Heavy Snow in Washington, December 24 to 25, 2017

Area Forecast Discussion
National Weather Service Pendleton OR
340 PM PST Sun Dec 24 2017

Updated Aviation Discussion

.SHORT TERM...Tonight through Tuesday...The warm front associated
with the latest Pacific system continues to move through the Pacific
Northwest this afternoon. The main snow band associated with this
warm front extends from north-central Oregon and extreme south-
central Washington southeast to Grant county in east-central Oregon.
This band will continue to move northeast through this evening,
stalling briefly over our northeastern most zones. Thus will see
generally 1 to 4 inches of snow across the Columbia Basin and Gorge,
north-central Oregon and the John Day basin. A period of extended
snow into the Blue mountains and Blue mountain foothills will result
in higher amounts. Also expect 3 to 5 inches to fall over the Grande
Ronde valley as snow slops over from the Blues. Have continued all
current advisories and warnings, plus have added an advisory for the
Grande Ronde valley. Temps are expected to rise this evening and
overnight across portions of central and east-central Oregon. Thus
could see the snow mix with rain or freezing rain, or change over to
rain. Overall precip amounts will be limited in central Oregon
tonight, and in east-central Oregon after midnight (when the warmer
air reaches this area). The system's cold front will move across the
region between midnight and noon Monday. This front will also cause
some precip. The main impact from this cold front will be the
lowering of snow levels in central and east-central Oregon, and
continue upslope induced snow showers into the northern Blue
mountains and the adjacent foothills Monday. Snow showers will also
continue over the Cascade east slopes Monday. Snow showers will
decrease areawide Monday night with generally dry conditions and
continued below seasonal temperatures on Tuesday. 90

.LONG TERM...Tuesday night through New Years Eve...The forecast for
the extended period was challenging. The run-to-run continuity of
the medium range models is poor, as models are flip flopping from an
amplified ridge pattern to a progressive flow pattern back to a
ridge pattern. Believe the progressive flow pattern is the way to go
based on the ensemble runs the past several days. The latest GFS was
the model with a more westerly flow. On Tues night through Wed
morning, a shortwave diving southeast across the forecast area will
bring numerous snow showers, especially across southeast WA and
northeast OR. There will be warmer air aloft bringing a chance of
snow, sleet, and freezing rain to central and north central Oregon
south of Warm Springs. On Wed, the shortwave exits the region but
orographic snow showers will continue over the mountains in a
northwest flow aloft. At least several inches of snow can be
expected over the Cascades and Blue Mountains. The threat of
freezing precipitation will extend farther north into the eastern
Columbia River Gorge and just south of the WA/OR border on Wed, but
only light ice accumulations are expected at this time. From Wed
night through Thu night, a warm front draping across WA state will
bring a significant increase in snow levels beginning in central and
southeast OR Wed night...northeast OR and southeast WA on Thu...then
south central WA Thu night. It will be a gradual warming trend for
areas in and around the Columbia Basin, particularly the Yakima and
Kittitas Valleys. Therefore, mixed precipitation will likely fall
for areas below freezing in the lower elevations. On Fri,
precipitation will transition to all rain for most areas below 5000
feet. The warm front is progged to lift north into far northern WA
Friday night with a transitory ridge this weekend that will bring
primarily dry conditions. Models are hinting at another Pacific
front bringing snow on Sun night. Wister

&&

.AVIATION...00Z TAFs...MVFR, IFR and LIFR conditions will prevail
for the next 24 hours.  Although snow has ended at KRDM and KBDN,
low stratus with CIGS AOB 1000 feet AGL will remain through
Christmas Day. The same will be true at KDLS when snow tapers off in
the next hour or two.  At KYKM, KPSC, KALW, and KPDT...snow is now
developing and there will be accumulating snow along the runways.
Snow will taper off or end at KYKM and KPSC around 14Z-18Z, but MVFR
CIGS will remain.  KPDT and KALW will likely observe periods of snow
through 00Z/26th.  Winds will be 10 knots or less through the
forecast period. Wister

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  18  25  20  28 /  90  60  20  10
ALW  21  27  22  31 /  90  70  20  10
PSC  22  29  22  29 /  90  50  10  10
YKM  22  28  18  29 /  70  20  10   0
HRI  21  29  20  29 /  90  50  10  10
ELN  19  27  18  28 /  80  20  10   0
RDM  20  38  12  37 /  40  10   0  10
LGD  23  32  20  30 /  80  50  20  10
GCD  25  37  19  34 /  80  20  10   0
DLS  24  36  27  33 /  70  30  10  10

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...Winter Weather Advisory until 4 AM PST Monday for ORZ041-505-508-
     510.

     Winter Weather Advisory until 4 PM PST Monday for ORZ502.

     Winter Weather Advisory until 6 AM PST Monday for ORZ044.

     Winter Storm Warning until 10 AM PST Monday for ORZ507.

     Winter Weather Advisory until 10 AM PST Monday for ORZ049.

WA...Winter Weather Advisory until 4 AM PST Monday for WAZ024.

     Winter Weather Advisory until 4 PM PST Monday for WAZ030.

     Winter Weather Advisory until 6 AM PST Monday for WAZ026>028.

     Winter Storm Warning until 10 AM PST Monday for WAZ029.

&&

90/85/85