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Area Forecast Discussion (AFD)

Issued by Pendleton, OR (PDT)

FXUS66 KPDT 181728

Area Forecast Discussion
National Weather Service Pendleton OR
928 AM PST Mon Feb 18 2019

.UPDATE...A large trough remains over the western states this
morning with a cold northerly flow aloft over our area. A few 
light snow showers remain over the Blue Mountains but should end 
by this afternoon. The NAM12 shows a weak disturbance reaching 
the Cascades this afternoon. I'm not too impressed with it but 
weak disturbances have been fairly efficient snow producers the 
last few days, so have kept a slight chance of snow showers along 
the Washington and northern Oregon Cascades and Simcoe Highlands 
this afternoon. Snow amounts should be no more than a half inch in
general with localized amounts up to an inch. Overnight 
temperatures were warmer than expected and the sun is finding some
holes in the clouds, so have bumped up temperatures a couple of 
degrees in places. Winds will be light aside from north to
northwest 10 to 15 mph winds in the southern Grande Ronde Valley.
Forecast update already out. Perry


.AVIATION...18Z Tafs. Some patchy low clouds with IFR conditions at 
DLS/BDN/RDM  will be improving through the afternoon as northerly 
flow continues over area.  All others should be VFR through the day 
and into the evening.  There will be IFR cigs 005-015k ft agl after 
10z at YKM/PSC/ALW/PDT with some patchy fog. Winds will be generally 
under 10 kts through the period. 93 


.PREV DISCUSSION... /issued 205 AM PST Mon Feb 18 2019/ 

SHORT TERM...Today through Wednesday. More snow is on the way,
and winter storm watches will be posted for the Blue Mountains and
the foothills for Tuesday evening through Wednesday evening. For
the next 24 hours, no major weather concerns are anticipated but
there will be isolated snow showers. The forecast area is on the
backside of an upper level trough and under a northerly flow
aloft. A few upper level disturbances will bring isolated snow
showers. Although the snow showers have been hit-and-miss since 
yesterday, the orographic lift and weak instability have caused 
snow to fall at rates of 1-2 inches per hour per reports from 
weather spotters. For today through Tuesday morning, the main 
areas that have a potential for snow showers will be over the 
Blue Mountains and foothills, the Wallowas and Elkhorns, the east 
slopes of the Cascades, and the Simcoe Highlands with light 
accumulations expected.

A stronger shortwave riding south along the B.C. coast Tuesday
then across WA/OR Tuesday night and Wednesday will bring snow to
all elevations (rain/snow mix in the Eastern Columbia River
Gorge). There is some weak warm air advection ahead of the
shortwave trough that may bring just enough warmer air aloft for
areas of sleet mixed with snow, mainly across south central WA and
north central OR. Heavy snow is likely over the northern Blues
during this time. The strong orographic lift and upper level
support in addition to snow-to-liquid ratios that should be around
1:15 or 1:20 will bring around a foot of snow to areas north of 
Ukiah. The Blue Mountain Foothills, especially east of Walla Walla
and Pendleton, are forecast to receive 3-5 inches. Amounts in the
Grande Ronde Valley and Wallowa County are expected to meet 
advisory criteria of around 3-5 inches as well, and the upcoming 
shifts may decide to issue highlights for these areas. Several 
inches of snow are expected for central Oregon, the John Day- 
Ochoco Highlands, and the east slopes of the Cascades. Models 
continue to indicate precipitation shadowing off the Cascades, 
therefore only light accumulations are expected for the Lower 
Columbia Basin, the Kittitas and Yakima Valleys, and north central
OR. Winds are not expected to be strong but there may be local 
blowing and drifting of snow where winds may gust to around 20 
mph. Wister

LONG TERM...Wednesday night through Sunday. A low pressure system 
will move across the two state area Wednesday night and the 
accumulating snow will persist across many locations especially in 
the area mountains although this will begin to wind down. The low 
pressure drifts south of the region Thursday as drier air invades 
the region from the north and west. By Thursday night a weak ridge 
of high pressure brings drier conditions but another storm system 
races toward or/wa on Friday that could bring another round of snow 
followed by a low pressure system on Saturday and Sunday. The low 
pressure remains stationary to our north and will continue to spin 
disturbances across the region and snow showers will persist for the 
weekend. Temperatures will remain below normal through the period. 


PDT  30  16  32  27 /  20  10  30  60 
ALW  32  17  33  26 /  10  10  30  80 
PSC  31  18  32  26 /  10  10  20  30 
YKM  32  18  31  23 /  20  10  20  20 
HRI  32  19  34  28 /  10  10  20  40 
ELN  32  15  32  20 /  10  10  20  30 
RDM  35  19  37  26 /  20  10  10  40 
LGD  32  13  32  26 /  20  10  30  90 
GCD  32  17  34  27 /  20  20  30  70 
DLS  38  27  39  32 /  10  10  20  40 


OR...Winter Storm Watch from Tuesday afternoon through Wednesday 
     evening for ORZ502-503-507.

WA...Winter Storm Watch from Tuesday afternoon through Wednesday 
     afternoon for WAZ029-030.