595
FXUS66 KPDT 151615
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
915 AM PDT Thu Aug 15 2024

.MORNING UPDATE...RED FLAG WARNINGS IN effect until 11 PM this
evening for the central mountains of OR, southern Blues and
Strawberry mountains as well as the central Blue Mountains and
the Wallowa District.

Slight change to the forecast this morning as the radar continues
to show showers creeping up and around Wallowa County and
sweeping towards the west. However, looking at ground
observations, very little rain is currently making it to the
ground as of yet. Thunderstorms are still expected to pop up
through much of the day stretching from central OR across the
eastern mountains (60-80%). The storms expected across central OR
will be dry in nature, however, the storms over the eastern
mountains will be wet and could produce heavy amounts of rainfall.
Bennese/90


&&

.PREV DISCUSSION... /issued 455 AM PDT Thu Aug 15 2024/

SHORT TERM...Today through Saturday...Overnight satellite imagery
showing a low pressure system centered over south central Oregon,
currently inching north and east, bringing with it a shield of
moisture with light showers currently impacting our central Oregon
zones. Earlier in the evening, thunderstorm activity had begun to
ramp up over Lake and Klamath Counties, however as the cluster of
storms approached our forecast area, its strength tapered off,
leaving us with light showers instead. Upper-level support seems to
be better situated over the southernmost counties of Oregon, however
we could still see some lightning strikes over the southernmost
portion of our forecast area as this low continues its northeastward
push through the Interior Northwest.

Based on latest CAMs, the current disorganized band of showers will
move toward the eastern mountains by daybreak, before models suggest
another cluster of convection developing primarily over the
Strawberries and Grant County, likely due to the center of the low
being overhead at the time, thus maximizing lift. Confidence is on
the lower end (20-30%) as to whether or not what the CAMs are
suggesting for the afternoon will verify, which does not bode well
for our current Red Flag Warning, but plans for now are to keep it
in effect in case convection starts to ramp up across our forecast
area with the low overhead.

The trend for Friday continues to be that of drier conditions as mid-
level moisture falls off quite a bit. The current low lifts
northward as is eventually absorbed by a larger, developing low that
looks to center itself just off the PacNW coast, staying in place
due to a blocking pattern. Models suggest this system will dominate
our weather through as long as the midweek next week. Greatest
forecast challenge heading into the weekend will be where exactly
the strongest axis of moisture advection sets up aloft, with models
right now suggesting a more SSW orientation. This would maximize
PoPs and storm chances along the Cascades, while drier, hotter
desert air from the Great Basin moves in over the eastern half of
the forecast area. Saturday thus looks to be warmer east of the
Cascade east slopes, with scattered shower and thunderstorm chances
elsewhere. Whether or not these storms will warrant any fire weather
headlines is to be determined once this ongoing low moves out of our
area. Evans/74

LONG TERM...Sunday through Thursday...Models are in very good
agreement in having an upper low and trough over the Pacific
Northwest through the long term period though there are differences
as to the strength and location of the trough axis. In general,
temperatures will be a few degrees below normal and despite a
southwest flow over the area through the period, light showers and a
few thunderstorms will be confined to the Washington Cascade crest.

Sunday through Tuesday, models are in good excellent agreement in
having the trough centered just offshore with a closed low wobbling
about the base of the trough, moving from the Washington coast on
Monday north to Vancouver Island on Tuesday. By Wednesday, the GFS
favors the trough off of the Oregon/Washington coast while over half
of the ECMWF ensemble members have a weaker trough further offshore
though this should not change the forecast significantly. On Sunday,
there is significant uncertainty in the temperatures as the NBM has
a 6-11 degree temperature spread between the 25th and 75th
percentiles. Looking at the individual model ensemble members, The
GFS is cooler with highs in the mid 70s to mid 80s while the ECMWF
is mainly in the 80s. The NBM determinant temperatures favor the
cooler GFS temperatures and given the trough overhead, have accepted
those temperatures but confidence is not high. This yields
temperatures in the mid 70s to mid 80s over the area. Monday warms
up a couple of degrees to the upper 70s and 80s and then on Tuesday
cools back down to Sunday`s temperatures. For each day, the NBM has
a greater than 85 percent chance of reaching 80 degrees in the lower
elevations, but the chance of reaching 90 degrees generally less
than 30 percent, except in the lower Columbia Basin where the Tri-
Cities has a 40-60 percent chance each day. Instability is nearly
nonexistent Sunday through Tuesday aside from some weak instability
along the Washington Cascade crest Sunday and Tuesday afternoon so
have a limited area with a slight chance of showers and
thunderstorms there. Rain amounts will be just a few hundredths of
an inch at most. Winds will be fairly light aside from breezy
afternoon winds of 10-20 mph through the Cascade gaps on Monday and
Tuesday.

By Wednesday, models begin having bigger differences. The ECMWF
favors a shallower weaker trough with the closed low north of
Vancouver Island. The GFS favors a stronger trough with the low off
the Washington coast and the Canadian is split between the two with
no solution being dominant. The GFS does have some instability over
the Washington Cascades and the NBM does put a slight chance of
mainly showers all along the Washington and Oregon Cascade crest in
the afternoon. Temperatures remain several degrees below normal in
the mid 70s to mid 80s.

On Thursday, the GFS continues to favor a trough off the Pacific
Northwest coast though the deterministic run sends the closed low
into British Columbia and weakens the trough, while the ECMWF and
Canadian have a stronger trough as a reinforcing low arrives out of
northern British Columbia but both struggle with the location of the
trough with it either well offshore, near the coast or directly
overhead. With models favoring a deeper trough, temperatures cool a
degree or so from Wednesday. A slight chance of light rain is once
again confined to the Washington Cascade crest in the afternoon.
Perry/83

AVIATION...12Z TAFs...An upper low is moving northeast into the
area this morning with a band of showers moving north through
central Oregon and the eastern Oregon mountains. There are isolated
thunderstorms moving towards of the mountains but further west,
thunderstorms ended a few hours ago. Showers should end at KBDN and
KRDM in the next couple of hours but BKN-OVC clouds will continue to
move north this morning at about 10K feet AGL. Showers and perhaps a
few thunderstorms will develop and affect all TAF sites from 20Z-
03Z. Due to low confidence have limited mention of precipitation to
a mix of VCSH and PROB30 -SHRA at all TAF sites. Showers will end
and skies will start clearing after 04Z. Winds at KDLS will increase
to northwest at 15-20 kts with gusts to 30 kts from 17Z-06Z. Other
TAF sites will increase to west to northwest at 10-15 kts with gusts
to 20 kts from 22Z-04Z. Perry/83

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  80  53  81  53 /  20  10   0   0
ALW  84  59  85  57 /  20  20  10   0
PSC  87  62  86  59 /  10  10   0   0
YKM  85  54  84  55 /  20  20   0  10
HRI  85  59  85  56 /  10  10   0   0
ELN  85  56  83  54 /  10  10  10   0
RDM  74  47  80  49 /  30  10   0  10
LGD  80  49  82  51 /  40  40  20  10
GCD  80  49  83  53 /  70  40  10  10
DLS  80  58  83  60 /  10   0   0  10

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...Red Flag Warning until 11 PM PDT this evening for ORZ644-645.

     Red Flag Warning until 11 PM PDT this evening for ORZ611-640-642.

WA...None.
&&

$$

SHORT TERM...74
LONG TERM....83
AVIATION...85