821
FXUS66 KPDT 160907
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
207 AM PDT Sun Jun 16 2024

.SHORT TERM...Today through Tuesday night...Models are in
excellent agreement on the evolution of the 500 mb pattern the
next few days. The GEFS ensemble moves a developing upper low over
SRN BC across Oregon and Washington tonight and Monday morning
leaving broad troughiness in its wake for Monday night through
Tuesday night.

For this afternoon and evening as the upper low develops/
approaches there will be a 30-60% chance of showers and a slight
chance (10-20%) of TSTMS along the east slopes of the Cascades.
Thermodynamic profiles from NAM forecast soundings show steep low
to mid level lapse rates and surface based CAPE of around 500
J/kg.

This evening scattered to numerous showers (30-60% POPS) will
develop over central Oregon and then move NE across the E/NE
mountains overnight. QPF amounts are forecast to be .10-.15
inches on average. Light snow of 1-2 inches is expected at
elevations above 5000 feet.

On Monday the upper low will move east of the region with the
bulk of the remaining showers confined to the northeast mountains
and Wallowa County...with a 10-15% chance of TSTMS. A cold front
will cross the area which will cause west-northwest winds to
increase in the Cascade gaps. Strongest winds are forecast to be
in the Simcoe Highlands north of he Columbia River and in the
Kittitas Valley. The NBM probabilities of wind gusts exceeding 45
mph is 40-70%. All 50 of the ECMWF ensemble members are predicting
max wind gusts exceeding 40 mph at KELN on Monday but none are
forecasting 50 mph. Thus the potential need for wind highlights is
moderate on Monday (40-50% chance).

On Tuesday there will just be a slight chance of showers along the
Washington crest and Wallowa County (15-20% chance). Otherwise it
will be dry. It will be chilly today with high temperatures 10-15
degrees below normal, but it will warm up each day so that by
Tuesday it will be just 3-5 degrees below normal for high
temperatures. Low temperatures below freezing can be expected in
the mountain valleys Monday and Tuesday morning.

.LONG TERM...Wednesday through Sunday...Models are in good agreement
with the synoptic pattern through Saturday, but differences begin to
emerge for Day 8 (Sunday).  A dry forecast is in store during this
time with the exception of a few showers along the east slopes of
the central WA Cascades next weekend.  The biggest weather impacts
over the forecast area will be the temperatures that are more
typical for late July rather than late June on Friday and Saturday.
Models have consistently advertised a weak marine push late Saturday
that will result in increasing wind and elevated fire weather.

The PacNW will be under a weak WSW flow aloft through the end of the
week.  It will be mostly dry but cannot rule out that sprinkles or
an occasional orographic shower will develop over the mountains.
Winds will predominantly be light and terrain driven, but there is a
chance of localized gusts around 20 mph (40% confidence) or 25 mph
(20% confidence).  An inverted surface thermal trough building east
of the Cascades Thursday night through Saturday will cause
temperatures to climb to 10-15 degrees above seasonal average.  The
experimental HeatRisk product shows minor to moderate heat.  Will
there be 100 degrees during this time?  NBM would suggest `no` based
on the 10% or less probability.  The probability of 90+ degrees from
the NBM is 60-80% across the Lower Columbia Basin, the Yakima
Valley, and portions of the John Day Basin.

Forecasters will keep an eye on the thermal trough moving into ID
and allowing cooler marine air to seep through the Cascades late
Saturday.  With forecast max temperatures in the 90s and forecast
RHs in the teens, the increase in wind will elevate fire weather
concerns.  Confidence for the marine push on Saturday is around 30%.

The forecast for Sunday will show cooler and breezy weather with a
20% chance of showers along the WA Cascades. The ensembles differ on
the amplitude and position of an offshore trough, and 50% of the
GEFS members maintain a westerly flow across the eastern Pacific and
the PacNW.  Differences are not major but are signs of
inconsistencies that extend into the long range forecast for early
next week. Wister/85

&&

.AVIATION...06z TAFs...VFR conditions expected. Winds will
continue to die down overnight, leaving all sites with light,
terrain-driven winds less than 10 kts. Winds will pick back up again
across many sites by late tomorrow morning, with mid-level sct-bkn
clouds building in as well. Light isolated showers may develop
across the forecast area tomorrow afternoon, however rain is
expected to be too light to cause any real aviation impacts, at
least at the surface. Evans/74

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  65  43  67  43 /   0  40  30  10
ALW  68  45  71  46 /   0  30  40  10
PSC  71  48  74  48 /  10  20  10  10
YKM  68  43  72  43 /  30  20  10   0
HRI  71  46  73  47 /  10  30  20  10
ELN  65  45  68  45 /  30  20  10   0
RDM  60  35  61  34 /  20  30  10   0
LGD  62  40  60  39 /  10  50  50  20
GCD  64  38  60  37 /  20  70  50  10
DLS  68  50  68  48 /  20  10  10   0

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...Freeze Warning until 9 AM PDT this morning for ORZ050.

WA...None.
&&

$$

SHORT TERM...78
LONG TERM....85
AVIATION...74