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FXUS65 KPIH 062127

Area Forecast Discussion
National Weather Service Pocatello ID
227 PM MST Fri Dec 6 2019

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 


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


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 

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