Area Forecast Discussion
Issued by NWS Pendleton, OR

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FXUS66 KPDT 150626 AAA

Area Forecast Discussion
National Weather Service Pendleton OR
1010 PM PST Sat Dec 14 2019

updated the aviation discussion

.UPDATE...Snow showers were persisting across portions of the
region and the forecast was expanded to reflect these showers
over south central Washington. Elsewhere patchy freezing fog had
also expanded into the Yakima and kittitas valleys. A disturbance
was moving near to the region overnight and the showers are
expected to continue mainly across the north and east and the
showers may also develop over central Oregon around La Pine.
Otherwise minor changes were also done to sky conditions and now
the short term forecast appears on track.

.AVIATION...Areas of mvfr and ifr conditions can be expected
especially at taf sites kykm...kalw...kpdt. Also taf sites kpsc
will experience areas of mvfr and lcl ifr into Sunday morning before
some improvement is likely around 18z.


.PREV DISCUSSION... /issued 534 PM PST Sat Dec 14 2019/

UPDATE...Patchy freezing fog was developing across the columbia
basin and cloud ceilings were lowering along with the visibility.
Thus the short term forecast was updated to include freezing fog
in the forecast overnight. Otherwise showers were persisting over
the eastern mountains and a few showers may work its way into
Deschutes county overnight. Otherwise the present short term
forecast appears on track.

PREV DISCUSSION... /issued 220 PM PST Sat Dec 14 2019/

SHORT TERM...Tonight through Tuesday...An upper level trough will
move mainly south of the area tonight. Another shortwave trough
will move across NE Oregon/SE Washington on Sunday. The net result
will be just a few rain/snow showers mainly over the NE mountains
and foothills with light QPF and no winter highlights. There will
be some patchy freezing fog overnight and Sunday morning in the
Columbia Basin and and adjacent valleys. Sunday night through
early Tuesday a rather amplified upper ridge will build eastward
across the inland Northwest promoting dry weather and seasonable
temperatures. Fog/freezing fog and stratus will likely become more
widespread across the lower elevations with time as low level
inversions strengthen. By late Tuesday afternoon an upper level
trough will be approaching the region from the west and some light
snow may develop along the Washington Cascade east slopes. Snow
levels in this region at that time will be 1200-1800 feet MSL. 78

LONG TERM...Tuesday night through Saturday...Active winter weather
is on the way later in the week, and winter highlights may be in
order for several zones. Tues night will be relatively quiet with
areas of freezing fog as the only weather concern.  An upper low
will develop off the OR coast Tues night then will decay as it
tracks northeast on Wed, bringing a chance of snow to the
WA/OR Cascades and east slopes.  A secondary low will follow
Wed night and Thurs, accompanied by a moist southwest flow
aloft. The warm front Wed night and Thurs may bring heavy
amounts of snow to the east slopes of the Cascades, particularly
over the WA side of the Cascade range where cold air damming will
maintain low snow levels.  The OR side of the Cascades will have
increasing snow levels to around 4500 feet, therefore many of the
towns like Sunriver, Sisters, and Camp Sherman will observe snow
changing to rain by Thurs. The eastern mountains may also have
moderate to heavy snow during this time with a gradual rise of snow
levels to around 3000-3500 feet.  With cold air trapped in the
Kittitas Valley and possibly the Yakima Valley, a mixed bag of snow
and freezing rain is expected. The strong southwest flow aloft and
tightening surface pressure gradients could mean it will be a
windy day for many areas across central and northeast OR on Thurs.

The moist southwest flow continues Thurs night and Fri, and snow
levels will increase to around 4500 feet for most of the region, but
models continue to hint of cold air remaining trapped along the east
slopes of the WA Cascades north of Trout Lake where snow levels may
only rise to around 2500-3000 feet.

Models solutions differ on the next offshore low Fri through
Sat. The ECMWF is farther north with the low, and keeps the
moist warm front over the northern half of WA with far less
precipitation compared to the GFS.  The Canadian is more in
agreement with the GFS but not as strong with the low.  Due to these
differences, confidence in how much precipitation is fairly low
for this time period.  Wister


PDT  27  37  24  39 /  10  10   0   0
ALW  29  36  28  39 /  20  30  10   0
PSC  28  37  26  37 /  10  10   0   0
YKM  22  38  21  35 /  10   0   0   0
HRI  28  39  25  39 /  10  10   0   0
ELN  23  35  20  33 /  10   0   0  10
RDM  21  41  21  43 /  10   0  10   0
LGD  27  36  21  36 /  20  20  10   0
GCD  23  38  19  39 /  10  10  10   0
DLS  31  43  31  42 /  10   0  10  10




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