Area Forecast Discussion
Issued by NWS Spokane, WA

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
000
FXUS66 KOTX 041227
AFDOTX

Area Forecast Discussion
National Weather Service Spokane WA
427 AM PST Sun Dec 4 2016

.SYNOPSIS...
Deep low pressure will bring rain, wind, and heavy mountain snow
to the Inland Northwest today. The heaviest snow will occur over
the Cascade Crest, Northeast Blue Mountains, and the mountains of
the Idaho Panhandle. A second round of light snow is forecast for
the Columbia Basin, Palouse, and southern Idaho Panhandle on
Monday. More snow will possible Thursday into Friday in most
areas.


&&

.DISCUSSION...

...HEAVY MOUNTAIN SNOW TODAY FOLLOWED BY COLD TEMPERATURES FOR
MUCH OF THIS WEEK...

Today and Tonight: Early morning radar and satellite imagery shows
our mountains being buffeted by heavy snow. A cold low pressure
trough over the Gulf of Alaska is descending southward this
morning. The interaction between a deep plume of Pacific moisture
with this advancing area of low pressure has produced widespread
precipitation over western Washington. The strong westerly flow
accompanying this system has contributed to an abrupt rain shadow
in the lee of the Cascades which extends well into eastern
Washington. Precipitation today will mainly be driven by
orographic ascent with the Cascade Crest and mountains of the
Idaho Panhandle receiving the brunt of the precipitation.

* Snow: This storm will likely produce up to 2 feet of snow over
  the Cascade Crest. Snow will continue in earnest through the
  morning hours, then decrease this afternoon as moisture sinks
  south into Oregon. Web cameras show a couple inches of
  accumulation as far east as Lake Wenatchee. The Leavenworth and
  Stehekin areas have remained too warm for accumulation. For the
  Central Idaho Panhandle, we expect 5 to 11 inches with most of
  the accumulation above 3000 feet. The heaviest precipitation
  will occur this morning through early afternoon. The snow level
  on the Camas Prairie is around 3500 feet with web cams at
  Winchester and Craigmont showing some wet accumulations. Snow
  levels will remain around 3500 feet until late in the afternoon.
  Look for 2 to 4 inches of wet accumulation on the Camas Prairie.
  Snow levels over the Blue Mountains will also be in the
  3500-4000ft range with 4 to 7 inches by early this evening.

* Winds: Breezy southwest winds this morning will transition to
  north or northwest this afternoon and evening. Strong cold
  advection from the north will steepen our low level lapse rates
  this afternoon. Sustained winds in the 15 to 20 mph range will
  be common over much of central and eastern Washington with gusts
  up to 35 mph in the Okanogan Valley, Upper Columbia Basin, and
  over the Palouse. Surface pressure gradients are expected to
  relax this evening leading to decreasing winds region-wide.
  Light winds and clearing skies will lead to rapidly falling
  temperatures. Afternoon low temps will fall into the teens and
  low 20s...our coldest night so far this season.

Monday: Light snow accumulations will be possible on Monday as low
pressure currently over the Gulf of Alaska moves across Washington
during the afternoon and evening. Our temperatures on Monday will
only reach the upper 20s and lower 30s. Chilly surface
temperatures combined with steep lapse rates under the cold upper
low will lead to relatively high snow to liquid ratios. At this
time, the GFS and ECMWF forecast the track of a 850-700mb low from
Ellensburg to Connell to Walla Walla. This track has the potential
to produce a band of 1 to 2 inches of accumulation over the
Columbia Basin, Palouse, and Camas Prairie. Dry north/northeast
winds will limit the northern extent of the snow with up to an
inch for Spokane and little to no accumulation for Colville,
Sandpoint, and Bonners Ferry. /GKoch

