Area Forecast Discussion
Issued by NWS Spokane, WA

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FXUS66 KOTX 290013
AFDOTX

Area Forecast Discussion
National Weather Service Spokane WA
413 PM PST SUN DEC 28 2014

.SYNOPSIS...
Generally light snow showers will continue tonight and into early
Monday morning. Bitterly cold northeast winds will develop Monday
morning and continue into Monday night. Temperatures will be well
below average with dry conditions most of next week as arctic high
pressure settles over the Inland Northwest.


&&

.DISCUSSION...
Update sent to allow the last of the winter weather highlights to
expire. Snow showers will be common across the region tonight with
brief periods of moderate snow possible but overall accumulations
will be light. Any location that receives multiple showers could
see upwards of 2 inches with the Blue Mtns and Camas Prairie
carrying the highest probabilities and forecast in these areas
will keep likely wording for snow through the overnight periods.
/sb

&&

.AVIATION...
00Z TAFs: Scattered snow showers will continue over and near the
eastern TAF sites through 06Z as an arctic front surges into the
region from the northeast. Most eastern TAF sites will experience
MVFR conditions and possibly brief IFR conditions as showers pass
through this evening. After 06z, an arctic cold front will surge
southward bringing increasing north/northeast winds and the
potential for a period of light snow with the front to the KMWH
and KEAT TAF sites while the eastern TAf sites begin to clear out.
The easterly upslope component of the low level wind may produce
some light runway accumulations at Wenatchee toward sunrise on
Monday. After 12Z-15Z Monday much drier air will lead to improving
conditions at all TAF sites...likely to VFR after 18Z. /MJF


&&

.PREV DISCUSSION... /ISSUED /



DISCUSSION...

LINGERING SNOW SHOWERS FOLLOWED BY WINDY CONDITIONS AND BITTER
COLD TEMPERATURES.

Tonight through Monday night...An upper level trough continues to
carve into the region from the north. While the risk of more light
snow depositing light accumulations still exists especially
tonight over the deep basin and Cascades...the main weather threat
will become the arrival of an Arctic high pressure bearing bone
chilling cold and locally very windy conditions as this bitter and
dry dense arctic air surges into the region through northern
mountain gaps.

Residual snow threat: The incoming Arctic front moving in from the
northeast will interact with a weak follow-on upper wave visible
on satellite dropping out of southern BC tonight. Although moisture
availability is waning as much drier arctic air filters in...not
much moisture is necessary in this cold air mass to enhance into
an area of snow showers as these two systems interact...augmented
by upslope flow into the Cascades as winds become northeasterly. A
period of light snow is expected to develop west of a line from
Moses Lake to Republic tonight. Up to an inch or so of fluffy dry
snow is probable over the basin and 1 to 3 inches in the Cascade
Valleys. East of this line scattered snow showers will prowl along
the arctic front with local further minor accumulations (dusting
to 1/2 inch) possible just about anywhere. The entire region will
dry out on Tuesday with sunshine likely especially over the
northern zones as bone dry air moves in. Current winter storm
highlights for the Idaho Panhandle will be allowed to expire at 4
pm with no more snow related highlights expected.

Wind and cold: The mass of dense dry Arctic air will begin to
squeeze through the Okanogan Valley and the Purcell Trench of
North Idaho tonight. Gradient analysis from local studies suggest
the incoming high pressure constricting the flow of this low level
dense air through the gaps will lead to Wind Advisory criteria (30
to 40 mph with gusts to 50 mph) being achieved during the day
Monday and Monday evening...certainly in the Okanogan Valley and
probably in the Purcell Trench. Wind Advisories will be issued for
these locations with the afternoon package. Breezy to windy
conditions out of the northeast will develop over the entire
region on Monday as the air mass exchange takes place. This also
raises the second threat which will be frigid wind chills by
Tuesday morning over much of the area. Fortunately the winds will
be slackening as the temperature really plunges late Monday
night...not quite achieving the Wind Chill Warning criteria of
-20 but coming close in the Purcell Trench and Coeur D`Alene area
by dawn Tuesday. /Fugazzi

