Area Forecast Discussion
Issued by NWS Pendleton, OR

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FXUS66 KPDT 172135

Area Forecast Discussion
National Weather Service Pendleton OR
235 PM PDT Sat Mar 17 2018

.SHORT TERM...Tonight through Tuesday...Main part of the upper level
low impacting the region will be moving off to the east overnight
while a secondary low forms off the coast of Washington. There will
continue to be some instability showers over the western portion of
the forecast area through this evening and then dissipating after
sunset. The eastern portion of the forecast area will remain under
some moisture wrapping around the low pressure over Idaho. This will
place the mountains of northeast Oregon and southeast Washington
under continued snow through most of tonight before tapering off
early Sunday. Winter weather advisories for snow have been issued
through Sunday morning.

The low off the coast of Washington will remain there through Sunday
and then slowly weaken and fall apart on Monday as a flat ridge of
high pressure pushes north from California. There could still be
some lingering showers Sunday and Monday mainly over the higher
terrain but overall will see improving conditions. By Tuesday some
moisture begins to be advected by a southwest flow into Oregon from
a deep low pressure system well off the coast of California. At this
point there is a low chance of precipitation but it could also just
be some mid level clouds passing over the region.

.LONG TERM...Tuesday Night through Saturday...Unsettled weather is
expected through the long term period. Tuesday night a ridge will be
moving off to the east as a pair of upper lows approaches the
Pacific Northwest. The stronger of the two will be coming south out
of the Gulf of Alaska while the second will have been several
hundred miles off the California coast for a few days. The flow will
turn towards the southwest Tuesday night and Wednesday and a
disturbance out of the southern low will move into the area Tuesday
night with a stream of moisture that will bring a chance of rain to
central Oregon and the eastern mountains then across the whole area
on Wednesday. The two lows will merge by Wednesday night and develop
a deep and strong trough offshore. Models are in good agreement in
having the trough offshore Wednesday night through at least Friday
and sending a series of disturbances through the area. That will
keep a chance of rain over the area Wednesday night and Thursday.
Thursday night snow will drop significantly from the previous 5000
foot levels and reach 500 to 1500 feet on Friday and remain there
through Saturday. This will give most of the area a chance of mainly
snow though the Columbia Basin will have a slight chance of a
rain/snow mix. Models diverge Friday night and Saturday with the GFS
keeping the trough offshore and the ECMWF moving it ashore and to
our east by Saturday with a ridge building offshore. For now have
leaned towards the GFS and kept a chance of mostly snow showers with
a slight chance of rain and snow in the Columbia Basin. Temperatures
will start out in the mid 50s to lower 60s with mid 40s to lower 50s
in the mountains Wednesday then slowly cool to the lower to mid 50s
with upper 30s and 40s in the mountains by Saturday. Perry


.AVIATION...Previous discussion...18Z TAFs...A broad upper level low
will keep conditions moist and unstable through 03-06Z. Rain showers
will be possible near or over most TAF sites at some point today.
Best chances of showers will occur during the morning and early
afternoon hours at KRDM, KBDN, KALW and KPDT. Have -SHRA for those
sites through 21Z-23Z and VCSH through about 03z. Shower chances
will be more limited at KDLS, KPSC and KYKM and have not mentioned
it in the TAFs for those sites. Cigs and visibilities will mainly be
VFR. MVFR conditions will be possible at KALW and perhaps KPDT due
to upslope flow and the shower activity through this afternoon.
Winds will generally be 5 to 15 kts through the period, though KDLS
may have winds of 10 to 20 kts with gusts to 30 kts and KPDT may
have gusts to 20 kts this afternoon. Perry


PDT  35  52  32  54 /  40  10   0  10
ALW  37  53  33  55 /  50  10   0  10
PSC  35  58  29  59 /  20  10   0  10
YKM  31  57  32  58 /  20  10  10  10
HRI  35  56  32  58 /  20  10   0  10
ELN  30  54  30  55 /  20  10  10  10
RDM  26  48  26  51 /  30  10  20  20
LGD  32  45  29  49 /  90  20  10  20
GCD  31  44  30  50 /  50  10  10  20
DLS  35  57  34  59 /  20  10  10  10


OR...Winter Weather Advisory until 8 AM PDT Sunday for ORZ050-502.

WA...Winter Weather Advisory until 8 AM PDT Sunday for WAZ030.



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