Area Forecast Discussion
Issued by NWS Pocatello, ID

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FXUS65 KPIH 211057
AFDPIH

Area Forecast Discussion
National Weather Service Pocatello ID
357 AM MST Thu Feb 21 2019

...VERY COLD TEMPERATURES AND DANGEROUS WIND CHILLS EXPECTED
FRIDAY MORNING...

.SHORT TERM...Today Through Saturday.
Southeast Idaho remains positioned underneath the axis of a deep
trough, with a low-level low pressure center currently located
over northern Nevada. This is the same storm system that brought
some moderate to significant snowfall to portions of the forecast
area 24 hours ago, particularly around Pocatello. Luckily, the
brunt of the moisture associated with this system has been pushed
well to our south, and we are NOT expecting any high-impact
precipitation over the next couple of days.

Radar early this morning does show some light echoes across the
Snake River Plain, possibly some moisture being "squeezed out" of
a stratus deck across the area. Hard to confirm that much is
actually reaching the ground looking at regional webcams, and
grip levels from ITD road sensor data generally look good, but
can`t rule out some flurries. Extended a low 15-20% chance PoP
west of the I-15 corridor throughout the morning. Accumulations
will be limited to a half inch or less.

Models generally take the low pressure center slowly to the
south, and then eastward into the Four Corners region between
today and Friday. This leaves most of our region under a cold
northerly flow at 850mb. The models do maintain a bit of an east
to southeast component to the regional flow down into our
southeast highlands and the Bear Lake region on the northeast side
of the low. The setup isn`t perfect from a pattern recognition
standpoint, particularly with how far south the low is expected to
be. However, this overall setup and wind trajectory MAY allow for
a hint of upslope enhancement to generate a few light snow
showers across this region from this afternoon into tonight,
generally south of Driggs and east of a line from Idaho Falls, to
Pocatello, to Holbrook. The 00z and 06z runs of the 12km NAM, as
well as the 09z RAP and some of the 00z HREF ensemble members do
HINT at this potential, although this didn`t translate well into
any of the PoP blends. Manually introduced a 15-30% chance of snow
showers across this region with a few hundredths of an inch of
QPF, yielding a dusting to an inch or so of snow. Thus, if these
snow showers do materialize, expecting them to be relatively low-
impact. Cold temperatures and high snow ratios running close to
20:1 will again yield very light, fluffy, low-density snow which
will accumulate efficiently, so can`t rule out someone picking up
2 inches. This activity should quickly settle down by early Fri
morning as the overall trough axis finally starts pushing east of
our region. Overall forecast confidence is high today and Friday,
with the exception of whether or not these snow showers develop
over SE areas.

Next, our attention turns to the potential for very cold wind
chill values across northern portions of our forecast area Friday
morning. Blends such as the NBM were running warm compared to
MAV/MET guidance, which seems to be a recurring theme. Felt
quite comfortable pushing forecast low temps toward those MOS
values, especially considering good model agreement on extensive
clearing across the northern half of the CWA, which would support
good radiational cooling. Our forecast paints low temps below zero
north of a line from Hailey, to Pocatello, to Soda Springs, with
values potentially approaching -10 to -15 from the Arco Desert
into the Upper Snake Highlands and Teton Valley. (As usual,
normally colder locales such as Stanley and the Copper Basin will
likely overperform at -15 to -30.) Coupled with northerly winds
of about 7 to 15 MPH, wind chill values of -20 to -30 are
expected over some pretty large real estate from the central Snake
Plain north and east. Research shows these are dangerous values
that can cause frostbite in about 10 to 20 minutes for exposed
skin. A lot of our confidence "nudge" factors are in place here
for a notable event, including potential clearing, recent snow on
the ground, and overall good model agreement on overnight winds.
Thus, we issued a WIND CHILL WATCH across the Arco Desert region,
Big Lost and Little Lost River Valleys, and Beaverhead/Lemhi
Highlands. Unless something significant changes, we are pretty
confident this will be upgraded to a WARNING by early this
afternoon. Further east across the Upper Snake Highlands and Teton
Valley, similar wind chill values are expected, but will be supported
more by cold temperatures and a bit less by the wind, which comes
in just shy of our 10 MPH guideline. Thus, we did not include
these areas in the watch, but safety messaging is the same: dress
in layers, cover all exposed skin including weather hats and
mittens, and bring pets indoors or to warmer shelter if possible.
It`s worth noting too that even across the Upper Snake Plain
including Idaho Falls and Rexburg, wind chills may reach -15
degrees or colder.

A dry day is expected Friday region-wide as we lie in between
storm systems, with our next disturbance (a low along the British
Columbia coast) spreading snow showers across the region by early
Sat morning. Sat morning lows look warmer, coupled with
increasing cloud cover to help insulate the surface and lighter
winds, so wind chills do not look nearly as dangerous. Saw little
reason to deviate from an NBM/GFS/NAM blend for PoPs and QPF.
Snowfall amounts Sat appear light, generally a dusting to an inch
for valley/plain areas and 1-3 inches in the western central mntns
and eastern highlands from Island Park to Palisades, again with
good model agreement. High temperatures will nudge up Sat, but
will remain solidly below freezing with no precip-type concerns at
this time. - KSmith/Huston

.LONG TERM...Saturday Night Through Next Thursday.
The GFS and ECMWF are in fairly good agreement through Tuesday
morning as a low pressure system sets up over Vancouver Island
ushering in an extended period of Pacific moisture across Idaho.
Currently, successive 12-hour accumulations across the CNTRL and
NRN mountains would support a Winter Weather Advisory that would
run from Saturday night through Monday night. Starting Tuesday and
continuing through Thursday, the GFS tries to maintain a drier
and much colder northerly flow across the region whereas the ECMWF
gradually rotates a Pacific Low into the NW coastal waters while
continuing to transport moisture and warmer temperatures into the
region. The best I could do here is to split the difference by
maintaining some mention of precipitation throughout the period
while also moderating the cold arctic temps evident in the GFS
numerical guidance. - Huston

&&

.AVIATION...
Early morning satellite imagery depicted an upper low digging SSE
through Nevada with attending moisture rotating about the low
into the SW Idaho border region. Areas of low clouds (MVFR)
remained over SE Idaho and the models appear to support at least
spotty light snow showers over the region today as the low
continues to dig south. Drier and colder conditions will spread
into the region from the north tonight with partial clearing.
Huston

&&

.PIH WATCHES/WARNINGS/ADVISORIES...
Wind Chill Watch from late tonight through Friday morning for
IDZ052-067-068.

&&

$$


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