Tuesday and Wednesday: We switch to north-northwest flow Tuesday
as a weak upper ridge approaches from the west. Some snow showers
could linger in extreme southeast Washington on Tuesday, but
otherwise the region will dry out with clearing skies on Tuesday
and into Wednesday, before the next system arrives Thursday. The
biggest story for this period will be the extreme cold
temperatures ushered in by this northerly flow, certainly the
coldest temperatures we`ve seen so far this season. Though models
have been trending slower with the passage of the next system,
temps still look to bottom out Wednesday morning with lows in the
single digits to teens. This puts us 10-15 degrees below normal
for this time of year. I dropped temps for Thursday morning a bit
as clouds will likely be slower to move in overnight giving us
more time to radiate. /bwilliams

Thursday through Sunday: Accumulating snow looks to return to the
Inland NW late in the week as temperatures gradually warm. The
jet stream will send a series of moist Pacific storms systems into
the region.

One minor change looks to be a slightly slower onset of the
precipitation. ECMWF has slowed down the onset of the precipitation
by almost 12 hours. Compared to the bullish GFS. Although the 00z
GFS has lowered its qpf so it`s total qpf seems more in line with
the ECMWF solutions. Each of these models has been following its
own independent solutions. The GFS has trended with a stronger
upper low settling over Vancouver Isle Thursday afternoon, while
the ECMWF has trended toward a more westward placement of the low
and a stronger ridge over the region. In fact, the 00z GFS seems
to be one of the more aggressive solutions within the ensemble
members. At this point, will follow a compromise among the
solutions.

Given the cold snap early in the week and very low snow levels,
the precipitation type looks to be snow in most areas initially.
The main question is how much. Models again are not consistent on
amounts beyond day five. Yet the warm air advection and strong
isentropic lift brings a good possibility of widespread
significant snowfall for Thursday into Friday morning in most
locations. Showery and locally breezy conditions will develop by
Friday night into Saturday with the mountains receiving more of
the snow and a possible rain/snow mix in the Columbia Basin,
Wenatchee Valley and LC Valley. There is the potential for another
winter system to arrive by the end of the weekend with another
round of snow and low elevation rain. Overall temperatures will be
trending to slightly above normal levels by the weekend. /rfox.

&&

.AVIATION...
12Z TAFS: A strong westerly jet will produce orographic
precipitation this morning. The rain shadow in the lee of the
Cascades is extending as far east as the Idaho border.
Precipitation over the Panhandle will be primarily rain below 3000
feet this morning. The arrival of a cold front early this
afternoon will shunt the plume of Pacific moisture southward, and
precipitation will transition to rain/snow showers as drier cooler
and drier air advects southward. The mountains will be pommeled
with heavy snow today, but our most of our airports will get
little to no accumulation. Gusty west or northwest winds will
impact most airports today through the 00z-02z time frame./GKoch

&&

.PRELIMINARY POINT TEMPS/POPS...
Spokane        38  21  27  17  24   8 /  20  10  40  50  10  10
Coeur d`Alene  38  21  27  15  21   6 /  80  10  30  30  10  10
Pullman        40  20  29  19  26  11 /  90  10  50  50  20  10
Lewiston       47  27  34  24  31  18 /  90  20  30  50  20  10
Colville       40  17  28  15  25   9 /  20   0  10  30  10  10
Sandpoint      37  20  26  13  19   3 / 100  10  20  20  10  10
Kellogg        35  18  25  11  19   3 / 100  20  20  30  10  10
Moses Lake     47  22  33  22  30  15 /  10   0  50  40  10   0
Wenatchee      45  23  32  22  29  15 /  10   0  50  40  10   0
Omak           38  17  28  17  26  11 /  10   0  30  30  10   0

&&

.OTX WATCHES/WARNINGS/ADVISORIES...
ID...Winter Weather Advisory until 10 PM PST this evening above 4000
     feet for Lewis and Southern Nez Perce Counties.

     Winter Storm Warning until 7 PM PST this evening above 3000 feet
     for Central Panhandle Mountains.

WA...Winter Weather Advisory until 10 PM PST this evening above 4000
     feet for Northeast Blue Mountains.

&&

$$



USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.