483 FXUS65 KPIH 241209 AFDPIH Area Forecast Discussion National Weather Service Pocatello ID 509 AM MST Sun Feb 24 2019 ...LONG-DURATION WINTER STORM HAS ARRIVED, AND WILL BRING WIDELY VARYING IMPACTS ACROSS SOUTHEAST IDAHO... .SHORT TERM...Today Through Tuesday. The NAM, GFS, and ECMWF continue to exhibit very good agreement this morning, showing a low pressure center that has set up near the Washington coast and will remain fairly stagnant/stationary in that general area right into the middle of the upcoming week. This system is tapping into significant Pacific moisture, and the counter-clockwise flow around the low (with westerly winds to the south of the low) with direct a continuous "fire hose" or river of moisture and precipitation inland across Washington, Oregon, and Idaho, relentlessly pummeling portions of our forecast area with moderate to heavy snow and some of the most significant accumulations we have seen with an event this season. A few weak shortwave disturbances may eject inland from the parent low and enhance precipitation, but these are hard to pick out and time in the guidance, and overall few breaks are expected in the above- mentioned stream of moisture. It is important to note, however, that due to meteorological factors such as areas of moist upslope enhancement, areas of downsloping/drying, and warming temperatures over the next several days introducing some precipitation-type concerns, NOT ALL LOCATIONS WILL BE HIT EQUALLY by this event. This will likely be one of those scenarios where folks in some of our major, lower-elevation population centers (particularly across the eastern Magic Valley and Lower Snake Plain) will be asking "what storm?!", while other communities get buried. Let's quickly run through each component of this event, with a focus on what has and hasn't changed in the forecast. TOTAL SNOW ACCUMULATIONS: Once again used a blend of the latest runs of the NBM and NAM12 and the previous forecast for PoPs, and the NBM/NAM12/GFS/previous forecast for QPF. Again made extensive manual edits to accommodate expected downsloping/drying off the southern highlands into the eastern Magic Valley and Lower Snake Plain, partial shadowing of the Wood River Valley, and upslope enhancement against the Soldier/Smoky/Sawtooth mountain ranges and into the Upper Snake Highlands. Looking at 6-hour forecast blocks for both of these elements (PoP and QPF), the overall model agreement in precipitation placement and projected, relentless pounding of some of our mountain regions is nothing short of incredible, with the majority of these 6-hour forecast blocks looking almost identical from today right through Tuesday. Our confidence was high enough to push PoPs to 90-100% across portions of the central mntns, Upper Snake Highlands, and Big Hole Mountains through Tuesday, and we also added mention of heavy snow to the weather grids in these areas. Overall, all of this resulted in no significant changes to expected 3-day storm totals today through Tuesday, with perhaps a slight nudge upward up toward St. Anthony, Island Park, and Ashton. For projected totals by region, please refer to our WINTER STORM WARNING and WINTER WEATHER ADVISORY products. The NBM, and to a lesser extent the GFS, were cranking out some very high totals in excess of 1/2 inch of liquid per 6 hours for some isolated locales in the central mntns and Big Hole Mountains. This seemed a bit high, and we capped some of these high values in the forecast grids at 0.30-0.45 inches, yet still came up with isolated 3-day totals in excess of 50 inches in the highest elevations above pass level! The next few days will be a VERY dangerous period for anyone to head out into the backcountry in these areas. Heavy SNOW LOADING on roofs and powerlines may also become an issue by Monday and Tuesday in the central mntns and Upper Snake Highlands/Big Hole Mountains. In fact, the snow load component of our experimental Winter Storm Severity Index (WSSI) product is lighting up these areas for "extreme" impacts, the highest level. Added mention of this and isolated power outages to the Winter Storm Warning products for these areas. STRONG WINDS: No significant changes were noted in model-projected winds this morning either, and thus our confidence in the overall wind forecast remains high as well. South to southwest winds of 15-25 MPH, with gusts of 30-45 MPH, are expected from this afternoon right through Tuesday across almost the entire region (slightly lower out toward the Shoshone Lava beds and Wood River Valley). Gusts in excess of 50 MPH cannot be ruled out near peaks and exposed west to southwest-facing slopes of the central mntns. So, even for areas that only see a few inches of snow, extensive blowing and drifting of new and preexisting snow is expected to enhance travel impacts and create localized whiteouts. Highway closures are very possible starting as early as this afternoon. Travel may become nearly impossible across the Island Park region, and over mountain passes in the central mntns and Upper Snake Highlands. We again liberally added mention of blowing snow to the weather grids this morning. The winds will make it quite difficult if not futile trying to measure snow during this event. WARNING/ADVISORY DECISIONS: Evaluating the latest projected snow accumulations and winds, no changes were made to the ongoing configuration of warnings and advisories this morning. We discussed adding the south central highlands to the advisory due to several inches of snow combined with strong winds, but we could also see some rain mixing in across the valleys here and reducing impacts, especially Monday and Tuesday. Thus, our confidence was low. This zone could be added later. PRECIPITATION TYPE CONCERNS: Forecast temperatures have changed very little over the past 24 hours, reaching the upper 30s to near 40 this afternoon in the eastern Magic Valley, Lower Snake Plain, and adjacent valleys, and possibly the low 40s by Tuesday. This will allow any snow showers to mix with or change to rain, further limiting already meager expected snow accumulations in this region. A rain/snow mix may reach as far north as Idaho Falls and Carey by Tuesday. The warmer temperatures should also take the edge off the blowing snow threat across this region as time goes on. Minor ponding of water is possible, but at this time we believe the threat of any flooding is low as QPF amounts look light. - KSmith/Huston .LONG TERM...Tuesday Night Through Next Sunday. The GFS and ECMWF have come into better agreement with this model cycle as the low positioned off the NW coast continues to pump a very moist Pacific airmass ENE into SRN Idaho through Wednesday night. By Thursday, the models drive the low east through Idaho with a fairly potent frontal boundary and colder temperatures advancing through the region. A drier and colder NW flow is left in the wake of the exiting low Friday and Saturday. By Sunday, a weak undercutting Pacific disturbance is swept up in the confluent flow over the NW states resulting in an increasing chance for snowfall. At the moment, the models have the main band of precipitation skimming east through NRN Nevada and Utah while the National Blend of Models is a little broader with the impacts. Fully anticipate that Winter Highlights (Warnings and Advisories) will be extended in some shape or form into Wednesday and perhaps even Thursday due to a threat of blowing and drifting snow associated with the frontal passage. Due to the agreement in models, have trended warmer early in the forecast period and then colder following the aforementioned frontal passage. - Huston && .AVIATION... Early morning observations, radar, and satellite imagery have the main precipitation band aligned from McCall to Butte with temporary encroachments as far south as Sun Valley to Idaho Falls. Generally favor the NAM idea of VFR/MVFR conditions at BYI and PIH throughout most of the day and then introducing lower MVFR/IFR CIGS and increasing chances of rain and/or snow overnight. As for SUN, IDA, and DIG, given their proximity to the main precipitation core, we will likely see a greater preponderance of MVFR/IFR CIGS through the day and then IFR/LIFR CIGS in snow through the night. - Huston && .PIH WATCHES/WARNINGS/ADVISORIES... Winter Weather Advisory until 5 PM MST Tuesday for IDZ051>053- 058-061-067>070. Winter Storm Warning until 5 PM MST Tuesday for IDZ060-062>066- 071>075. && $$