162 FXUS65 KPIH 310255 AFDPIH Area Forecast Discussion National Weather Service Pocatello ID 755 PM MST Wed Dec 30 2020 .UPDATE...Have updated the forecast through the overnight hours to nudge the forecast more in the direction of both observational trends as well as 00Z high-resolution model guidance trends. The update was necessitated because snow showers had already entered the Sawtooths and Wood River Valley, and radar echos were capturing developing snow showers across the Magic Valley and south hills as well. The 00Z models, the HRRR specifically, have captured this trend nicely and have updated the forecast to reflect these trends. The recently developed snow showers should continue on an eastward trajectory towards the Snake Plain over the next hour or two before the main area of snow showers arrives across the eastern Magic Valley, southern Snake Plain and south hills after midnight. The latest high-resolution guidance indicate a re-intensification of a broad shield of snow showers as the mid-level trough approaches from the mid-morning through early afternoon hours. These snow showers are depicted generally along and east of a Twin Falls to Island Park line. Have nudged the forecast in this direction as well. The previous discussion follows. AD && .PREV DISCUSSION... /issued 133 PM MST Wed Dec 30 2020/ SHORT TERM...Tonight through Fri night. An incoming trough is packing some moisture and will leave some light to moderate snow accumulations in eastern Idaho starting this evening in the central Idaho mountains and before midnight spreading to the ID-WY border. The trend from the previous forecast this morning is only slightly higher, with 95 percent of the mountain/highland area receiving less than 6 inches of snow, and Snake River plain/eastern Magic Valley receiving 0.5 inch in the warmer Magic Valley to 3 inches in the north end of the Snake River plain. The associated front is expected to sweep through during the morning hours on Thu, but wind on either side of the front should stay light. There is some southerly wind tonight that will bring in a warm frontal inversion that could allow at least partial melting and then a partial re-freeze for a few hours this evening, making for icy conditions on roads and highways. The area most affected by this will be the Wood River valley and Stanley basin, and the eastern Magic Valley. This should be over by midnight for all locations. Much lighter snow falls during the day Thu, with only the area north and east of Ashton in the northeast corner of the forecast area receiving more than an inch; most of the rest of the eastern highlands could receiving 0.5 to 1 inch, and the central Idaho mountains, south central highlands, and the valley and plain locations getting less than 0.5 inch during the day Thu. Thu night will be dry, with the clearer skies and northerly airflow dropping temperatures 10 to 15 deg F from tonight's lows, with single digits returning to the Snake River plain. A drier Fri should see temperatures about the same as Thu, then Fri night the next storm starts spreading cloudiness during the evening and a chance of precipitation for the northern half of the forecast area, but the brunt of this storm arrives in the extended forecast. This will put Fri night lows in the teens for the populated areas. Have re-issued the Special Weather Statement with an emphasis on the potential for icy road conditions in the west, and the heavier snowfall in the northeast. Did not add any forecast zones to the Statement. Messick LONG TERM...Saturday through Wednesday. A constant flow of moisture will lead to better chances of mountain snow and a mix in the lower elevations almost daily heading into next week. For Saturday, light precipitation is expected mainly for the Sawtooths and eastern highlands...although there is much smaller chance elsewhere. Sunday through early Tuesday looks very wet as we get a stronger surge of moisture potentially working east of the Cascades into our area. While the mountains look to see a fairly steady stream of moisture, the Snake Plain/Magic Valley should see breaks between each individual storm moving across the state during that timeframe. This will be a warmer and windy pattern. This will lead to a mix of rain and snow across portions of the Magic Valley and Lower Snake Plain. The wind MAY create some blowing and drifting issues at higher elevations. The caveat here limiting that would be a "wetter" snow vs powder. Snow amounts have been trending down a bit, but favored areas of the Sawtooths could see 5-10" with a little more than that in valley locations. For the eastern highlands, we could 3-7" for the Big Holes and Island Park area. For the Shoshone Lava Beds area, 1-4" is not out of the question. Elsewhere, downslope and warm temperatures will limit snowfall potential. These numbers are NOT SET IN STONE, as we will need to see just how much moisture ends up getting past the Cascades...and with these types of storms we usually don't get a better picture until we get a little closer to the event. Another bigger push of moisture arrives sometime later Wednesday. Keyes AVIATION...Conditions have improved at all TAF sites through the morning, as we turn attention toward potential impacts from snow and lower ceilings through tomorrow. We are currently VFR, awaiting to see if the first band of precipitation actually falls enough to impact airports. It seems a little bit too dry for snow to fall, BUT we are still thinking some impacts are possible with this first round. Confidence is LOW for the first few hours, and IF it does start snowing, look for MVFR/high end IFR conditions. Trends are showing that impacts are more likely with the band of snow moving in later this evening through tomorrow morning. We are forecasting MVFR/IFR due to snow and low clouds with this band. The ban should clear SUN closer to sunrise with improving weather. The band should stay going through at least the morning hours at BYI, PIH, IDA and DIJ...with IFR conditions. We could see a brief dip to LIFR if snowfall temporarily increases intensity-wise. Conditions should slowly improve tomorrow afternoon. Keyes && .PIH WATCHES/WARNINGS/ADVISORIES... None. && $$