Heavy Snow in Idaho, February 18 to 19, 2012

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SPOKANE WA
348 PM PST Sat Feb 18 2012

.SYNOPSIS...
Snow showers and windy conditions will continue to impact the
region through Sunday as an upper level trough moves through the
forecast area. These showers will linger across the Idaho
Panhandle, followed by another round of lighter snows Monday as a
series of quick moving systems continue through much of the
upcoming week.

&&

.DISCUSSION...

Tonight...A cold upper trough will pass through the Inland
Northwest tonight bringing snow showers to the Cascades, Eastern
Washington, and North Idaho. 850mb winds out of the southwest-west
at 25-35 knots will result in a strong upslope component into the
mountains of Northeast Washington, Blue Mountains, and North Idaho
further enhancing snowfall amounts. Very moist low levels and
upslope into the Coeur D'Alene area and Idaho Palouse will also
enhance snow totals with model qpf and snow ratios around 15:1
yielding 2-3 inches. Meanwhile bands of heavy snow showers may
develop due to deep instability and a vort max passing through.
Thus the winter weather advisories will be expanded to include
these areas. Meanwhile strong downslope flow in the lee of the
Cascades will result in a precipitation shadow with Wenatchee and
Moses Lake likely remaining dry tonight. The NAM and MM5 GFS
models also indicate a rain shadow in the lee of the Blue
Mountains with Lewiston possibly remaining dry as well. Concerning
winds...Cold air advection this evening along with a weakening sfc
low passing through Southern British Columbia will continue to
produce gusty south-southwest winds except in the Cascade valleys
which given west-east valley orientation will remain sheltered.
Then as the low pulls away from the area overnight winds would
should gradually decrease. JW

Sunday through Tues: Following the exit of the short-wave trof
thats the main snow producer for the Sunday morning heavy snow
across the Id Panhandle, rising hts and transient short-wave
upper ridging will accompany a very brief break in the pcpn
threat before the next quick-moving wave crosses the region Monday.
Due to its low-amplitude/quick transit through cntrl Wa, we're not
expecting a large amnt of pcpn. However, steep mid-level lapse
rates accompanying the compact vort max will allow for a
respectable response for any forcing for ascent from any large-
scale (vort advection, waa/isentropic) lift, or even deep orographic
forcing for ascent. The result will be high enough pcpn rates to
produce a couple tenths liquid equivalent on average for far Ern
Wa and the Id Panhandle Mon into Tues morning. Very broad upper
ridging will then build over the region Tues. Though this will
favor dry wx for the Columbia Basin, deep west/nw flow embedded
in a somewhat rich moisture plume along the warm conveyor belt
and maritime cold front will produce a steady episode of pcpn
near the Cascade crest as well as across the Cntrl Id Panhandle
Mtns. Rising snow levels however will limit any significant snow
accumulations to the mountains above about 3500 feet. Windy
conditions are still expected on Tues in this favorable Columbia
Basin shadow pattern. bz

Tuesday night through Saturday: after a broader threat of
precipitation at the start of the period, drying and a northwest
flow shifts the primary threat to the eastern fringes of the CWA
until the next frontal wave by the end of the week. Between
Tuesday night and Wednesday low pressure tracks east and southeast
through Canada toward the northern Plains. A trailing cold front
drops in while a jet streak skims across the northeast half of the
CWA. A plume of moisture drawn into this system (driving PWATS to
between 150-175% of normal) will bring a good threat of
precipitation to the Cascades, the eastern mountain zones and the
Palouse through Blues. Moderate chances will be found over the
remainder of the eastern third of Washington but wanes to just
slight chances in the lee of the Cascades and Columbia Basin in
the down-slope west/northwest flow. With 850 mb temperatures in
the single digits (above and below zero) and wet-bulb zero heights
near 1-2kft in the north and between 3-5kft over the south, look
for a snow or a rain/snow mix for the north and eastern mountains,
with primarily rain south.

From Wednesday night through Thursday night the cold front sags
to the WA/OR border and the atmosphere dries out (with PWATs
dropping to between 40-70% of normal). This will lead to a
declining threat of precipitation over much of the CWA. The
primary threat will shift into the central Panhandle through
Blues, with slight chances also lingering near the Cascades. This
will be accompanied by an overall cooling trend too as 850mb
temperatures drop to between -3 and -6 degrees C.

Friday through Saturday the next frontal wave comes into the
region. Models are in fair agreement with the overall trend but
precise timing issues are subject to change. Look agreement
indicates the leading warm front and overrunning moisture begins
to envelope the region through the day Friday, renewing the threat
of precipitation across the region by afternoon. The accompanying
cold front passes Friday night into early Saturday morning, while
the more unstable and showery upper trough enters through the
remainder of Saturday. This will keep the threat of precipitation
going though a transition to a more showery regime seems more
likely. With 850mb temperatures initially lingering below zero
(down to near -5 or -6 C), the primary precipitation type looks to
be snow for areas that see precipitation beginning earlier Friday.
The deepening southerly flow and milder air surging in ahead of
the accompanying cold front Friday afternoon into Saturday morning
will allow for the potential for rain or rain/snow mix into the
Upper Columbia Basin through Palouse and Spokane/C'DA area. The
precise details will continue to be fine-tuned as models come into
better agreement. /J. Cote'

