453 
FXUS66 KOTX 211744
AFDOTX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SPOKANE WA
945 AM PST Thu Feb 21 2013

.SYNOPSIS...An upper level disturbance will bring some light snow
to the Idaho Panhandle and portions of Northeast Washington today. A
much stronger storm will plow into the Pacific Northwest on Friday
and Friday night bringing windy conditions and heavy snow to the
mountains. Mountain snow showers will likely linger into Saturday
afternoon. A break in the active weather is expected on Sunday,
before another frontal system brings increasing chances for rain
and snow to the region on Monday.

&&

.DISCUSSION...
Today and tonight: 

A well-defined vort max will accompany a very swift moving frontal
passage this morning, with a few inches of snow expected in
mainly the mtns of NE Wa and the Id Panhandle. Though the low-
level flow backs briefly to the SW, there's not a whole lot of
waa/implied isentropic ascent to produce any heavy snow threat.
However, strengthening westerly upslope into the Id Panhandle will
translate into a few inches of snow especially for elevations
above 3000 feet. The beginning of a pattern change to a much
wetter, snowier, and windy fcst will begin with the first initial
onslaught of strengthening west winds Thurs Nt. This will help
maintain the briefly decreasing light snow threat for the Id
Panhandle Thur Nt. It's late Thurs Nt that this flow will rapidly
back to the S/SW ahead of the strong cyclone moving down the BC
coast that will begin to ramp up the deep and strong ascent over
the warm front that will be near the Cascades by late Thurs Nt. bz

Friday and Saturday: A potent winter storm system will impact the
Pac NW bringing a mix of rain and snow to the valleys and
potential for moderate to heavy snow in the mountain. The storm
system will also bring breezy to gusty winds across a majority of
the Basin...Palouse...West Plains and exposed ridgetops. Models
are generally in good agreement with the large scale features but
subtle differences do still exist with the related to the timing
of rising snow levels Friday morning...and snow amounts within the
convective post-frontal air mass Friday night and Saturday.

The storm system of interest is currently visible on this evening's 
Water Vapor imagery as a sharp area of darkening off the western
tip of the Aleutian Islands. This storm system will deepen quite a
bit over the next 24-48 hrs and dive southeast into the
Northwestern US. A rich fetch of 1.00" precipitable water will be
driven toward the region ahead of a strong warm front leading to
increasing precipitation Friday morning and afternoon as the
moisture rises along the 285-300K isentropic surfaces.
Precipitation will be at it highest intensity Friday afternoon
into the early evening as lift becomes enhanced by the approaching 
cold front. Some clearing will be possible behind the front but
the air mass will become increasingly unstable Friday night and
Saturday within the cusp of the upper-level trough leading to
continued mountain snow showers and hit or miss valley snow
showers. Some of this activity will be heavy at times and the most
pronounced orographics will focus over the Idaho Panhandle and
along the immediate Cascade Crest. We will be closely watching for
a Puget Sound Convergence Zone bringing burst of snow over the
crest into locations like Plain, Lake Wenatchee, and Stehekin.

Precipitation is expected to start off as snow for most lowlands 
north of I-90 and across the Palouse. As warmer air, associated
with the subtropical moisture fetch, punches northward within a
40-50kt low level jet, snow levels will be rapidly rising near
3000 feet and most valleys will transition to rain or at least
...non-accumulating snow. Exactly when this transition occurs for
locations along Hwy 2 and northward carries considerable
uncertainty and could have large ramifications on valley snow
amounts. Given the speeds within the LLJ and daytime timing...a
non NAM solution was preferred which brings the transition into
Spokane-CDA around late morning and toward the Canadian Border
early afternoon.

The main changes that we made to the previous forecast were to 
increase snow amounts in the Idaho Panhandle Mountains, including
IDZ001. As a result, we have included this zone into the current
watch which was already in place for the Central Panhandle Mtns.
There is a strong possibility that winter highlights will be
necessary for the Northeastern Mtns of WA, Cascade Crest, Camas
Prairie, and perhaps the Okanogan Highlands but amounts/coverage
were lower and did not warrant winter storm criteria...and we will
cover this with the HWO at this time. We also expanded the watch
for the Northern Blue Mtns through 11PM Sat night as post frontal
orographic flow orients to the west/northwest during this time.
Otherwise, inherited snow amounts were generally on track with the
heaviest amounts focusing over the Central Panhandle Mtns and Blue
Mtns.

The storm system will also bring the likelihood for breezy to gusty 
winds with our confidence low whether the 40-50kt LLJ will be able
to surface to valley floors. Typically...these winds remain just
off the surface under such stable profiles and Bufkit data would
agree with gusts largely in the 30 to 35 kt range. However, this
is a rather strong cold front...vertically stacked from the
surface to 500mb and this is worrisome. The track of the main
surface low (southern Alberta) along with a 16-18mb pressure
gradient expected between Portland and Kalispell is a common
pattern for wind highlights, mainly advisory speeds, across the
CWA and this is almost a certain. We will be taking closer look into the
higher resolution guidance tomorrow to determine if stronger gusts
will surface with the passage of the front between 00-06z Friday
evening to assure advisory speeds is all we can expect.

The weather will remain active following this storm system with 
at least three more storm systems through next week. /sb

&&

.AVIATION...
18Z TAFS: A TROF is currently moving through the region. The eastern 
TAF sites can expect to have MVFR to IFR ceilings throughout the 
next 24 hours with periods of snow that will drop vis around MVFR. 
KEAT and KMWH can expect mid to high level clouds to continue with 
MVFR to VFR conditions. KLWS will be impacted by low to mid level 
clouds with periods of rain showers. 

&&

.PRELIMINARY POINT TEMPS/POPS...
Spokane        37  31  42  29  40  24 /  80  50 100  90  30  20 
Coeur d'Alene  36  31  41  29  38  25 /  90  70 100 100  40  30 
Pullman        38  32  43  31  39  25 /  90  60  90  90  50  30 
Lewiston       44  35  51  34  47  29 /  20  20  60  80  60  30 
Colville       39  32  40  24  44  24 /  50  40 100  80  20  20 
Sandpoint      33  31  40  29  38  25 / 100  80 100 100  50  30 
Kellogg        36  31  40  29  37  26 / 100 100  90 100  70  40 
Moses Lake     45  30  52  31  51  26 /  10  10  70  30  10  10 
Wenatchee      41  31  47  30  46  26 /  10  10  80  30  10  10 
Omak           36  28  41  22  41  20 /  10  10  90  30  10  10 

&&

.OTX WATCHES/WARNINGS/ADVISORIES...
ID...Winter Storm Watch from Friday afternoon through Saturday 
     afternoon for Central Panhandle Mountains-Northern 
     Panhandle.

WA...Winter Storm Watch from Friday morning through Saturday evening 
     for Northeast Blue Mountains.

&&

$$