Area Forecast Discussion
Issued by NWS Pendleton, OR

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FXUS66 KPDT 230959

Area Forecast Discussion
National Weather Service Pendleton OR
258 AM PDT Wed Aug 23 2017

.SHORT TERM...Today through Friday night...Some fairly active
weather is expected for at least the first half of the short term
period. A weak upper level disturbance continues to spin over north-
central California...with the cyclonic flow around this feature
importing plenty of monsoonal moisture into south-central Oregon. A
highly amplified, long wave trough is now approaching the Pacific
Northwest coast, and this will continue to push onshore, through the
area later today into Thursday. As moisture and instability increase
out ahead of these systems, expect scattered t`storms to develop
this afternoon, and track northeast across the area this evening.
The highest probabilities to see t`storms during this period will be
across the eastern half of the forecast area...or along a line from
about Sunriver--Fossil--Pendleton--Dayton and points east. A few of
these storms could be on the stronger side, with abundant cloud to
ground lightning, brief gusty winds (to 35 mph), and locally heavy
downpours. PWATS are now actually forecast to reach 1.25-1.33" over
the forecast area by late this afternoon/evening...which is well
above average levels. Storm motion will be SW to NE around 10-20
mph. In anticipation of these t`storms a Red Flag Warning remains in
effect for portions of the forecast area...please see the fire
weather discussion below for more details. These showers and
t`storms will begin to shift off to the north and east after
midnight tonight, as the surface frontal boundary moves through. The
front is now expected to stall out over the far southeastern portion
of the CWA. Thus, an unstable airmass will remain in place through
Thursday afternoon/evening, and there will be a slight chance of
t`storms lingering over Grant, Union and Wallowa Counties. For the
remainder of the forecast area Thursday should be drier, under
mostly sunny skies and breezy westerly winds between 15-25 mph.
Temperatures will be on a cooling trend for both Thursday and
Friday...with highs only reaching the 80s...except upper 60s and 70s
mountains. This is actually close to, or just slightly below average
for this time of year. Thursday night, Friday and Friday will remain
dry, under mostly clear skies with the CWA in a general northwest
flow pattern. Overnight lows will be quite cool...dipping into the
40s or lower 50s...except 30s in the mountains and high valleys. 77

.LONG TERM...Saturday through Tuesday. Upper ridge building back
into the Pacific northwest Saturday with sunny skies and warmer
temperatures. The ridge will strengthen and remain centered over the
region Sunday with another sunny day. 850mb temperatures are
increasing and highs will be into the 90s. Overall trend of the
models is to shift the ridge axis to the east Mon and Tue. This may
allow some mid and high level clouds to move across the forecast
area. Temperatures remain warm with highs in the 90s. With strong
persistent high pressure over the region for several days and many
large fires burning expect smoke and haze will be an issue. Models
show fairly light winds from the surface to 800mb and mixing will be
limited. 94


.AVIATION...12z tafs. Mid and high level clouds lifting northward
will keep sct-bkn 100-200 over central and northeast Oregon
impacting rdm bdn pdt alw through the morning. Remaining taf sites
will be mainly clear. Models show an area of showers and scattered
thunderstorms developing over central Oregon moving northeast late
afternoon through the evening. This would result in lowering
ceilings to 040-080. Winds 5-15kt for most sites. 10-20kt at dls. 94


.FIRE WEATHER...For this afternoon and evening, thunderstorm
coverage will increase with the approach/passage of a cold front.
Scattered thunderstorms are expected across Central Oregon in the
afternoon, migrating eastward across the northeast Oregon mountains
in the evening. The latest hi-res, near term models are indicating
t`storm activity shifting slightly west of where it was originally
anticipated this evening. Therefore, there is now a chance of
t`storms along and east of a Fossil--Lexington--Pendleton---Dayton
line. with a slight chance of t`storms as far west as Shaniko,
Boardman, and Richland. Will keep the current Red Flag Warnings as
is for now, and let the dayshift decide if this trend warrants any
expansion of the warning area. Models continue to slow the
progression of the front on Thursday, with some potential for
residual thunderstorms across far eastern Oregon...will keep LALs 2
for that activity. Winds will increase in the wake of the front
Thursday, with breezy to locally windy conditions for the Columbia
Gorge and Basin. Model trends have been a bit lighter for winds, and
with the maximum winds likely occurring when RHs are beginning to
rise, will not issue any watches for Thursday at this time. 77/80


PDT  89  61  82  49 /  10  30  10   0
ALW  92  66  86  53 /  10  40  10   0
PSC  93  63  89  48 /  10  20   0   0
YKM  94  60  86  47 /   0  10   0   0
HRI  92  63  87  49 /  10  20   0   0
ELN  90  60  78  49 /   0  10   0   0
RDM  86  50  80  39 /  40  30   0   0
LGD  88  56  81  46 /  20  40  10   0
GCD  86  57  82  48 /  50  50  20  10
DLS  89  65  83  54 /  10   0   0   0


OR...Red Flag Warning from noon today to 10 PM PDT this evening for

     Red Flag Warning from 5 PM this afternoon to 10 PM PDT this
     evening for ORZ643.

     Red Flag Warning from noon today to 5 PM PDT this afternoon for

WA...Red Flag Warning from 5 PM this afternoon to 10 PM PDT this
     evening for WAZ643-645.



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