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FXUS65 KPIH 230807 AFDPIH

Area Forecast Discussion National Weather Service Pocatello ID 207 AM MDT Sat Sep 23 2023

SHORT TERM
Today and Sunday.
Today will be the first dry day that we have seen in a bit as the upper low that brought multiple days of cloudy and wet weather continues to depart to the east and high pressure builds over the region. This morning will be a chilly one, with FROST ADVISORIES AND FREEZE WARNINGS in effect through 9 AM for the Upper and Lower Snake River Plain, Eastern Magic Valley, Shoshone, and Arco Zones. Low temperatures this morning will span the 30s and upper 20s across the entirety of southeast Idaho. While temps will predominately remain just above freezing in these aforementioned zones, the National Blend 25th Percentile indicates portions of the Arco Desert and Lower Snake Plain will just barely drop into sub- freezing territory. The limiting factor is the potential for lingering low-level cloud cover and fog formation which would help to insulate and keep temperatures a bit more elevated. After these last lingering clouds clear by late morning, you can count on mostly clear skies overhead this weekend with plenty of sunshine. While temperatures will run below climatological norms again today, high temperature will increase by over ten degrees as compared to yesterday in some locations. Temperatures will increase to around normal on Sunday following another 5 to 10 degree warm up. Cropp

LONG TERM
Monday through next Saturday.
Low pressure will drop southeastward out of the Gulf of Alaska, significantly deepening the upper-level low off the coast of British Columbia. This system will not only bring impactful rain to the PacNW, but it will help amplify the transient upper-level ridge building over eastern Idaho this weekend. We're still expecting dry conditions to remain in place, but winds and temperatures will be on the increase Monday as the periphery of the broad low moves in. NBM 4.1 showing better than a 60 percent chance of reaching 80 degrees in the Magic Valley including areas around and just east of Shoshone. Similar chances (greater than 60 percent) exist for Burley, American Falls, Pocatello, Blackfoot and even Idaho Falls to reach 75 degrees, which is near seasonal averages. Monday stands to be the warmest day, however, as the coastal low creeps a little further inland by the middle of next week leading to another cool down.

Through the bulk of next week, the ridge and coastal low will keep our area in southwest flow aloft. This southwesterly fetch will keep moisture streaming mostly northwest of our area. Our central Idaho mountains could see some potential for daily showers, but the current trends should keep the majority of eastern Idaho dry through Wednesday. While ensemble guidance still mostly agrees on the evolution of the low pressure system entering the northwest, cluster analysis shows there are some modest timing differences on the pattern breakdown toward the end of next week. Broad low POPS are reintroduced during that time. All indications point to this opening the door to cooler and wetter than average conditions again. TAX



AVIATION
As high pressure begins to build in behind the departing upper low, low-level clouds will linger through late morning with the potential for fog development across the Snake Plain and eastern Magic Valley. The HRRR model indicates a 20% probability of IFR VIS in the vicinity of KBYI, with a 40% probability at KIDA and even a 30% of LIFR VIS at KIDA through around 15z this morning. Otherwise, expect mostly clear skies, dry conditions, and light winds 10 KTS or less. Cropp



FIRE WEATHER
Finally beginning to see the pesky upper-level low move away from the area. We should quickly transition back to a ridging pattern developing over the area in the coming days. This means a trend to drier and warmer weather is expected through the balance of the weekend and into Monday. The ridge amplifies Monday leading to the warmest and driest day and with low pressure digging into the coastal PacNW, it will also be fairly windy. RH values will dip down into the 20s for most valley locations Monday, but zone 425, 475 and 476 could mix down into the upper teens. Winds across most of eastern Idaho could gust to at least 20mph, but zones 475, 476, and 422 gusts could even get to 35mph at ridgetop level.

After Monday, the coastal low begins to nudge a little further inland. This will lead to slightly cooler and more moist air arriving. Winds should also begin to decline through midweek.
TAX



PIH WATCHES/WARNINGS/ADVISORIES
Frost Advisory until 9 AM MDT this morning for IDZ051-053-055.

Freeze Warning until 9 AM MDT this morning for IDZ052-054.

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