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fxus66 kpdt 150134 

Area forecast discussion
National Weather Service Pendleton or
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.

Aviation...kpsc may return to LIFR overnight due to a light north
wind pushing the low clouds and fog from the upper Columbia Basin
into the Tri-Cities. A west-northwest flow along the boundary
layer will likely keep stratocumulus clouds from a low level
upslope flow along the blue mtn foothills tonight with areas of MVFR
and LCL IFR ceilings and visibility at kalw and kykm and possibly kpdt. A
low stratus deck will also develop at kdls overnight with a light
easterly gradient. Krdm, and kbdn are forecast to remain VFR for
the next 24 hours. Winds will generally be 10 kts or less at all
taf sites.

Previous discussion... /issued 220 PM PST Sat Dec 14 2019/

Short term...tonight through upper level trough will
move mainly south of the area tonight. Another shortwave trough
will move across NE Oregon/southeast Washington on Sunday. The net result
will be just a few rain/snow showers mainly over the NE mountains
and foothills with light quantitative precipitation forecast 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 mean sea level. 78

Long term...Tuesday night through 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
Washington/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 Washington 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 Washington 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 European model (ecmwf) is farther north with the low, and keeps the
moist warm front over the northern half of Washington 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


Preliminary point temps/pops...
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


PDT watches/warnings/advisories...

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