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fxus66 kpdt 272145 

Area forecast discussion
National Weather Service Pendleton or
245 PM PDT Sat may 27 2017

Short term...tonight through Monday 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 mean sea level. 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 European model (ecmwf). For
now...made little changes to the existing forecast and leaned more
toward the NAM/GFS solution...thinking that the European model (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 surface 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 County Warning Area 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


Aviation...00z tafs...VFR conditions will prevail at all taf sites
over the next 24 hours. There will be some cumulus buildups this
afternoon and evening over the mountains...generally between 070-
100. Otherwise skies will be clear through Sunday morning.
Additional mid-level cumulus will develop mainly over the mountains
Sunday afternoon...between 6-10k ft agl again. Winds will be light
and terrain driven at all taf sites, less than 12 kts. Visibility
will be unrestricted.


Preliminary point temps/pops...
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 0
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


PDT watches/warnings/advisories...

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