Area Forecast Discussion
Issued by NWS Pocatello, ID

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FXUS65 KPIH 072121

Area Forecast Discussion
National Weather Service Pocatello ID
221 PM MST Wed Dec 7 2016

.SHORT TERM...Tonight through Saturday. Pockets of stratus/fog
still linger over lower elevations in E Idaho this afternoon with
largest patch through Lower Snake Plain near Pocatello. Snow
appears to have finally ended in this region but could just be
temporary reprieve in afternoon sun. Downward forcing continues
overnight which could reinforce inversions. Favorable temp
profiles exist to support additional flurries/light snow if
stratus can reform. Have maintained compromise dry but cloudy
forecast through tonight.

Pattern rapidly shifts Thurs with consolidated band of moisture
lifting into the region and precip onset Thurs afternoon. Flow
aloft westerly but low level flow turns south, which tends to
force downslope drier/warmer conditions along I-84/86 corridor
stretching up along portions of I-15. Despite widespread high QPF
in models, have trended these areas slightly warmer and slightly
drier, resulting in reduced accumulations these areas especially
with warm up headed into Friday. Uncertainty in how much impact
current snow pack will have on strength of warming late Friday
especially in the downsloped areas. If cold air could stay
trapped at surface, could see portions of the area mix to freezing
rain. Overall accumulations through Friday remain generally 2-4
through Snake Plain though amounts in northern portions of Magic
Valley may be slightly higher. Much different story headed into
higher elevation zones. Most areas looking at 4-10 inches with
exception of some peaks through southern highlands where around a
foot will be possible. Since main band with first system appears
to be focused across southern highlands, areas closer to the
divide can expected to be on the lesser end of those amounts.

Second system drives through Fri aftn and overnight, packing a
similar punch. Model trends continue to be up and down especially
with where to focus highest moisture. Current round of solutions
again targets southern highlands and Snake Plain, but once again,
low level downsloping may limit potential. Warm up on Fri could be
enough to mix or shift precip to rain especially in lower
elevation areas and limit accumulation potential at higher
elevations due to decreased snow ratios. Colder temps filter in,
but generally behind the back end of the precip.

Overall, general expectation is for a very wet and unsettled end
to the work week and beginning of the weekend. Accumulated snow
totals Thurs-Sat may well exceed a foot in higher elevation and
along some passes. That said, there is still a LOT of model
uncertainty on QPF amounts, strength of the warm push especially
over recent snow pack, and amount of potential downsloping to
limit precip. Will leave winter storm WATCH in place, for now. DMH

.LONG TERM...Saturday night through next Wednesday.  We will remain
under a constant zonal/northwest flow type pattern into next week.
The consistent issue remains where we are having a hard time
pinpointing periods of "heavier" precipitation. This is more
significant for lower elevations whereas the mountains will see more
persistent snowfall. The key things to take from this (despite the
lack of confidence mentioned), is that mountains will see more snow
and breezy conditions through most of this timeframe. For the lower
elevations, there MAY BE a better chance of precipitation later
Sunday, then again Tuesday and Wednesday. We stress MAY because that
could change with the next forecast model update. Temperatures
should remain cool enough for all snow in the higher elevations.
For lower elevations, that will be more dependent on timing. Keyes


.AVIATION...We are finally seeing the low clouds and snow that`s
been plaguing KPIH and KBYI ending. It appears the upslope flow and
band of snow is finally eroding. The forecast for tonight calls for
no limited flight rules due to fog or low clouds, although we are
calling for at least some scattered or broken cloud deck that easily
could become MVFR/high end IFR if reality is more pessimistic that
the forecast. Even if things end up going a bit south tonight, we
should see a break until tomorrow afternoon at all TAF sites.
Conditions will quickly deteriorate from southwest to northeast as
clouds and snow with the next storm arrives. The current idea is for
the onset of things to occur at KBYI and KSUN between 18-22z, 22-01z
at KPIH/KIDA and KDIJ between 00z-03z. That said, this could easily
change as the forecast data we look continues to fluctuate with
this.  Keyes


Winter Storm Watch from Thursday morning through Saturday
afternoon for IDZ018-019-022>025-031-032.


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