Area Forecast Discussion
Issued by NWS Pendleton, OR

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FXUS66 KPDT 280513

Area Forecast Discussion
National Weather Service Pendleton OR
1009 PM PDT Sat May 27 2017

.SHORT TERM...Clear skies expected for the remainder of the
evening and overnight. Upper level ridge centered over the region
Sunday with sunny skies and high temperatures a few degrees warmer
than today. Scattered cumulus over the mountains in the
afternoon. An upper level trough will be approaching the Pacific
northwest Monday with the flow aloft becoming southerly.
Instability will be increasing and should have more widespread
cumulus over the mountains in the afternoon. Isolated
thunderstorms could develop over the Cascades late in the day. As
the upper trough pushes inland Tuesday into Tue night more
widespread showers and thunderstorms are expected. 94

.AVIATION...06z tafs. Clear skies tonight into Sunday morning. Sct
cumulus at 060-100 mostly over the mountains in the afternoon and
early evening. Winds 5-10kt.


.PREV DISCUSSION... /issued 245 PM PDT Sat May 27 2017/

SHORT TERM...Tonight through Monday night...An upper level ridge
will continue to dominate the weather conditions across the Pacific
Northwest through Sunday night. Thus expect mainly dry conditions
with above seasonal temps. The upper ridge axis will shift from west
of the Cascades this afternoon to a position over eastern Oregon and
Washington Sunday afternoon. A few high based cumulus clouds over
the mountains this afternoon will dissipate with sunset. Will see
high based cumulus redevelop over the higher terrain Sunday
afternoon. At this time expect there to still be enough subsidence
to cap off much of this convective development between 13 and 15
thousand feet MSL. The exception will be over the Siskiyous and
southern Cascades late Sunday afternoon and evening, where stray
thunderstorms may develop. One of these storms may reach extreme
southwest Deschutes county in the evening before dissipating. A
Pacific system will begin to approach the region Monday. This system
will push the upper ridge further east resulting in increasing
southerly flow over the region. Also capping will decrease across
the area. Moisture will increase Monday over our western zones, but
the eastern zones still look to be quite dry. Thus should see
isolated showers and thunderstorms develop over portions of central
and north-central Oregon, and south-central Washington late Monday
afternoon and evening. A few showers and/or storms may persist into
the overnight hours of Monday night/early Tuesday. Temps Monday will
continue well above seasonal. 90

LONG TERM...Tuesday through Saturday...The extended period will
start with very warm, well above average temperatures on Tuesday as
the area remains on the western periphery of the departing upper
ridge. Look for high temperatures in the upper 80s to mid-90s in the
lower elevations with upper 70s to mid-80s in the mountains. Most
locations should fall just shy of any record high temperatures.
Otherwise...expect increasing instability through the day Tuesday as
moisture advection continues in a deep southerly flow. A mid/upper
level trough is then progged to reach the OR coast Tuesday afternoon
and evening. There remains some significant differences in respect
to the strength of this trough moving into the area...with the
latest GFS/NAM being much stronger than the 12z ECMWF. For
now...made little changes to the existing forecast and leaned more
toward the NAM/GFS solution...thinking that the ECMWF may be too
weak with the incoming disturbance. Therefore continued with a
chance of thunderstorms over roughly the western half of the
forecast area for late Tuesday afternoon and well into the evening
hours. With the continued model uncertainty did not feel it was
prudent to add any mention of strong/severe t`storms into the
forecast at this time (i.e. mention hail and/or gusty
winds)...although if the NAM/GFS solution pans out stronger storms
could certainly develop. The GFS/NAM generally agree on about 800-
1300 J/Kg of SFC based CAPE over the area...along with LIs between
about -2 to -6. Deep layer shear is also progged to reach 35-50 kts
over the western half of the CWA as waves moves through Tuesday
evening. Heading into Tuesday night the trough moves east into the
forecast area. Continued with a chance of showers and a slight
chance of thunderstorms through the overnight period, as dynamics
increase and instability lingers. During the day on Wednesday much
of the activity should begin to shift east...with most of the
remaining instability (and thunder threat) now focused from the Blue
Mountain Foothills eastward into the Blues and Wallowa/Union
counties. Temperatures are forecast to be about 10-15 degrees cooler
on Wednesday as the cooler marine air mass moves in. Westerly winds
will also be breezy over the area...mainly between 15 to 25 mph.
Thursday through Saturday the forecast area will remain under a
broad upper level trough and cyclonic flow. Several embedded
disturbances are forecast to quickly move through the northern half
of the area. The GFS is slightly stronger with the upper trough and
almost closes off a 500mb low over northern Washington on Saturday.
For now took a blend of the latest available model guidance for the
official forecast...until the details can become clearer. This gives
mainly dry conditions over the Lower Columbia Basin and surrounding
valleys...with slight chances for showers and possibly thunderstorms
in the area mountains. The highest PoPs were placed along the east
Slopes of the Washington Cascades...closer to the lower heights and
increased moisture. The slight chance of thunderstorms were mainly
mentioned over the eastern mountains on Thursday and Friday
afternoon. Temperatures during this period should be close to or
slightly above the seasonal average. With highs in the mid 70s to
lower 80s in the lower elevations and 60s to lower 70s in the
mountains. 77


PDT  51  88  53  90 /   0   0   0   0
ALW  58  90  59  92 /   0   0   0   0
PSC  54  91  53  95 /   0   0   0   0
YKM  53  92  54  95 /   0   0   0   0
HRI  51  91  53  93 /   0   0   0   0
ELN  49  91  51  93 /   0   0   0  10
RDM  42  88  47  89 /   0   0   0   0
LGD  43  84  45  85 /   0   0   0   0
GCD  44  88  49  91 /   0   0   0   0
DLS  54  94  58  96 /   0   0   0   0




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