Tuesday through Thursday: The region will be under dry northerly
flow. The main story for this period will be the cold
temperatures. Overnight low temperatures will be bitterly cold
with below zero temperatures possible for areas that have snow on
the ground. Strong inversions will also set up across the Inland
Northwest. This will result in poor mixing through the period with
high temperatures only warming into the teens for most locations.
Tuesday is expected to be the coldest day. Then a gradual bump up
in temps is expected Wednesday and Thursday, but will still be
very cold each day. /SVH

Thursday night through Sunday...Subtle changes to the steering
flow from the persistent pattern consisting of a narrow yet highly
amplified ridge with axis placement off the coast. The ridge
appears to deamplify as disturbances run over the top and west
side of it starting Thursday night. The ridge may flatten out
Friday and beyond to allow for stratiform overrunning
precipitation regardless of elevation as Eastern Washington and
Northern Idaho will still be quite cold. The most favored ECMWF
model holds off on such an occurrence until Saturday night that
continues through the weekend. The GFS is a bit more aggressive in
a similar solution starting as early as Friday. With the timing
difference in mind may just continue the earlier trend in which
minor pops are introduced Thursday night that increase in
magnitude and areal coverage over the weekend. Latent heat release
from the precipitation should allow for slight warming trend
starting Thursday that runs through most of this interval.
/Pelatti

AVIATION...
18Z TAFs: Morning IFR and LIFR conditions at the KGEG...KSFF and
KPUW TAF sites will gradually improve as a cold unstable air mass
promotes mixing in the lower levels but also creates the
possibility of brief MVFR conditions in passing snow showers
particularly at the KPUW and KCOE TAF sites and possibly the KMWH
TAF site...but the scattered nature of these showers makes
confidence that any particular location will get hit only low to
moderate. Snow showers will be concentrated over the Idaho
Panhandle mountains today. After 06z, an arctic cold front will
surge southward bringing increasing north/ northeast winds and the
potential for a period of light snow with the front. The easterly
upslope component of the low level wind may produce some light
runway accumulations at Wenatchee toward sunrise on Monday. /MJF
18Z TAFs: Morning IFR and LIFR conditions at the KGEG...KSFF and
KPUW TAF sites will gradually improve as a cold unstable air mass
promotes mixing in the lower levels but also creates the
possibility of brief MVFR conditions in passing snow showers
particularly at the KPUW and KCOE TAF sites and possibly the KMWH
TAF site...but the scattered nature of these showers makes
confidence that any particular location will get hit only low to
moderate. Snow showers will be concentrated over the Idaho
Panhandle mountains today. After 06z, an arctic cold front will
surge southward bringing increasing north/ northeast winds and the
potential for a period of light snow with the front. The easterly
upslope component of the low level wind may produce some light
runway accumulations at Wenatchee toward sunrise on Monday. /MJF

&&

.PRELIMINARY POINT TEMPS/POPS...
Spokane        18  21   5  12   1  14 /  50  10   0   0   0   0
Coeur d`Alene  17  20   1  12   1  14 /  60  10   0   0   0   0
Pullman        19  23   2  11  -2  14 /  50  40  10   0   0   0
Lewiston       24  29   7  15   4  17 /  60  50  10   0   0   0
Colville       20  24   2  17   0  19 /  50  10   0   0   0   0
Sandpoint      18  20   0  12  -1  14 /  50  10  10   0   0   0
Kellogg        15  17  -4   8  -3  16 /  70  10  10   0   0   0
Moses Lake     23  29  11  20   4  17 /  50  20  10   0   0   0
Wenatchee      25  30  12  21  10  21 /  70  60  10   0   0   0
Omak           21  26   6  15   3  16 /  60  20   0   0   0   0

&&

.OTX WATCHES/WARNINGS/ADVISORIES...
ID...Wind Advisory from 8 AM to 10 PM PST Monday for Coeur d`Alene
     Area-Northern Panhandle.

WA...Wind Advisory from 10 AM to 10 PM PST Monday for Okanogan Valley.

&&

$$



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