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000
FXUS66 KPDT 181732
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
1033 AM PDT Wed Oct 18 2017

UPDATED AVIATION DISCUSSION

.UPDATE...A large upper low and trough in the Gulf of Alaska has
set up a westerly flow across the area this morning. A stream of
moisture extending all the way back to Asia is reaching our area
and will feed a warm front currently draped across the area. The
warm front is currently draped across North Central Oregon east
to the Grande Ronde Valley and Wallowa county. The front will be
lifting north today and tonight. Precipitation is filling in the
radar returns and expect rain mainly in the Washington portion of
the area and the Oregon Columbia Basin and Blue Mountain Foothills
today. The Yakima Valley will be in a rain shadow from the
Cascades and do not expect much rain there. Southwest winds will
pick up this afternoon to 10 to 20 mph then decrease this evening.
Tonight the warm front will lift north out of the area as the
cold front of the system begins to come ashore. This will end
precipitation across most of the area though rain will continue
along the Cascades through the night. The current Flash Flood
Watch for the east slopes of the Washington Cascades looks good,
especially with recent burn scars in that area, so no changes are
anticipated. Temperatures will continue to be warm with highs in
the 60s and lower 70s today. Lows tonight will be in the mid 40s
to lower 50s in the lower elevations as cloudy skies help keep
temperatures up while the mountains will be mainly in the mid 30s
to mid 40s. Further south where cloudiness will be less, locations
like Sunriver, Paulina and Seneca will be in the lower 30s.
Current forecast is in good shape and only made minor
temperature, rain and wind changes to the current forecast.
Forecast update already out. Perry

.AVIATION...18Z TAFs. A passing warm front will keep some mid levels
clouds 070-120 over the area before a cold front arrives on Thursday
bringing rain to all sites. There will be a few showers impacting
KALW and KPSC through this afternoon associated with the warm front
passage. Winds will generally be south to southwest 10-20kts today,
decreasing slightly overnight and then increasing again early
thursday ahead of the cold front.
.

&&

.PREV DISCUSSION... /issued 550 AM PDT Wed Oct 18 2017/

SHORT TERM...Today through Friday night...The weather pattern will
continue to be very active through the entire short term period. A
large upper level low pressure system is evident on the WV/IR
satellite this morning, centered over the Gulf of Alaska. A large
plume of moisture extends well to the south of this low pressure
system, along a series of frontal boundaries. For today, a strong
west-northwest flow aloft will bring clouds and moisture across the
northern half of the forecast area. The highest chances for rain
(and the highest amounts) will be along the East Slopes of the WA
Cascades where 0.50-1.50 inches of rain is possible over the next 24
hours. Elsewhere, there will be a chance of rain for much of
southern Washington and the northern Blue Mtns. Further south,
across most of our Oregon zones conditions are expected to remain
generally dry today, under mostly cloudy skies. There will be some
break of sun across the far south (Sunriver to Paulina and Seneca).
Southwest winds will be slightly breezy today, between 8-15 mph,
with a few higher gusts possible. Temepratures will remain mild,
with highs in the mid-60s to lower 70s this afternoon...except 50s
and low 60s mountains. Heading into tonight, the warm front lifts
north, putting much of our CWA in a temporary dry slot. This will
allow for decreasing clouds across the south and eastern portion of
the area. It will remain mostly cloudy across the north and west
(The Dalles, Yakima, Ellensburg, WA East Slopes). Overnight lows
will be mainly in the 40s to near 50...with some 30s in the
mountains. On Thursday, a surface cold front and associated stream
of moisture begin to move east across the forecast area. In the
morning the highest PoPs will be along the East Slopes of the
Cascades, extending into the Eastern Columbia Gorge. By Thursday
afternoon the band of rain spreads as far east as Spray, Pendleton
and the Tri-Cities. This area of rain then overspreads the remainder
of the area Thursday evening and night. Rainfall amounts could be
moderate to locally heavy with this system moving through. At this
time 24 hours amounts from 5am Thursday - 5am Friday are forecast to
range from 0.75 to 1.50" along the East Slopes of the Cascades, a
third to two-thirds of an inch in the Blue Mtns, with a tenth to
quarter inch elsewhere. We will need to keep a close eye on the
recent burn scar areas if heavy rain does indeed develop...as
increase runoff may cause localized flooding issues here. On Friday,
the upper level trough and associated cold pool of air will be over
the forecast area. Showers will be likely in the mountains, with a
slight chance or chance of showers for most other locations. As is
typically the case with this type of weather pattern, much of the
Lower Columbia Basin will likely remain generally dry. Snow levels
will be much lower on Friday, between 4000-4800 feet. An inch or two
of snow will be possible above 4500 feet on Friday. There could even
be enough instability for an isolated t`storm Friday afternoon, but
there was not enough confidence to add into the forecast just yet.
Westerly winds will be breezy to windy on Friday as well...between
15-30 mph, with a few higher gusts. The area sees a short break in
the precip Friday evening, before another frontal system approaches
later Friday night. 77

LONG TERM...Saturday through Tuesday...A warm front will move
into eastern WA and OR Saturday. Snow levels increase from 4500-5000
feet Saturday morning to 8000-9000 feet in the evening. Widespread
precipitation will occur Saturday as overrunning moist warm air
ahead of the front will be over the area. Strong southwest winds
will develop Saturday afternoon and evening as stronger winds aloft
mix to the surface behind the warm front. Chance of rain will
persist Saturday night...then decrease Sunday as the cold front
drops south over Oregon. A broad ridge will build offshore Monday
with rising heights over WA and OR. Subsidence from the ridge will
bring dry and warm conditions Monday and Tuesday. Coonfield



&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  70  48  65  43 /  20  10  60  90
ALW  69  52  65  46 /  30  10  50  90
PSC  65  51  62  46 /  20  10  60  80
YKM  62  47  60  39 /  10  20  30  20
HRI  69  47  62  44 /  20   0  60  80
ELN  59  42  58  37 /  20  30  30  20
RDM  71  42  61  36 /  10   0  60  60
LGD  66  44  70  40 /  10   0  20  70
GCD  71  45  71  41 /  10   0  20  80
DLS  68  52  60  45 /  20  30  80  50

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...Flash Flood Watch from 5 PM PDT this afternoon through Thursday
     evening for WAZ520.

&&

$$

83/91/91