Area Forecast Discussion
Issued by NWS Pendleton, OR

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266
FXUS66 KPDT 240544 AAC
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
1040 PM PDT Sun Apr 23 2017

updated the aviation discussion

.UPDATE...Earlier thunderstorms had weakened with the loss of
daytime heating but the latest radar was continuing to show
scattered showers mostly over the eastern half of the region. A
low pressure system over the ne pac will spin additional moisture
into our region overnight and the showers will expand in coverage
toward morning. Adjustments to the forecast made earlier remain
and the present short term forecast is on track.

.AVIATION...vfr conditions will give way to lcl mvfr conditions
after 08z when clouds and showers will increase in coverage.
Mvfr conditions are expected after 12z.

&&

.PREV DISCUSSION... /issued 554 PM PDT Sun Apr 23 2017/

UPDATE...Quick update to the zfp this early evening after tsra
fired up along the foothills of the Oregon blue mountains. Tsra
were added to the Washington blue mountain foothills into all of
the blue mountains as well. Now the current forecast is more on
track.

PREV DISCUSSION... /issued 500 PM PDT Sun Apr 23 2017/

SHORT TERM...Tonight through Wednesday...A weak upper trough is
moving across the CWA this afternoon and will continue this evening.
There has been some breaks in the overcast to allow solar heating to
take place. Both the NAM and GFS models are showing some modest
instability over northeast portions of the CWA, so I have decided to
include isolated thunderstorms from the northern Blue Mountains
eastward from 00Z-03Z this evening. There probably won`t be much
thunder but there may be a few strikes in this portion of the CWA.
This system will move on out of the region only to be replaced by a
stronger system moving in from the west later overnight. This next
system will have abundant moisture for almost a continuous chance to
likely showers across the entire forecast area from Monday through
Tuesday. The models are again showing some instability Monday
afternoon and early evening so included a slight chance of
thunderstorms again for Monday afternoon and early evening over the
Blue Mountains eastward...which this time covers a greater area that
includes the eastern John Day Highlands northeast across the Blue
Mountains eastward. However, am not expecting any severe
thunderstorms today or Monday. Showers will persist Monday night
through Wednesday as an upper low pressure system drops down out of
Canada and moves southeast over the Forecast area. There will be
some increase in downslope westerly winds along the Cascades by
Tuesday which will cause a rain shadow over the adjacent terrain
east of the Cascade east slopes...and therefore have left this area
dry for Tuesday. Another weather system will then move across the
region Tuesday night and Wednesday with more showers covering the
entire forecast area. However the amount of moisture does not look
to be as much as Monday night and Tuesday`s system. It will be cool
with lowering snow levels to around 4000-5000 feet at night and 4500-
5500 during the day on Tuesday and Wednesday. As such there may be
some light snow accumulation in the higher mountains/passes...
especially Tuesday night. However, at this time am not expecting any
needed snow highlights. Temperatures through the short term look to
be cooler than normal. It will also become breezy to windy at
times...especially on Wednesday behind a cold front mainly in the
Lower Columbia Basin and adjacent valleys and also in central/north
central Oregon. Elsewhere can expect winds of 10 to 20 mph at times
through the period...mainly in the afternoons and early evenings. 88

LONG TERM...Wednesday night through Sunday...A weak mid/upper
level shortwave moves across eastern sections of the forecast area
Wednesday evening and then exits far eastern Oregon by midnight
followed by decreasing PoPs overnight over the eastern mountains and
dry conditions in the sections of the forecast area immediately
leeward of the Cascades. Northwest flow aloft over the region
Thursday, ahead of a mid/upper level trough moving southeast out of
British Columbia and into the interior Pacific Northwest Thursday.
This system produces likely PoPs from sunup to sundown on Thursday
in the Blue Mountains and the Wallowa Mountains with snow levels
lowering to 3500 feet on Thursday for a snow above 4000 feet and a
rain/snow mix 3500-4000 feet in the mountains of eastern Oregon.
Thursday night and Friday snow levels remain around 3500 feet while
the aforementioned system tracks across the forecast area. The
system exits the region Friday evening for just a slight chance of
snow showers in the eastern mountains Friday evening, then dry
conditions kick in overnight by virtue of a flat mid/upper level
ridge of high pressure moving into the region through the weekend.
Polan

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  37  52  37  54 /  60  70  40  20
ALW  41  54  41  57 /  60  70  50  20
PSC  44  61  41  63 /  30  60  30  10
YKM  39  60  36  63 /  30  50  20  20
HRI  41  59  38  62 /  50  60  30  20
ELN  37  57  36  59 /  30  50  20  20
RDM  33  53  31  54 /  40  70  20  60
LGD  36  51  35  54 /  60  80  70  40
GCD  37  52  34  56 /  60  80  60  30
DLS  42  57  41  60 /  50  70  30  60

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...None.
&&

$$

97/97



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