Area Forecast Discussion
Issued by NWS Pocatello, ID

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FXUS65 KPIH 210153
AFDPIH

Area Forecast Discussion
National Weather Service Pocatello ID
653 PM MST Mon Feb 20 2017

.UPDATE...

Updated forecast to upgrade winter weather advisory for upper
snake highlands to winter storm warning as I15 is now closed north
of Dubois because of blowing and drifting snow. Also, lowered
snowlevels in both warnings based on reports of significant snow
falling roughly around 5500-6000 feet.

Valle

&&

.PREV DISCUSSION... /issued 338 PM MST Mon Feb 20 2017/

UPDATE...

Updated forecast to issue wind advisory through Tuesday afternoon.
Wind observations are already exceeding forecast levels. Expect
winds to diminish some tonight, especially in the valleys, but
will probably increase during the overnight hours or early Tuesday
morning. Gusts to 45 mph are highly likely with 50 mph possible
especially Tuesday afternoon.

Valle

PREV DISCUSSION... /issued 236 PM MST Mon Feb 20 2017/

SHORT TERM...Tonight through Wednesday night. Atmospheric river
continues today with strong shortwave circulation over OR/WA.
Vigorous 700mb jet and deepening surface low shifting northeast
across the region with ample moisture plume aimed at east ID.
Winds have strengthened considerably as surface gradient has
tightened this afternoon in response to first 700mb jet with some
areas nearing advisory criteria. Higher elevations also picking up
speeds as well. Widespread rainfall with embedded weak convection
may be contributing to increase in winds as well. Snow levels
still generally above 6500 ft and should stay elevated most of the
night. Stronger 700mb jet noses into region ahead of next vigorous
shortwave reaching CA coast, and will help to reinforce
precipitation across the region. Snow band will enhance overnight
and into Tuesday morning mainly targeting higher elevations. Snow
levels remain elevated until frontal boundary pushes through
during the afternoon. Have also added thunderstorms for the
afternoon across the entire area in unstable airmass ahead of the
front. Will continue current winter storm headlines as they are,
and anticipate that a wind headline will be necessary for Tuesday
and possibly even into Tuesday night as surface gradient remains
tight. Upper trough axis shifts through Idaho during the day on
Wednesday. Heaviest precipitation tapers Tuesday night through
Wednesday but higher elevations could remain orographically
influenced in still-unsettled westerly flow. Thus have kept precip
chances going through remainder of short term. DMH

LONG TERM...Thursday through next Monday. The theme continues
to be wet, but much cooler weather is expected heading into the
weekend. Periods of light snow (mixing with rain at lowest
elevations during the day) are possible. This type of pattern still
lends itself to issue trying to map out when each "wave" move
through eastern Idaho. By late in the weekend and early next week,
another storm develops to our west, which will allow for southerly
flow to return and helping push temperatures back up a few degrees
especially in the afternoon. Keyes

AVIATION...A deep pacific low is bringing ample moisture to the
area. The first of a couple of waves is moving through our area
currently bringing showers and gusty Southerly winds. KBYI and KPIH
will likely remain VFR with strong downslope winds and just see rain
showers. The rest of the TAF sites are having IFR/MVFR CIGS and VIS.
KIDA will likely just see rain through the period as well. KSUN and
KDIJ will start as a rain/snow mix and may briefly see rain this
afternoon before turning back to snow early this evening and
remaining as snow through tomorrow morning. Other than KSUN and KDIJ
there will be a brief break in rain and snow showers late this
afternoon into early evening before the next wave of moisture moves
through late this evening through early tomorrow morning. With this
wave this evening the other sites should join KSUN and KDIJ with
MVFR/IFR CIGS. With this wave tonight, models are indicating a
strong low level jet that will increase the possibility of low level
wind shear. Tomorrow afternoon looks like another continuous round
of moisture particularly for KSUN and KDIJ. Wyatt

HYDROLOGY...Moderate precipitation amounts and mild low to mid
elevation temperatures the past several days have maintained and
potentially worsened low land flooding threat across the region.
Minidoka county has, in particular, seen increased low land
flooding. Will continue all low land flood headlines. With
significant rain expected overnight through tomorrow below 6500
feet, some of the flood advisory areas may see increased flooding.
These areas, to include Eastern Custer, Central Blaine,
Jefferson, Southeast Butte, Northwestern Bonneville, Western
Madison, and Southwest Caribou counties, may need to be upgraded
to flood warnings. Some rises have been seen in area creeks, and
continuing to see small responses on the rivers as well. Have also
issued a flood warning for Portneuf River at Pocatello with river
forecasted to rise to flood stage Tuesday morning. Current trend
is to decrease crest and shorten time the gage spends above flood
stage. Wyatt

&&

.PIH WATCHES/WARNINGS/ADVISORIES...
Wind Advisory until 8 PM MST Tuesday for IDZ017-020>022.

Winter Storm Warning until 3 PM MST Tuesday for IDZ018-019-031.

&&

$$



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