Weather Service Report

FXUS66 KPDT 282231 AAA

Area Forecast Discussion...Update
National Weather Service Pendleton OR
330 PM PDT SUN AUG 28 2016

Updated Aviation Discussion

.SHORT TERM...Tonight through Wednesday...The large upper level
trough over western Canada will shift westward and take up position
along the west coast with a closed low positioned in the trough off
the Canadian coast. This will place the forecast area under a
southwest flow aloft through Wednesday with a ridge of high pressure
building to the east over the northern Plains. This will keep the
forecast under a dry weather pattern except for a shortwave coming
around the bottom of the trough is expected to pass over the
forecast area on Tuesday. It will bring some increasing clouds and
slight instability to the forecast area providing a slight chance of
thunderstorms from central Oregon to southeast Washington.   

.LONG TERM...Wednesday Night Through Sunday Night...A broad upper
level trough will settle over the Pacific Northwest for much of the
extended period and even beyond. An anomalous...and nearly
stationary upper ridge of high pressure will be centered near 35N,
150W out over the Pacific during this time. This will again act to
keep our area under lower than average heights. For sensible weather
expect more clouds...chances for showers and much cooler
temperatures. In fact, high temperatures are only forecast to be in
the 70s to around 80 in the valleys/basins...with 60s in the
mountains for Thursday through Sunday...this is about 5 to 8 degrees
below the seasonal average. Low temperatures will generally be in
the mid-40s to mid-50s through the period in the lower
elevations...with 30s and lower 40s mountains.

Wednesday night through Friday morning a slow moving upper level low
will slide southeast along the British Columbia Coast. This will
bring more clouds...along with a slight chance of showers and
afternoon thunderstorms mainly along the East Slopes of the Cascades
but also at times to the Northeast Oregon Mountains and Washington
Blue Mtns. After the first trough axis passes through midday Friday
some drier mid level air may try to filter into the region as the
flow briefly turns more zonal. Thus...only left slight chances for
showers or a thunderstorm along the immediate Cascade Crest Friday
afternoon...with the rest of the area looking to stay dry. Then
significant timing and intensity discrepancies develop within the
latest model guidance for next weekend's potential weather
system(s). The latest 12z GFS brings a closed upper level low
pressure system over the forecast area later Saturday into Sunday.
This would mean a better chance for showers across much of the
CWA...especially north. Meanwhile the latest European model brings a
weaker system through on Saturday with just enough moisture to
produce some showers over the eastern third of the area. It then has
a break in the action until a much stronger system, with abundant
moisture moves in Sunday night and Monday. The GFS holds this
stronger...wetter system off until beyond the extended period (next
Tues/Weds). Therefore it is clear the models are having difficulties
timing the individual shortwaves embedded within this larger, mean
upper level trough. For the official forecast went with a blend of
the latest available guidance but also trended higher with PoPs into
next Monday with some agreement on the potential for a stronger
system approaching the area by this time. 77


.AVIATION...00z TAFS...VFR conditions will prevail over the entire
area over the next 24 hours. Expect only a few or sct high clouds
into tonight with mainly clear skies on Monday. Winds will be light
through the period...diurnally terrain driven and generally AOB 12
KTs for most locations. A few gusts could reach 15 KTs at
KRDM..KBDN and KPDT through about 29/04-06z. 77


.FIRE WEATHER...A Red Flag Warning is in effect Monday afternoon
and Monday evening for the John Day-Ochoco Highlands (fire zone 642)
due to Haines of 6...low humidity...and locally gusty winds. This
includes the Rail Wildfire.  As the upper flow shifts to the
southwest and the ridge breaks down...the air mass will become
increasingly unstable on Monday.  Dry air aloft can also be seen on
water vapor spreading into southeast Oregon.  The Haines will reach
a high category of 6 in the warning area during the peak heating of
the day when the humidity will be 8-15 pct.  The Haines will also
reach a 6 as far north as the Wallowa-Whitman National Forest...but
this will not be until Monday evening when the humidity will be
higher and the Haines will have less of an impact on any existing
fires. After close watches will be issued for
thunderstorms on Tuesday.  The two main inhibiting factors that will
limit lightning activity will be the cloud cover Monday morning that
will be broken or overcast and limit surface heating as well as
relatively high CIN and mid-level cap on forecast soundings.  Will
keep the LAL of 2 for Tuesday for Central and Northeast Oregon.


PDT  60  90  60  83 /   0   0   0  10 
ALW  64  91  64  84 /   0   0   0  10 
PSC  58  93  60  87 /   0   0   0   0 
YKM  57  92  58  88 /   0   0   0   0 
HRI  58  92  60  85 /   0   0   0   0 
ELN  57  90  57  85 /   0   0   0  10 
RDM  49  91  49  80 /   0   0   0  10 
LGD  53  92  53  88 /   0   0   0  10 
GCD  54  92  56  89 /   0   0   0  20 
DLS  62  90  61  82 /   0   0   0   0 


.PDT Watches/Warnings/Advisories...
OR...Red Flag Warning from 2 PM to 9 PM PDT Monday for ORZ642.




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