Area Forecast Discussion
Issued by NWS Pocatello, ID

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FXUS65 KPIH 170425 AAA
AFDPIH

Area Forecast Discussion...UPDATED
National Weather Service Pocatello ID
925 PM MST Thu Nov 16 2017

.UPDATE...We are not looking to make changes to the forecast or
headlines tonight. We are seeing a distinct break over most of the
Snake Plain and central mountains, but more precipitation is
moving in. A band of moderate to heavy rain/snow persists over
the southern and eastern highlands, as well as the interstate
corridor. Under that band, we are starting to see some localized
accumulations down to 5500-6000ft. This band will remain most of
the night over the southern and eastern highlands, slipping just
south and east of the interstate corridor. We will be watching to
see if we need to adjust the advisory to a lower elevation or even
replace it with a warning. We also considered dropping the
warning in the central mountains but with colder temperatures and
moisture starting to fall, we decided it wouldn`t be prudent.
Keyes

&&


.PREV DISCUSSION... /issued 130 PM MST Thu Nov 16 2017/

SHORT TERM...Tonight through Sunday. Early afternoon satellite
imagery continued to show a moist sub-tropical fetch streaming
across the Great Basin into SE Idaho. Numerical models show the warm
sub-tropical fetch slipping into the SE highlands overnight as the
upper low off the NW coast begins to advance inland. Dynamical
support associated with the trough is expected to generate a second
albeit lighter round of snowfall across the CNTRL mountains tonight.
The trough sweeps through the region Friday with snow levels
lowering to the valley floor and ongoing snowfall across the upslope
favored regions of the CNTRL and SE mountains. Thus have extended
the Winter Storm Warning for the CNTRL mountains to 4 AM Friday
morning and the Warning/Advisory for the ERN highlands through 3 PM
to cover the additional snowfall. Strong gusty winds are also
expected to develop across the Snake River Plain by Friday afternoon
possibly requiring an advisory. We will let the mid-shift take one
last look at this item to see if the models maintain some
consistency with the strength. For Friday night through Sunday, a
ridge of high pressure expands across the NW states resulting in
drier conditions. Did not stray to far from guidance which had
daytime highs running near climatic norms through the period. Huston

LONG TERM...Sunday night through next Thursday night. Overall theme
of the extended portion of the forecast is a continuation of the
active weather pattern. There are many uncertainties, however, such
as location, timing and snow levels. Forecast guidance is in
somewhat better agreement that an incoming Pacific Low Pressure
Through will spread precipitation across SE Idaho on Monday,
beginning first over the Sawtooths/Pashimeroi then spreading
eastward across the remainder of SE Idaho. Temperature trends with
this system indicate another Valley Rain/Mountain snow event, with
Snow Levels perhaps lower than our current ongoing system, which may
mean more snow for some of the higher elevation valleys, and
potentially even the Upper Snake Plain. This will bear watching over
the next couple of days. Forecast trends then indicate a warming
trend through the week with a continuation of moist, Southwest flow.
Best chances for Precipitation favor late Monday into early Tuesday
over the Wyoming border region with the above-mentioned departing
system, followed by Valley Rain/Mountain snow for the
Sawtooths/Pashimeroi and Montana Border south to Driggs region
Wednesday through Friday with varying levels of intensity as
embedded waves of Low Pressure work through the region. Chances for
Precipitation are lesser through the Snake Plain through this time
than compared with the higher terrain areas. Many details to iron
out as we head through the next couple of days. AD/Huston

AVIATION...Trough of Low Pressure is forecast to work across SE
Idaho through Friday with a myriad of Aviation issues. Many
terminals across the Snake Plain have been VFR through the day, but
conditions will be deteriorating late this afternoon and evening as
an expanding low Stratus deck works its way from BYI to PIH to IDA.
Uncertainty as to just how low CIGS will go, but am currently
carrying a transition to MVFR CIGS by 00Z at PIH and IDA, although
this transition may happen sooner. A band of Rain is expected to
develop from BYI-PIH-IDA beginning around 00Z with some potential
for IFR CIGS to develop beneath the band. Surface winds have been
southerly at PIH and IDA but should turn more Southwesterly as the
trough passes through around 00Z. Winds at BYI have already turned
westerly behind the trough passage. For DIJ, the band of heavier
precipitation should work into this region as well, with a later
transition for CIGS and winds. CIGS should decrease to MVFR and
possibly IFR between 03-06Z along with a changeover from Rain to
Snow. Much uncertainty at KSUN as VIS and CIGS have been higher than
forecast, although there is some potential for MVFR and IFR CIGS to
re-develop as expanding low Stratus across the Eastern Magic Valley
may advect into KSUN this evening before northerly winds develop at
KSUN overnight and push lower clouds out. Some potential for -SHSN
and associated MVFR and IFR conditions at all TAF sites Friday
morning, with KSUN and KDIJ favored. Windy conditions expected to
develop across the Snake Plain, with SW gusts to 35 kts possible at
KPIH and KIDA during the afternoon Friday. Should see improving CIG
and VIS all TAF sites during the afternoon. AD/Huston

&&

.PIH WATCHES/WARNINGS/ADVISORIES...
Winter Storm Warning until 3 PM MST Friday for IDZ019.

Winter Weather Advisory until 3 PM MST Friday for IDZ023-025.

Winter Storm Warning until 4 AM MST Friday for IDZ018-031.

&&

$$



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