Freezing Rain Advisory 3, NWS Spokane, WA

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SPOKANE WA
348 PM PST Sat Nov 30 2013

.SYNOPSIS...
Big changes will come to the Inland Northwest as a strong low
pressure system begins to impact the region late tonight.
Initially this system will deliver warm conditions and widespread
precipitation. Snow will be possible over the valleys of northern
Washington and north Idaho early Sunday...however snow levels
will rise above the valley floors during the day. Heavy snow will
likely continue over the mountains. The widespread precipitation
will gradually come to an end Monday night...as a cold arctic air
mass pushes into the region. Temperatures will plunge to some of
the coldest the region has experienced over the past couple years.
The cold will likely persist through the week.

&&

.DISCUSSION...
Tonight: This evening will be the calm before the storm. The Gulf
of Alaska low is starting to move south. A warm front will push
towards Eastern WA through the evening and overnight hours.
Temperatures tonight will only vary about 3-5 degrees from our
afternoon highs. Temperatures will likely remain near stationary
through the night. Very nice isentropic lift occurs overnight with
the chance of precipitation increasing through the night. Most
locations will see rain. However locations up across northern WA
and north ID...ie. Republic, Colville, Sandpoint, etc. will be
hovering right around freezing where they could pick up around an
inch of snow overnight. /Nisbet

...STRONG WINTER STORM TO DELIVER HEAVY SNOW TO THE
MOUNTAINS...GUSTY WINDS...AND SIGNIFICANTLY COLDER TEMPERATURES
TO THE INLAND NORTHWEST...

Sunday through Monday...Complex weather period still awaits much
of the Inland Northwest as nice plume of sub-tropical moisture
pools ahead of unusually strong upper level trough and sends a
series of fronts through the area. The latest water vapor
satellite loops are showing a broad plume of moisture extending
from about 160w to just off the Washington coast and moving
steadily eastward. Meanwhile...there was a strong arctic boundary
plunging southward from the Gulf of Alaska. Looks like this event
can really be split into two regimes. First is the sub-tropical
air mass which will become the focus for late tonight through
Sunday night or early Monday morning. This is when the bulk of the
precipitation is expected to fall via a well-defined isentropic
ascent pattern and mid-level warm front. Precipitable water
amounts are expected to nearly double the normal values for this
time of year. As is usually the case with such a moist air
mass...its quite warm and as a result...snow is generally not an
issue except for the mountains. More on that below. Given the
strength of the isentropic ascent...it looks like all locations
will be subject to measurable rain...even in the lee of the
Cascades as the 850-700 mb westerly flow will be accompanied by
strong warm air advection which generally cancels out the
downslope flow impacts. Looks like this isentropic ascent pattern
will give way on Sunday evening as the models are consistent on
bringing the back edge of the moisture plume through the Cascades
with very little if any warm air advection present. This will
likely bring an end to the widespread precipitation in the lee of
the Cascades...however there will be a slight chance associated
with the passage of the Pacific cold front. Meanwhile...the sub-
tropical moisture plume will continue to sag slowly south as the
Gulf of Alaska low pushes across the WA/BC border on Monday. This
low will open the door to an arctic air mass which will begin to
surge across the International border. While this won't entirely
end the threat of precipitation...as light wrap around will be
possible...the heaviest of the precipitation will have ended
except perhaps over the southern half of the Idaho Panhandle.
Temperatures will likely fall or remain steady through the day
once the front moves in. There are big model differences in timing
the arctic front into the region...with the GFS being the
fastest...while the Canadian and EC keep the upper level low over
Washington through most of Monday...resulting in potential
wraparound precipitation and the development of upslope
showers...especially over the Idaho Panhandle. So here are the
impacts

Snow: Snow will not be an issue in most valley locations.
The exception will occur across northern valley locations
beginning tonight and continuing through Sunday. Locations such as
Republic...Colville...Sandpoint and Bonners Ferry will likely
begin to see snow during the night and continuing into the first
half of Sunday. However as the warmer air streams into the
region...these locations will gradually change over to rain. Its
possible some of these valley locations could see snowfall ranging
from 2 to 5 inches...with locally heavier amounts possible near
the Canadian border. Meanwhile the mountains will accumulate snow
from late tonight through Monday. Snowfall totals in the mountains
of NE Washington and the Idaho Panhandle could see snowfall
amounts ranging from 12 to 24 inches...with the heaviest amounts
expected over the central Idaho Panhandle. Heavy snow will also
fall in the immediate lee of the Cascades...however its generally
at elevations above pass levels. The exception will be the north
Cascade Highway...which will likely be subject to periods of heavy
snow.

