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166
FXUS66 KPDT 292143
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
243 PM PDT Mon Apr 29 2024

.SHORT TERM...
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This afternoon through Wednesday night...Latest
water vapor imagery shows an upper trough moving across the region
at the present time. Scattered showers and high mountain snow
showers have been associated with this feature and expect a
gradual diminishing trend this evening as the trough moves east of
the area. There have been a few lightning strikes over the
mainly over the eastern Mountains already and the SPC mesoscale
analysis page shows 250-500 J/KG surface-based CAPE over northern
Oregon and southeast Washington. Best potential for thunderstorms
(15-30%) through early this evening will be from the eastern part
of the Columbia Basin of Washington through the Blue Mountain
Foothills of WA/ORE and the northeast mountains.

Winds will continue to increase this afternoon across the lower
elevations peaking 23-01Z and then slowly diminishing in the
03Z-06Z time frame. West winds of 20 to 25 mph with gusts to 30 to
40 mph are expected but the potential for much stronger winds is
rather low due to a lack of low-mid level jet support. The
probabilities of 40 mph wind gusts in the Columbia Basin of Oregon
(where winds are forecast to be the strongest) are 30-70% from
the NBM but the probabilities of wind gusts exceeding 45 mph are
only 5-30%. Patchy blowing dust may occur across the Oregon Basin
through early this evening but confidence is low (40% chance).

Ensembles are in good agreement on dropping another upper
trough/low across the region on Tuesday. This will bring another
round of scattered showers and high mountain snow showers. SPC
HREF shows a slight potential (15-25% chance) for thunderstorms
over northern Oregon and extreme southern Washington Tuesday
afternoon and evening. Surface Based CAPE on Tuesday is similar to
today. Thus a slight chance of TSTMS was added to the forecast in
the aforementioned areas. There will be light to moderate snow
along the Oregon east slopes Tuesday but likely to be sub
advisory amounts. The probability of 4 inches of snow in the 24
hour period ending 12Z Wednesday for Santiam Pass is 40%.

On Wednesday a drier NW flow will be over the region with just a
low chance of mountain showers (15-30%). The next upper low will
approach the region on Wednesday night but with most of the
impacts occurring in the extended period. 78

.LONG TERM...Thursday through Monday...Models in good agreement
through Friday night but diverge in the Saturday-Monday synoptic
pattern. This leads to significant uncertainties in the Days 5-7
forecast.

Thursday will be met with cool showery conditions as a deep upper
low off the WA coast travels to the southeast across the forecast
area. There will be steep lapse rates and cold air aloft (H5 temps
averaging around -23C) for a chance of thunderstorms, mainly over
eastern Oregon. QPF does not look overly impressive...mainly 0.1" or
less. Therefore, snow amounts will be light, although snow levels
will be near pass levels at 3500-4500 feet.  There will be a break
in precipitation Thursday night and Friday as the forecast area lies
between systems, and precipitation for most of Friday night will be
west of the Cascades.  Models are in agreement with a deep low
offshore Friday night but differ considerably on the track of the
low starting Saturday.  The deterministic GFS and a majority of the
GEFS solutions keep a deep low offshore Saturday, however there is a
fair amount of spread on the position of the low.  The deterministic
ECMWF and around 55% of the EC ENS solutions weaken the low and push
it across WA/OR, leaving us in a westerly flow on Sunday. The
Canadian and its ensembles are somewhere in between the GFS and
ECMWF. The model variations are more pronounced Sunday with either a
trough or a ridge over the PacNW.  The uncertainty bleeds into
Monday as well, although they seem to be in slightly better
alignment advertising another offshore trough.

Chances of precip at the low elevations are low (<30%) with a 30-40%
at the mountain areas for Thursday morning into night as the broad
low passes before precip starts to decrease when the ridge occurs.
Friday onward, precip increases with a 30-45% chance in the forecast
area. Gusty winds will occur between 20-35 kts for late Saturday
morning into Monday, induced from the surface pressure gradient due
to the troughing. Snow levels from Thursday into Monday night will
be around 3500-5500ft as the systems move over PacNW.

Unfortunately, all of the model discrepancies will affect most of
the sensible weather forecast--temperatures, precipitation, sky
cover, and wind.  Despite these differences, there doesn`t appear to
be any major anomalous conditions for our CWA. The long term can be
summarized simply as average spring weather.  Feaster/Wister
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&&

.AVIATION...18Z TAFS...VFR currently at all sites with cloudy skies
(FEW to OVC, 040-120 kft) for this period. Wind gusts will be the
primary concern for all sites as the frontal boundary passes through
PacNW, inducing strong surface pressure gradient. KDLS/KALW are
currently having rain showers until tonight while rain showers at
KPDT might occur around late morning into afternoon. However, there
is a 20-30% chance of thunderstorms happening at the Blue Mountains
late Monday morning into night, but was excluded from the TAFs due
to low confidence (20%). Expect light precip for the remaining TAF
period (80% confidence). Winds will be moderate at 10-20 kts for
most sites during morning into tonight for KDLS/KRDM/KYKM/KALW/KPSC
with wind gusts at 20-30 kts, but wind gusts vary for KPDT/KBDN
until Tuesday morning. Feaster/97

&&

.PRELIMINARY POINT TEMPS/POPS...
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PDT  33  54  34  60 /  10  40  30  10
ALW  38  57  37  63 /  20  40  30  10
PSC  40  62  39  68 /  10  20  10   0
YKM  32  59  33  67 /  10  30   0   0
HRI  37  60  37  66 /  10  30  20   0
ELN  32  56  36  62 /  10  20   0   0
RDM  26  50  26  57 /   0  30  10   0
LGD  30  50  32  55 /  10  60  50  20
GCD  27  50  30  55 /  10  50  40  10
DLS  37  57  40  62 /  20  60  10   0
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&&

.PDT WATCHES/WARNINGS/ADVISORIES...
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OR...None.
WA...None.
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&&

$$

SHORT TERM...78
LONG TERM....85
AVIATION...97