Area Forecast Discussion
Issued by NWS Pendleton, OR

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000
FXUS66 KPDT 221709 AAA
AFDPDT

Area Forecast Discussion...Updated
National Weather Service Pendleton OR
1008 AM PDT Tue Aug 22 2017

Updated Aviation Discussion

.SHORT TERM...Today and tonight...A ridge of high pressure will be
moving over the region today and then off to the east by Wednesday.
This will allow temperatures to climb 5 to 10 degrees today. This
will also allow a southwest flow to develop which will eventually
pull some moisture from California and Nevada into southern Oregon
late today. There is a slight chance some of this could reach
central Oregon this evening and and overnight. There is not a lot of
forcing in the atmosphere so convective threat is minimal but worth
mentioning of slight chance in the forecast. The southwest flow is
also responsible for pulling some of the smoke from fires over
southwest Oregon over the forecast area. This will create hazy
conditions today and could moderate temepratures down a couple of
degrees in some areas.

&&

.FIRE WEATHER...Other than for areas of smoke/haze, quiet weather
expected through this afternoon. An initial slight chance of
thunderstorms will enter Central Oregon this evening in advance of
an upper level trough. Thunderstorm coverage will increase Wednesday
afternoon/night with the approach/passage of a cold front, with
scattered thunderstorms expected across Central Oregon in the
afternoon, migrating eastward across the northeast Oregon mountains
Wednesday night. Will continue a Fire Weather Watch for zones 611,
640, 642, 643, 644 and 645 for the potential of abundant lightning
and gusty thunderstorm winds. Winds will increase in the wake of the
front Thursday, with breezy to locally windy conditions for the
Columbia Gorge and Basin. Model trends have been a bit lighter for
winds, and with the maximum winds likely occurring when RHs are
beginning to rise, will not issue any watches for Thursday at this
time.

&&

.AVIATION...18z tafs. Clear skies expected over the next 12-18 hours
with increasing high clouds after that. Smoke is overspreading the
region from fires in southwest Oregon and northern California. Will
likely have an elevated smoke layer and possible reductions in
surface visibility especially at KRDM KBDN. Winds 5-15kt. 76

&&

.PREV DISCUSSION... /issued 244 AM PDT Tue Aug 22 2017/

SHORT TERM...Today through Thursday night...An active weather
pattern will develop through the short term period, as an
approaching upper level trough impacts our region. For this morning,
things start off fairly quiet, although some haze/smoke will likely
move across the CWA in the southwest flow pattern. By the afternoon
hours, instability and moisture begin to increase over south-central
Oregon in association with the weaker upper level low over central
California. The latest hi-res model guidance indicates some pop-up
showers and t`storms for locations south of Bend and John
Day...mainly after 3 PM. These isolated showers/storms may linger
into the evening hours. Temperatures will be quite warm today, about
10 degrees above average...with highs well into the 90s in the
valleys/basins. For tonight, much of the area will see mainly clear
to partly cloudy skies. There will still be a slight chance for a
lingering shower or storm...mainly in the Ochoco-John Day highlands
and south-central Oregon. On Wednesday, the stronger upper level
low/trough currently located in the Gulf of Alaska will begin to
impact the area. Heights will begin to fall, and a frontal boundary
is forecast to move onshore late in the day. Out ahead of this
system out forecast area will remain in a general southwest flow.
This will allow moisture and instability to continue to increase.
PWATS rise to over 1" in the afternoon, and most unstable CAPE
values could reach 500 J/kg in some areas. This will mean an
increasing chance for showers and t`storms Wednesday afternoon and
evening, especially from Central Oregon northeast to the Blue
Mountain, Wallowa and Union Counties. Cannot rule out a t`storm
making into the Blue Mountain Foothills during this time as
well...have this covered with slight chc PoPs for now. Storms that
do form are not expected to be severe, as deep layer shear values
are only 20-30 kts and instability is only marginal. A few storms
could however produce abundant lightning, and therefore a fire
weather watch has been issued for fire weather zones 611, 640, 642,
643, 644 and 645 (see fire weather discussion below for more
details). Further north and west over the Columbia Gorge, Kittitas
Valley, Yakima Valley, and the western Lower Columbia Basin
conditions are forecast to remain dry, under partly cloudy skies on
Wednesday. Temperatures will start a cooling trend Wednesday and
westerly winds will turn breezy (15-20 mph) in the Columbia Gorge
and Kittitas Valley. The latest guidance disagrees on just how fast
the shower/t`storm activity exits the area Wednesday night as the
front pushes through. The 00z GFS would linger the activity into the
overnight hours in the Northern Blue Mtns/Wallowa County. Meanwhile,
the NAM would move the activity off to the north and east...out of
the area by midnight Weds night. For now, leaned toward continuity
and the slower GFS solution...but this may need to be adjusted in
future updates. Thursday is now looking mostly dry, besides from a
stray shower/t`storm over the far eastern mtns. Westerly winds will
be quite breezy...between 15-25 mph in the afternoon. This could
cause some localized, patchy blowing dust over the Lower Columbia
Basin and Blue Mtn Foothills. Temperatures will be much cooler, with
highs only reaching the 80s, except upper 60s and 70s mountains.
Thursday night will be mostly clear and chilly, with overnight lows
dipping into the 40s and lower 50s...except 30s mountains and high
valleys. 77

LONG TERM...Friday through Monday. Models in fairly good
agreement through the extended. Dry westerly flow aloft Friday as
upper level trough moves to the east. Expect sunny skies and cooler
temperatures. Winds will be diminishing. Heights start rising
Saturday as a strong upper level ridge begins to build into the
Pacific northwest with a warming trend. The ridge will be centered
over the region Sunday and Monday with temperatures well above
normal. High will be in the 90s with some locations approaching 100.
Skies will remain mostly clear through the period.  94

FIRE WEATHER...Quiet weather expected this morning. Isolated
thunderstorms will be possible across mainly central Oregon this
afternoon as a trough of low pressure begins to approach.
Thunderstorm coverage will increase Wednesday afternoon/night with
the approach/passage of a cold front, with scattered thunderstorms
expected across Central Oregon in the afternoon, migrating eastward
across the northeast Oregon mountains Wednesday night. Will issue a
Fire Weather Watch for zones 611, 640, 642, 643, 644 and 645 for the
potential of abundant lightning and gusty thunderstorm winds. Winds
will increase in the wake of the front Thursday, with breezy to
locally windy conditions for the Columbia Gorge and Basin. Model
trends have been a bit lighter for winds, and with the maximum winds
likely occurring when RHs are beginning to rise, will not issue any
watches for Thursday at this time. 80/77

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  93  58  91  60 /   0   0  10  20
ALW  95  62  93  64 /   0   0  10  20
PSC  96  60  94  61 /   0   0   0  10
YKM  94  56  93  58 /   0   0   0   0
HRI  95  59  93  62 /   0   0  10  10
ELN  93  56  90  59 /   0   0   0   0
RDM  92  51  87  49 /  10  10  20  10
LGD  92  54  89  55 /   0  10  20  50
GCD  94  57  88  56 /   0  20  30  30
DLS  94  63  89  64 /   0   0   0   0

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...Fire Weather Watch from Wednesday afternoon through late
     Wednesday night for ORZ642>645.

     Fire Weather Watch from Wednesday afternoon through Wednesday
     evening for ORZ611-640.

WA...Fire Weather Watch from Wednesday afternoon through late
     Wednesday night for WAZ643-645.

&&

$$

91/76/76


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