&&

.AVIATION...
00Z TAFS: Unstable upper trough migrating through the region will
bring periods of rain and snow to the eastern WA/nrn ID TAF sites
this evening and overnight. Expect MVFR/lcl IFR conditions to
develop over the KGEG-KCOE and KPUW areas toward 01-09Z with these
increasing showers. The precipitation threat will begin to wane
later overnight into early Sunday, with conditions improving toward
VFR in these locations between 17-21Z. KMWH, KEAT and KLWS will
primarily see isolated shower chances tonight, with VFR conditions.
Breezy conditions Saturday evening will dissipate after 03-06Z,
with some weak renewal in winds Sunday afternoon. /J. Cote'

&&

.PRELIMINARY POINT TEMPS/POPS...
Spokane        29  38  27  35  27  40 /  60  20  10  50  30  40
Coeur d'Alene  30  37  26  35  27  39 /  90  30  10  50  30  50
Pullman        30  38  29  37  28  47 /  70  50  10  50  40  50
Lewiston       35  45  32  42  31  50 /  40  40   0  50  30  20
Colville       22  40  27  35  25  42 /  80  20  20  50  30  30
Sandpoint      29  37  26  35  26  38 /  90  50  20  40  40  50
Kellogg        28  36  27  34  25  38 / 100  80  20  60  50  70
Moses Lake     27  45  28  42  29  53 /  10  10   0  30  10  10
Wenatchee      27  42  27  38  29  47 /  20  10  10  20  20  20
Omak           20  38  26  36  23  42 /  20  10  30  40  20  10

&&

.OTX WATCHES/WARNINGS/ADVISORIES...
ID...Winter Weather Advisory from 7 PM this evening to 10 AM PST
     Sunday for Coeur d'Alene Area-Idaho Palouse.

     Winter Weather Advisory until 10 AM PST Sunday for Central
     Panhandle Mountains-Northern Panhandle.

WA...Winter Weather Advisory until 10 PM PST this evening for East
     Slopes Northern Cascades.

     Winter Weather Advisory from 7 PM this evening to 10 AM PST
     Sunday for Northeast Blue Mountains.

     Winter Storm Warning until 4 AM PST Sunday for Northeast
     Mountains.

&&

HAZARDOUS WEATHER OUTLOOK...UPDATED
NATIONAL WEATHER SERVICE SPOKANE WA
316 PM PST SAT FEB 18 2012

IDZ026-027-WAZ032>036-038-041-043-044-192315-
LEWISTON AREA-LEWIS AND SOUTHERN NEZ PERCE COUNTIES-
LOWER GARFIELD AND ASOTIN COUNTIES-WASHINGTON PALOUSE-
MOSES LAKE AREA-UPPER COLUMBIA BASIN-SPOKANE AREA-
OKANOGAN HIGHLANDS-WENATCHEE AREA-OKANOGAN VALLEY-
WATERVILLE PLATEAU-
316 PM PST SAT FEB 18 2012

THIS HAZARDOUS WEATHER OUTLOOK IS FOR PORTIONS OF NORTH CENTRAL
IDAHO...CENTRAL WASHINGTON...EAST CENTRAL WASHINGTON...NORTH
CENTRAL WASHINGTON...NORTHEAST WASHINGTON AND SOUTHEAST
WASHINGTON.

.DAY ONE...REST OF TODAY AND TONIGHT

NO HAZARDOUS WEATHER IS EXPECTED AT THIS TIME.

.DAYS TWO THROUGH SEVEN...SUNDAY THROUGH FRIDAY

NO HAZARDOUS WEATHER IS EXPECTED AT THIS TIME.

IDZ001>004-WAZ037-192315-
NORTHERN PANHANDLE-COEUR D'ALENE AREA-IDAHO PALOUSE-
CENTRAL PANHANDLE MOUNTAINS-NORTHEAST MOUNTAINS-
316 PM PST SAT FEB 18 2012

THIS HAZARDOUS WEATHER OUTLOOK IS FOR PORTIONS OF NORTH IDAHO AND
NORTHEAST WASHINGTON.

.DAY ONE...TONIGHT AND SUNDAY

WINTER STORM WARNINGS AND ADVISORIES HAVE BEEN ISSUED FOR THESE
AREAS. THE THREAT OF MODERATE TO HEAVY SNOW SHOWERS WILL
CONTINUE ITO SUNDAY. THE HEAVIEST SNOW AMOUNTS WILL OCCUR OVER
THE MOUNTAINS AND WILL BE ACCOMPANIED BY BLOWING OR DRIFTING SNOW
ON RIDGES FROM OCCASIONAL WIND GUSTS OF 35 TO 50 MPH.

.DAYS TWO THROUGH SEVEN...MONDAY THROUGH FRIDAY

NO HAZARDOUS WEATHER IS EXPECTED AT THIS TIME.

WAZ031-042-192315-
NORTHEAST BLUE MOUNTAINS-EAST SLOPES NORTHERN CASCADES-
316 PM PST SAT FEB 18 2012

THIS HAZARDOUS WEATHER OUTLOOK IS FOR PORTIONS OF NORTH CENTRAL
WASHINGTON AND SOUTHEAST WASHINGTON.

.DAY ONE...TONIGHT THROUGH SUNDAY

A WINTER WEATHER ADVISORY HAS BEEN ISSUED FOR THE EAST SLOPES OF
THE CASCADES...ESPECIALLY FOR LOCATIONS NEAR THE CASCADE CREST...AS
WELL AS THE NORTHERN BLUE MOUNTAINS OF SOUTHEAST WASHINGTON.
PERIODS OF MODERATE TO LOCALLY HEAVY SNOW SHOWERS WILL CONTINUE
OVERNIGHT WITH HEAVY ACCUMULATIONS EXPECTED.

.DAYS TWO THROUGH SEVEN...MONDAY THROUGH FRIDAY

NO HAZARDOUS WEATHER IS EXPECTED AT THIS TIME.