Area Forecast Discussion
Issued by NWS Pendleton, OR

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FXUS66 KPDT 290526
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
1022 PM PDT Sun May 28 2017

.SHORT TERM...Scattered cumulus over the mountains dissipating
as the airmass becomes more stable with sunset. Expect clear
skies overnight. The upper ridge will remain over the area Monday.
Expect another sunny and warm day. 850mb temps indicate highs
about 1-4 degrees above today with mid 80s to mid 90s. A little
deeper instability will result in more cumulus development over
the mountains in the afternoon. Might have some showers or
isolated thunderstorms over the Cascades late in the day. Models
still show upper level trough and cold front moving into the
region Tuesday afternoon and evening with showers and scattered
thunderstorms. 94

.AVIATION...06z tafs. Clear skies tonight into Monday morning. Sct
cumulus 060-090 developing in the afternoon mainly over the
mountains. Winds 5-10kt.

&&

.PREV DISCUSSION... /issued 246 PM PDT Sun May 28 2017/

SHORT TERM...Tonight through Tuesday night...An upper level ridge
will continue to affect the Pacific Northwest through Monday. This
upper ridge will gradually shift east with it`s axis running along
the Idaho/Washington/Oregon border area by late Monday afternoon.
This will result in the flow to become increasingly southerly
tonight through Monday. Dry conditions are mainly expected tonight
into Monday morning. There is an outside chance that a stray shower
or thunderstorm may develop over the southern Cascades and move into
extreme southwest Deschutes county this evening. The increased south
flow across the area will lead to increased moisture and instability
along and near the Cascade crest Monday afternoon and evening. Thus
will continue to indicate chances of showers and/or thunderstorms
over portions of our central and north-central Oregon, and south-
central Washington zones for this period. Limited moisture will keep
the areas to the east dry despite the increased instability. High
temps will be well above seasonal Monday afternoon. A Pacific system
is still expected to move into the region Tuesday and Tuesday night.
This system will not be strong, but there will be moisture and
plenty of instability for it to cause the development of isolated to
scattered showers and thunderstorms. The bulk of the storms can be
expected during the afternoon and evening hours of Tuesday. Temps
will remain above seasonal, though a marine push ahead of the main
cold front will cause some cooling through the Gorge and along the
Cascade east slopes.

The very warm temps over the next couple days will increase high
elevation snowmelt and run off into the Naches River in south-
central Washington. There is the potential for some localized
flooding and a flood watch is in effect. 90

LONG TERM...Wednesday through Sunday...There will be a somewhat
diffuse upper level trough moving into the Pacific NW on Wednesday.
This will bring several weak frontal boundaries through the area.
Deciding where the highest chances for precipitation across the CWA
will be with this system was challenging...due to the separate
disturbances embedded within the trough. There appears to be some
model agreement that the greatest chance for shower activity will be
across the southeastern half of the forecast area Wednesday
morning...and this will basically remain in place into Wednesday
afternoon/evening. During the afternoon/evening there will also be a
slight chance of thunderstorms over the Blue Mtns east into
Union/Wallowa Counties as instability remains here. A few t`storms
could produce small hail...mainly over Wallowa County in the
afternoon as SFC CAPE values are near 500 J/Kg and deep layer shear
values are around 30-35 kts. Not quite enough confidence in this to
add into the official forecast yet though. Wednesday night and
Thursday a broad upper level trough remains over the forecast area.
This will keep a chance of showers and a slight chance of
thunderstorms in the forecast...mainly in the mountains. The lower
Columbia Basin and surrounding valleys will likely be mainly dry
during this time. Temepratures will be closer to the seasonal
average...with highs in the mid-70s to lower 80s...except 60s and
lower 70s mountains. For Friday through next Sunday there is rather
significant model differences. This uncertainty is making for a
lower than normal confidence forecast. The latest GFS/CMC models
show a shortwave ridge building over the area on Friday into
Saturday, before a modest upper trough eventually moves onshore
Saturday night and Sunday. This would keep conditions mainly dry
(except few mtn showers)with seasonably warm temperatures. Meanwhile
the latest ECMWF quickly brings a deep mid/upper level closed low
into the PAC NW Friday night...and keeps it overhead through Sunday.
This would keep chances for precip higher...with ample mtn showers
as the atmosphere becomes unstable under the associated cold pool.
Snow levels/temperatures would also be much lower. For now, decided
to lean toward the more consistent and WPC favored GFS/CMC solution
for the official forecast. Will need to closely monitor future model
runs to see how this eventually plays out. For now the official
forecast for next weekend has partly cloudy skies...slight chances
of mtn showers with high temps in the 70s to lower 80s. 77

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  53  90  56  92 /   0   0   0  10
ALW  57  92  61  92 /   0   0   0  10
PSC  52  95  55  97 /   0   0   0  10
YKM  53  95  59  94 /   0   0  10  10
HRI  51  94  54  94 /   0   0   0  10
ELN  50  93  55  88 /   0   0  10  20
RDM  46  90  49  82 /   0   0  20  30
LGD  47  85  47  91 /   0   0   0  10
GCD  48  90  52  92 /   0   0  10  20
DLS  58  96  60  79 /   0   0  10  20

&&

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

$$

94



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