363
FXUS66 KPDT 100240
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
740 PM PDT Thu May 9 2024

.UPDATE...A quiet weather evening is underway with clear skies
aside from some mountain cumulus which should dissipate later on.
Winds are beginning to decrease and should be light overnight.
High pressure will help keep temperatures warmer tonight than last
night - as much as 10 degrees warmer in the Columbia Basin.
Current forecast looks good and no changes were made.

One mild concern going into this weekend and next week are
increased snow melt from the warmer temperatures in those parts of
the mountains that still have a snowpack. Several rivers and
streams in the mountains are forecast to rise though near all stay
well below bankfull. The exception is the Naches River at
Cliffdell which is forecast to rise slightly above bankfull
Saturday evening and the streamflow data shows the beginnings of
a rise in the last few hours. Probabilistic forecasts for the
Naches at Cliffdell have been steadily rising from an 88 percent
chance of exceeding bankfull in the next 10 days a couple of days
ago to a greater than 95 percent chance in the latest forecast.
The good news is that the Naches River at Cliffdell is expected to
crest at just a few inches above bankfull Monday morning though
it is expected to remain barely above bankfull through Thursday.
At these levels, there should be few, if any, impacts.

Further downstream, the Naches River at Naches is forecast to
rise as well but remain below bankfull. However, the probabilistic
forecast for the Naches River at Naches does give a 33 percent
chance of exceeding bankfull at some time in the next 10 days, so
that`s another location to be monitored through then. 83

&&

.PREV DISCUSSION... /issued 455 PM PDT Thu May 9 2024/

SHORT TERM...Tonight through Saturday night...A large ridge of high
pressure will build over the Pacific Northwest through Saturday.
This ridge will flatten a bit through Saturday night. Dry and
increasingly warm conditions are expected through the weekend. High
temperatures will be 15 to 20 degrees above normal on Saturday, on
average across the area.

Probabilities of high temperatures >=90 degrees on Saturday range
from about 60% in the lower Columbia Basin of Oregon to about 60
to 70% in the Yakima Valley and 70 to 80% in the Columbia Basin of
Washington. None of the GFS ensemble members reach >=90 degrees
on Saturday for Tri-Cities whereas approximately 50% of the ECMWF
ensemble members >= 90 degrees.

High temperatures will range from the upper 70s to mid 80s on
Friday, then generally in the 80s to around 90 degrees on
Saturday. Overnight lows will be in the 40s tonight and the 40s
to lower 50s Friday night.

LONG TERM...Sunday through Thursday..A shortwave trough will
approach the area on Sunday and move across the region Sunday night.
The ensemble clusters favor a flat wave compared to the higher
amplitude depicted in the deterministic models. Sunday will be the
last very warm day with high temperatures in the mid 80s to lower
90s in the lower elevations. Probabilities of 90+ degree high
temperatures Sunday are 30-70% across the Columbia Basin and Yakima
Valley. A cold front will cross the area Sunday associated with the
shortwave and it will become breezy through the Cascade gaps in the
afternoon and evening. The winds will continue on Monday along with
cooler temperatures (8-10 degrees cooler than Sunday.) The 12Z GFS
is forecasting the surface pressure gradient between PDX and GEG to
exceed 10 mb on Sunday night and Monday which supports windy west
winds in the lower elevations. Probabilistic NBM for the 24 hours
ending at 11 PM Monday shows a 50-70% chance of winds exceeding 45
mph in the usual windy locations under strong westerly flow regimes.

On Tuesday into early Wednesday the GEFS and ECMWF ensemble build a
ridge into the region from offshore with NWLY flow aloft over the
area. This will result in dry and warm conditions on Tuesday and
possibly Wednesday though the uncertainty in the forecast begins to
increase on Wednesday and even more on Thursday. For Thursday the
ensemble clusters are split with roughly half the ensemble members
favoring a trough solution and the other half favoring a ridge. This
is creating significant uncertainty in the temperature forecast for
mid week. For example, on Thursday at Pendleton the NBM 25th
percentile high temperature is 66 degrees and the 75th percentile is
85 degrees. The GFS and ECMWF ensemble temperatures are cooler than
the official forecast so it is possible that temperature will have
to be nudged downward in later forecasts. The chance for
precipitation appears quite low through the period. Except for
occasional POPS 15-20% along the Cascade crest, Northern Blues and
Wallowas, the chance for measurable precipitation elsewhere is 10%
or less. 78

AVIATION...00z TAFS...Little to no aviation concerns the next 24-
hrs. The latest satellite imagery shows SCT-BKN mountain cu across
parts of central OR mountains to the eastern mountains that should
diminish with the loss of daytime heating around 02-06 UTC this
evening. High confidence (>80%) in limited sky cover over the basin
and lowlands through the TAF period, though mountain cu development
is expected again. Current thinking is similar to today but leaning
towards a somewhat larger geographic area across the higher terrain
of central OR, Ochocos, John Day Highlands, and eastern mountains
with bases >6 kft and higher. Otherwise, an upper ridge aloft will
remain and dominate our local sensible weather. A light surface
gradient in place with an innocuous surface pressure pattern
prevailing that will promote light winds and breezes through the
period. Of which, 24-hr daily max sustained winds chance of
exceeding 11 kts is sub-60 percent at all TAF sites thus high
confidence in place for this outcome. 80

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  47  79  51  84 /   0   0   0   0
ALW  50  82  54  86 /   0   0   0   0
PSC  52  86  55  91 /   0   0   0   0
YKM  48  85  51  90 /   0   0   0   0
HRI  49  84  53  90 /   0   0   0   0
ELN  49  83  53  87 /   0   0   0   0
RDM  40  78  46  83 /   0   0   0   0
LGD  44  76  49  81 /   0   0   0   0
GCD  43  77  47  81 /   0   0   0   0
DLS  53  86  57  89 /   0   0   0   0

&&

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

$$

EVENING UPDATE...83
SHORT TERM...77
LONG TERM....78
AVIATION...80