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Area forecast discussion 
National Weather Service Pendleton or
233 PM PDT Wednesday may 27 2015

Short term...tonight through Saturday...upper level closed low
pressure system over Wallowa County this afternoon will continue its
slow drift to the southeast overnight and be out of the area by
Thursday. As such can expect the showers and few thunderstorms to
continue into this evening and then decrease and die out after
sunset. A ridge of high pressure will begin to build over the region
on Thursday but with some lingering instability from the departing
low there will continue to be some showers and possible
thunderstorms mainly over the mountains during the afternoon and
early evening period. On Friday the weak ridge will remain in place
with a another closed low pressure system over British Columbia.
This low will send a small shortwave through Washington late Friday
which will trigger some showers and possible thunderstorms again
mainly over the mountains. Some of this activity could continue
through Friday night mainly over northeast Oregon and southeast
Washington. The upper level ridge will begin to intensify on
Saturday in response to yet another upper level closed low advancing
towards the West Coast from the eastern Pacific. This will produce
some instability over central Oregon extending to northeast Oregon
for more chances of showers and thunderstorms. High temperatures
will be gradually warming over the next three days with lowers 90s
in the basin Friday and Saturday.

Long term...Saturday night through Wednesday night...the weather
begins to turn much more active by Sunday as a closed upper level
low pressure system begins to approach the area. By Sunday afternoon
the region will be under a diffluent upper level pattern with a
south to southwesterly flow advecting warm air and moisture into the
County Warning Area. The current thinking is that there would be enough moisture and
instability to ignite scattered showers and thunderstorms over most
of our Oregon zones by the afternoon and evening hours Sunday as the
system continues to move northeastward. Cape values between 500-1000
j/kg appear possible over much of our Oregon zones...with 0-6km bulk
shear values of 30-40kts. Sunday night some nocturnal showers and
embedded thunderstorms would remain possible as synoptic scale lift
moves over the region...therefore left chances probability of precipitation in the forecast
and also began to shift some activity north into Washington. If the
current guidance remains consistent Monday would likely be the best
day for more widespread shower and thunderstorm development. Current
guidance is suggesting a strong vorticity maximum along with a
closed middle to upper level circulation moving into our County Warning Area later
Monday into Monday night. This combined with cape value between 500-
1000 j/kg and Li of -1 to -5 will lead to chance/low end likely probability of precipitation
for showers and thunderstorms over much of the area. 0-6km bulk
shear values will again be between 30-40kts...which could initiate
some isolated stronger convection...this is something that will need
to be watched closely in the coming days. On another the
system approaches the area several frontal boundaries will move
through...behind these fronts expect breezy conditions to develop
both Monday and Tuesday afternoon...early indications are sustained
winds of 15-25kts with higher gusts possible. The upper low is then
forecast to move directly over the County Warning Area during the day on
Tuesday...which will again lead to a good chance for showers and
isolated thunderstorms. Conditions begin to dry out by Tuesday night
as a drier northwesterly flow develops. This system will eject east
into the intermountain west by Wednesday and Thursday with a
building ridge and drier weather conditions over our region.
Temperatures will start out above average Sunday before falling back
to near or slightly below average Monday and Tuesday with the clouds
and precipitation Wednesday and Thursday expect a
gradual warming trend to take hold. 77


Aviation...18z tafs...VFR conditions expected to prevail over all
taf sites through the period. An upper level low centered over
northeastern Oregon will keep the air mass over the region rather
unstable. As a result, showers and possible thunderstorms will
develop this afternoon. The best chances for a shower, with an
outside chance of a thunderstorm, will be seen in vicinity of kalw and kpdt.
Thunderstorms will more likely occur over the mountains east of kalw
and kpdt. Showers and thunderstorms are also expected to develop
over the east slopes of the cascades(possibly in vicinity of kykm mainly
after 27/22z). Direction of movement of these showers/storms is
expected to be from the north. Thus at this time expect showers and
storms to stay west of kdls, krdm and kbdn. Ceilings, when they
occur, will mainly be between 7000 and 10000 feet above ground level. Winds are
forecast to remain light at or below 12kts at most taf sites...the only
exception to this will be kdls and krdm where winds may reach 15kts
for a time later this afternoon and into the early evening hours. 77


Preliminary point temps/pops...
PDT 52 81 58 86 / 20 0 0 10
alw 57 83 63 88 / 30 10 10 10
psc 56 87 59 91 / 20 0 0 10
ykm 55 85 59 87 / 20 10 10 10
hri 55 85 58 90 / 20 0 0 10
eln 53 85 58 87 / 20 10 10 10
rdm 44 80 49 82 / 10 0 10 10
lgd 47 77 52 81 / 40 20 10 20
gcd 46 80 50 84 / 30 10 10 20
dls 57 88 61 90 / 10 0 0 10


PDT watches/warnings/advisories...

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