Scientific Forecaster Discussion
000 FXUS65 KPIH 062127 AFDPIH Area Forecast Discussion National Weather Service Pocatello ID 227 PM MST Fri Dec 6 2019 .DISCUSSION... A storm system remains on track to impact SE Idaho this weekend but impacts will vary greatly based on location and elevation. For tonight, expecting the low stratus that has plagued much of the region for the past few days to return although perhaps not quite as widespread. This low stratus, combined with incoming clouds from the aforementioned storm system approaching the region, may enable a seeder-feeder process to develop across southeastern portions of the CWA so have introduced low PoP snow potential into the forecast tonight to account for this although confidence in this is quite low. It's entirely possible that this does not occur and may just persist as low clouds or patchy fog in spots but felt it wise to at least mention the potential. Any accumulations would be low but with temps below freezing this could create some icy surfaces on area roads if it does pan out. Shifting gears into the weekend, the much advertised storm system remains on track as an area of low pressure pushes into the Pacific Coast later tonight with the associated moisture reaching our area, namely the Central Mountains, around daybreak Saturday with the rest of our area getting in on the precip during the afternoon and evening hours. Similar to our last event, 850 mb flow will be out of the south so this will support a downslope flow off the mntns along the Snake Plain from Oakley, to American Falls, to Pocatello, to Blackfoot, to Idaho Falls, and perhaps into the Teton Valley. Additionally, temperatures will be marginal for this event in the valleys as highs are expected to top out in the upper 30s and lower 40s with snow levels in the 5300-5900 ft range for much of the day. So for much of the day in the lower elevations precip will be mostly rain or perhaps a rain/snow mix with a potential changeover overnight to more snow but even then temps/snow levels look marginal. Evaporative cooling processes may play a role in precip type as well so needless to say it's a very tricky forecast across the Snake Plain. All that being said, have seen a trend in hi-res model guidance delaying the onset of precip across the Snake River Plain until very late in the evening or perhaps into early Sunday morning. Haven't gone this extreme with the forecast yet but something to keep an eye on. Multiple hi-res model solutions have been showing the first wave of precip progressing through the Central Mountains and into the Lemhi Highlands and Island Park area keeping much of the Plain dry through the day Saturday. Something to monitor. Across the mountains no major changes to the ongoing forecast as we still expect 4-10 inches of snow across the Central Mntns west of Challis and Mackay (including the Wood River Valley due to upslope enhancement), and similar values across the Upper Snake Highlands, Teton Valley, Caribou Highlands, and Bear River Range. Valley locations can expect anywhere from a dusting to around 2 inches through this event but again it's a very tricky call with so many factors in play. Overnight Saturday into Sunday as temps cool and snow levels drop may see a bit more snow than rain across valley locales but again it is marginal but this would be the mostly likely time for any accumulating snows. During the day Sunday, temps again look to warm into the upper 30s and lower 40s so similar forecast challenges exist there. Later in the day, seeing the potential for an enhanced convergence band to set up as cooler NW flow filters into the region late Sunday so this may provide more potential for accumulating snow across the Snake Plain. As we move into the start of next week, the remnants of the upper trough stick around Mon over higher elevations east of I-15. There is low confidence chance of a few showers lingering Monday night into Tuesday as well, but if so these are expected to be light and fairly isolated. Cooler temperatures will be in place under influence of colder northerly flow behind trough axis as well with limited precip potential until late in the forecast period when models are showing the potential for the next weather maker to reach our area. McKaughan && .AVIATION... Fog and stratus has thinned out with slightly drier air at PIH and BYI this afternoon. That "dry punch" is trying to work north toward IDA as well as SUN, but confidence is not high it will actually make it to either airport. We did show some slight improvement to MVFR/low end VFR weather. DIJ is barely VFR right now but should stay that way even if clouds remained socked into the Teton Valley. Tonight and Saturday morning will be interesting as the models try to redevelop fog and stratus where things have cleared or could clear across the Snake Plain. Persistence would say YES, but it depends on how much we cool with the drier air working into that area. We went with persistence and dropped conditions slowly back through MVFR to IFR overnight at BYI, PIH, and IDA. We did the same for DIJ as well. With the potential return of stratus, we did add VCSH for IDA and DIJ due to possible light snow developing ahead of the main storm. We are expecting the a bigger wave of light snow to impact the central mountains late tonight and Saturday. We kept with the trend of snow with MVFR/IFR conditions at SUN because of this through the day. We COULD see at least VCSH for the other sites during the afternoon, but that likely will hold off until tomorrow evening/night. Keyes && .PIH WATCHES/WARNINGS/ADVISORIES... Winter Weather Advisory from 4 PM Saturday to 5 AM MST Monday for IDZ060-062>066. Winter Weather Advisory from 8 AM Saturday to 11 PM MST Sunday