depicted by ensemble clusters show the presence of a more prominent trough over the Great Basin, represented by almost half the GFS ensemble members. Other scenarios show a more prominent upstream ridge. Hinsberger && .AVIATION...Precipitation ahead of the next upper trough will begin making its way to the Central Mountains early this afternoon, and spread to the rest of the state by late afternoon and early evening. Temperatures supportive of rain/snow at PIH and IDA with BYI remaining just rain. Ceilings and visibilities this afternoon and evening will be at their lowest at SUN and DIJ where precip type favors snow. With a cold front pushing through late this evening, we may see a change-over to snow at BYI, PIH and IDA, along with some gusty winds. HRRR runs are supporting the idea of a convergence band impacting IDA and DIJ towards the end of the TAF period. Hinsberger && .HYDROLOGY...Warmer air will gradually filter into the eastern Magic Valley, Snake plain and valleys across the south hills on today. As a result, rain is expected to develop in these areas early Monday afternoon and continue through much of Monday evening, possibly moderate in intensity at times Monday afternoon. While confidence is high in rain falling as the primary precipitation type across the eastern Magic Valley, southern Snake Plain and valleys of the south hills (mainly below 6,000 feet), confidence in the precipitation type diminishes as we head north through the Snake Plain. From Idaho Falls northward towards Rexburg, a rain/snow mix is expected, with the possibility of a complete changeover to rain in the Idaho Falls area. Rainfall amounts have increased slightly from previous forecasts, with a range of around 0.25-0.40" of rainfall now expected across the eastern Magic Valley, southern Snake Plain and south hills. With soils frozen across much these areas, and with snow cover present across much of the upper Snake Plain from Blackfoot to Rexburg, rainfall may cause some ponding of water and locally enhanced runoff. We will continue to monitor for the potential for minor sheet flooding, but rainfall amounts are likely too low to cause significant impacts at this time. Temperatures will quickly cool below freezing late Monday evening changing any lingering rain to snow. AD && .PIH WATCHES/WARNINGS/ADVISORIES... Wind Advisory from 11 PM this evening to 11 AM MST Tuesday for IDZ051-054-055. Winter Weather Advisory from 1 PM this afternoon to 11 AM MST Tuesday for IDZ052-053-056>059-061-062-065. Winter Storm Warning from 9 AM this morning to 5 AM MST Tuesday for IDZ071>075. Winter Storm Warning from 1 PM this afternoon to 11 AM MST Tuesday for IDZ060-063-064-066-067. Winter Weather Advisory from 9 AM this morning to 5 AM MST Tuesday for IDZ069. &&
The national radar mosaic for the moment the first event began.
3 heavy snow. Local standard time, as filed.
One foot of snow was recorded by the Ketchum COOP observer. Other estimated SNOTEL amounts were: Chocolate Gulch 9 inches, Dollarhide Summit 11 inches, Galena and Galena Summit 9 inches, and Lost Wood Divide 14 inches.
Estimated snow amounts were 10 inches at the Emigrant Summit and Franklin Basin.
Estimated snow amounts were 9 inches at Chocolate gulch and 10 inches at Swede Peak.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in Idaho in 2021Every state in 2021This day in weather historyWarnings in Idaho, January 2021
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: 937160, 937161, 937162.