Winds:  Winds will generally intensify through Sunday...which
should eliminate our air stagnation issues of the past week. For
much of the area...south to southwest winds will increase through
the day and into the night as a strong low pressure area drops
through BC during the period. South to southwest winds will
intensify as the N-S pressure gradients steadily grow. If we will
need a wind advisory is questionable at this time...however if we
did...it would generally be for the SE corner of
Washington...impacting the Palouse...eastern sections of lower
Columbia Basin...as well as adjacent portions of the Idaho
Panhandle. Typically we do not achieve wind advisory criteria in
the warm sector of a low...and not too sure this will be an
exception. Nonetheless we expect to see widespread winds of 15 to
25 mph with gusts from 35-40 mph possible in these areas. Wind
speeds will likely peak late Sunday night early Monday morning.
Looks like the winds will transition to a north to northeast
orientation once the arctic air moves in late Monday. Then
blustery conditions will develop down the Purcell Trench and
Okanogan Valley. fx

Monday night through Thursday night...A cold upper level low will
slide south across the forecast area Monday night. This will bring
an area of steady snow to the southeast portion of the forecast area
along with scattered snow showers to the remainder of the Inland
Northwest. The most favorable areas will be the NE Blues, Camas
Prairie and the central panhandle mountains of Idaho. Light to
moderate snowfall amounts will be possible Monday night into
Tuesday. There is the possibility of a deformation zone forming over
the southeast zones, bringing the threat of heavy snow bands
developing along this low level boundary. Confidence in the
development, timing and location of such a feature is low at this
point but remains a possibility. As the low moves south the synoptic
flow will shift to the north, ushering in an arctic airmass.
Moisture will be limited in this continental airmass but the
snow-to-liquid ratio will increase dramatically.

These northerly winds will be channeled down the northern valleys,
especially the Okanogan Valley and Purcell Trench. Expect wind
gusts of 30 to 40 mph at times, producing very cold wind chill
temperatures.

By Tuesday night the threat of snow showers will be minimal but
temperatures will continue to plummet. Low temperatures for the
valleys will be in the teens to single digits with little warming
during the day, especially for those locations with fresh snowfall.
The cold and mainly dry conditions will last until at least Thursday
night.

Friday through the weekend...medium range model solutions diverge
for this portion of the extended forecast period. Run-to-run
consistency and model agreement both suffer. For now the weekend
forecast is trended toward climo pops with moderating but still cold
temperatures. /Kelch

&&

.AVIATION...
00Z TAFS: IFR cigs and vsby will continue at most TAF sites this
evening. The well advertised system will begin to spread mainly
-shra over the area late tonight with IFR cigs and vis. Precip for
KGEG and KCOE may start out as -SN or -RASN before changing to
rain as warmer air moves in from the south overnight. Steady
precip will continue through around 18Z for KEAT/KMWH and possibly
through 00Z Monday for remaining TAF sites. /EK

&&

.PRELIMINARY POINT TEMPS/POPS...
Spokane        33  43  36  36  21  23 /  80 100  80  70  30  10
Coeur d'Alene  33  42  36  37  19  21 /  90 100 100  80  30  20
Pullman        36  47  38  39  23  24 /  70 100  80  90  40  30
Lewiston       38  49  41  44  26  28 /  40  90  80  90  50  30
Colville       31  38  33  36  17  20 /  90 100  70  70  30  10
Sandpoint      32  37  35  36  16  18 /  90 100 100  90  30  10
Kellogg        34  40  36  36  15  16 /  90 100 100 100  60  30
Moses Lake     33  47  37  41  24  27 /  50  80  20  10  20  10
Wenatchee      33  42  32  40  23  26 /  60  70  30  10  20  10
Omak           32  39  30  36  16  20 /  60  70  20  10  20  10

&&

.OTX WATCHES/WARNINGS/ADVISORIES...
ID...Air Stagnation Advisory until 10 AM PST Sunday for Central
     Panhandle Mountains-Coeur d'Alene Area-Idaho Palouse-Lewis
     and Southern Nez Perce Counties-Lewiston Area-Northern
     Panhandle.

     Winter Storm Warning from 10 PM this evening to 4 PM PST Monday
     for Northern Panhandle.

     Winter Storm Warning from 10 PM this evening to 4 PM PST Monday
     for Central Panhandle Mountains.

WA...Air Stagnation Advisory until 10 AM PST Sunday for East Slopes
     Northern Cascades-Lower Garfield and Asotin Counties-Moses
     Lake Area-Northeast Mountains-Okanogan Highlands-Okanogan
     Valley-Spokane Area-Upper Columbia Basin-Washington Palouse-
     Waterville Plateau-Wenatchee Area.

     Winter Weather Advisory from 10 PM this evening to 10 AM PST
     Monday for Okanogan Highlands.

     Winter Storm Warning from 10 PM this evening to 4 PM PST Monday
     for Northeast Mountains.

&&