Area Forecast Discussion
Issued by NWS Pocatello, ID

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438
FXUS65 KPIH 230342
AFDPIH

Area Forecast Discussion
National Weather Service Pocatello ID
842 PM MST Wed Feb 22 2017

.UPDATE...

Monitoring a band of heavy snow that is working its way up the
Snake Plain this evening. This band has been putting out around an
inch or so of snow but has been progressive. The center of the low
pressure appears to be based somewhere near Twin Falls on radar.
They`ve seen more steady snow this evening. This low will slowly
move east tonight and into Friday morning. Still think the best
snowfall amounts of 2 to 4 inches around Burley seem reasonable
through midday Friday. Higher amounts are possible south of there
where terrain will help to add to snow intensity. Increased
chances further north from Sun Valley to Idaho Falls as this band
of snow lifts northward. Once again expecting around an inch,
although wouldn`t be surprised if Sun Valley sees something closer
to 2 or 3 inches because of the terrain helping to increase lift
in the area. Will leave all advisories alone and amounts in the
advisories/warnings alone. 1 to 3 is likely through noon Friday
around Pocatello and around 1 around Idaho Falls. Low to medium
confidence in these amounts as the models are still trying to hone
in on the qpf amounts with this system. The update tonight was
heavily based on the HRRR which seem to better depict individual
elements with each hour (for example the progression of the band).
The new NAM may actually be somewhat reasonable although the qpf
amounts in the bands are way way overdone.

Valle

&&

.PREV DISCUSSION... /issued 224 PM MST Wed Feb 22 2017/

SHORT TERM...Tonight through Friday. Strong shortwave driving
across Nevada this afternoon per satellite imagery. Good moisture
north of circulation with cooling cloud tops in area of increasing
divergence aloft in southern ID. Models greatly increase the
amount of divergence aloft through the remainder of the evening,
then rotate region north into Snake Plain and weakening it through
the rest of tonight. Surface low still expected to rapidly deepen
near the state line this evening, but model consensus
considerably weaker on exact positioning of surface features
overnight. this is problematic because while divergence aloft will
drive much of the upward motion, snowfall overnight will likely
be enhanced by orographics driven by the lower level features.
Regardless, there is quite a bit of model consensus that a strong
band of snow fall will develop over the southern highlands then
develop/swing northeast into the Caribou highlands overnight.
Snowfall max extend into the Snake Plain as well. Consensus puts
several inches of snow fall along I-84/I-86 corridors along with
the southern highlands but there is much less agreement on whether
significant snow will develop much farther north than Pocatello.
First shortwave drives east during the day Thursday but inverted
700mb and surface trough remain draped over region Thursday
through early Friday with flow favorable for orographically
enhanced snowfall to continue. Current estimates maintain snow
amounts sufficient to provide significant impact on travel through
the region mainly across the southern portions of the forecast
area. There is much lower confidence on significant snows reaching
as far north as Idaho Falls, but given divergence of model
solutions cannot quite rule out yet. Have maintained previously
issued advisory headline, but also added the Pocatello region.
Have also upgraded IDZ022 to winter storm warning to support
higher snow amounts above 6000 ft. If system develops as expected,
more regions could be upgraded to warnings as well. DMH

LONG TERM...Saturday through Wednesday. Saturday will start off
mild but quickly become snowy with a series of shortwaves passing
through the region over the weekend. Snow accumulations are expected
to be light overall, with anywhere from 1 to 4 inches expected
across the mountains and along the Continental Divide. Snake
Plain regions may see a light dusting as well. Temperatures both
days will be in the mid 20s to low 30s for highs with areas below
0 for minimum temperatures in the Central Mountains and Bear Lake
regions. Late Sunday into Monday will bring a widespread wet
period with an anticipated closed low approaching from the
southwest. Snow is expected Monday morning through early Tuesday
across the region. Winds are expected to increase Tuesday into
Wednesday in response to a passing shortwave behind the main low.
Additionally, light snow is possible across Southeast Idaho with
low confidence in occurrence either day. NP/DMH

AVIATION...VFR conditions expected until the late evening/overnight
hours due to approaching trough. Ceilings will then lower to MVFR
through tomorrow with snow or a mix or rain and snow at BYI, PIH and
SUN. IDA and DIJ are not expected to be impacted by SN until
tomorrow morning. Winds are not forecast to be of impact today
either. NP/DMH

HYDROLOGY...Moderate precipitation amounts and mild low to mid elevation
temperatures the past several days have maintained and worsened low
land flooding threat across the region. Minidoka, Lincoln, Oneida,
Franklin, Southwest Bingham, and Southwest Caribou counties have, in
particular, seen increased low land flooding where flood warnings
are in effect.  Some minor low land flooding has continued for
Western Caribou, Eastern Bingham, Western Jefferson and Madison
counties where flood advisories remain in effect. Some rises have
been seen in area creeks as well as some small responses to rivers.
Goose creek from the Utah line to the Oakley reservoir has continued
to have significant flooding where a flood warning is also in
effect. The Raft river continues to have minor low land flooding
from Utah to the Snake River where a flood advisory is in effect.
Have also continued the flood warning for the Portneuf River at
Pocatello. The river is at minor flood stage currently and cresting
below moderate stage tomorrow afternoon before dropping below flood
stage on Saturday.

A cool down today through most of the weekend will aide to the slow
down of some flooding particularly for Northern areas, though it may
take some time to see the full effect. Though cooler temperatures
are expected to move into the area today into tonight, several
inches of snow is in the forecast mainly for our Southern areas
extending Northward into the Southern Snake river plain and Eastern
Magic valley. Snow will start this evening and continue through
early Friday. Temperatures will gradually start to rise Saturday and
daytime temperatures will approach the low to mid 30s for our
Northern areas and low 40s for our Southern areas Monday which will
aide to melting.
Wyatt

&&

.PIH WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until 11 AM MST Friday for IDZ017-021-
023>025.

Winter Storm Warning until 11 AM MST Friday for IDZ022.

&&

$$



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