Area Forecast Discussion
Issued by NWS Pocatello, ID

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134
FXUS65 KPIH 231339
AFDPIH

Area Forecast Discussion
National Weather Service Pocatello ID
639 AM MST Thu Feb 23 2017

.UPDATE...Update this morning to include Idaho Falls, Rexburg and
INL in Winter Weather Advisory. Snow has rotated into this area
around a strong area of low pressure centered near the
intersection of ID, UT and NV. Periods of snow will continue
across all of SE Idaho through mid day Friday, with the location
of the heaviest snow meandering around SE Idaho. Indications
suggest the heaviest band may set up along the Interstate 15
corridor and benches east of the Interstate overnight tonight with
an extension of this band extending westward along and south of
the Interstate 86 corridor. Can`t rule out pockets of occasionally
heavy snow outside of this band either, with trends suggesting the
heaviest snow today will set up across the Arco Desert southward
towards Burley. Roads will be snow-covered and slick for many
areas. Some improvement in driving conditions possible today when
snow intensity lets up, but surfaces may re-freeze overnight
tonight leading to a slick morning commute Friday as well.
Additional snow accumulations generally 1 to 4 inches with local
areas seeing 6 inches or more under heavier snow bands through
tonight. AD/RS


&&

.PREV DISCUSSION... /issued 304 AM MST Thu Feb 23 2017/

SHORT TERM...Today through Friday night. Challenging forecast
through tomorrow as forecast guidance struggling to capture
intricate details with our winter storm. As of 2AM MST, a strong
winter storm was centered over the Eastern Magic Valley, with a
dry slot rotating into eastern Cassia County and western Oneida
County, resulting in snow temporarily ending here. A band of
nearly stationary heavy snow has developed, aided by dynamical
processes on the northern flank of the dry slot, with a boost from
enhanced low-level convergence. Low-level winds are northerly from
Idaho Falls east to INL and north to the MT border, with winds
south of this line generally west to southwest. There is no
consistency among forecast guidance, with each model offering a
different idea of how features will evolve through Friday. This
lends low confidence in the details of how this system will evolve
through Friday.

At this time, have settled on a forecast that is between the
high-resolution NAM models and HRRR. The NAM wants to rotate
snowfall northward all the way to the MT border this morning while
the HRRR settles the current band southward through the day.
Believe reality will be somewhere in between. The center of the
circulation currently located over the Eastern Magic Valley should
gradually rotate eastward today, allowing the Caribou Highlands
to emerge into the dry slot, resulting in decreased precipitation
there. The heavy band between I-86 and HWY 20 across the Arco
Desert currently should become more SW-NE oriented with time, and
gradually wrap around the circulation center which should mean a
redevelopment of snow for Cassia and Oneida Counties by this
afternoon and evening. Considered adding Idaho Falls and INL to
the Winter Weather Advisory as the snow band, per radar, is trying
to inch north as moisture pivots around the low. Believe some
snow will work into Idaho Falls this morning, but with a northerly
feed of relatively dry low-level air, believe the northward
extent of this band may be impeded. Do not think the band will
make it as far north as Rexburg today, so thus have left this zone
out of an Advisory. If the northward push suggested by the NAM
materializes, may have to include Idaho Falls as well. For areas
between I-86 and HWY 20, expect snowfall rates of 1-1.5 inches per
hour to continue through much of this morning, resulting in 6-12
inches of snowfall for these areas. Similar totals expected for
the Southern Mountains as well, although heavier snow will hold
off here until this evening and overnight tonight. Bottom line is
snowfall amounts will be highly variable across SE Idaho, and
future forecast updates will likely be needed to hone in on
rapidly evolving conditions.

Should see westerly winds increase across the Eastern Magic
Valley, Southern Highlands and southern Snake Plain by this
afternoon, with gusts around 40 mph possible across the Southern
Highlands. This will result in blowing snow which will limit
visibilities, making travel hazardous. AD/RS

LONG TERM...Saturday through Wednesday. A couple of little waves
embedded in northwesterly flow on Saturday keep a chance of snow in
the forecast, but not with a lot of accumulation. The main focus is
Sunday and Monday with the GFS bringing the next disturbance in by
early Sunday morning and periods of off and on snow that could last
into Tuesday morning. So far model consensus is favoring this
timing. Wednesday and beyond would then favor week high pressure
ridging and the storm track slipping northward. RS

AVIATION...Snow this morning is circulating around a 700 mb low
centered a little northwest of KBYI with a dry pocket roughly over
KBYI. Center of the circulation is expected to drift south-southeast
this morning, then the 700 mb circulation gradually consolidates
more over Wyoming early this afternoon. Using the RAP model for
guidance, the snow moves over KBYI about 12Z and the dry slot shifts
over KPIH near 15Z. KBYI may not emerge from the snow until 03-05Z
this evening, and for KPIH there is some uncertainty if the field
will fall back under the snow 21-24Z this afternoon. The RAP also
develops a band over KIDA 13-18Z, fallowed by just vcsh this
afternoon. Although the GFS brings KIDA back under a snow band 22Z
and keeps it going through Friday morning. KSUN will have a risk of
snow 10-13z this morning, then the NAM is the only model with snow
moving back over KSUN this afternoon. So far the NAM looks a little
off the mark this morning. RS

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 Sunday and
Monday with daytime highs approaching 30 for our Northern areas and
middle to upper 30s for our Southern areas. That could bring
some melting. Wyatt/Survick

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 Sunday and
Monday with daytime highs approaching 30 for our Northern areas and
middle to upper 30s for our Southern areas. That could bring
some melting. Wyatt/Survick

&&

.PIH WATCHES/WARNINGS/ADVISORIES...
Winter Storm Warning until 11 AM MST Friday for IDZ017-021-022.

Winter Weather Advisory until 11 AM MST Friday for IDZ020-
023>025.

&&

$$



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