633
FXUS66 KPDT 011016
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
316 AM PDT Sat Jun 1 2024

.SHORT TERM...Today through Monday...

Key Messages:

1. Breezy to windy conditions Saturday and Monday.

2. Widespread rain Sunday afternoon through Monday afternoon.

3. Thunderstorm potential Monday afternoon.

Current radar and infrared satellite imagery showing some light
returns approaching the west slopes of the Washington Cascades as
high to mid-level clouds begin to encroach along the east slopes
of the Cascades. This is associated with the approaching upper
level shortwave following closely behind the departing transient
ridge that brought sunny skies and warm conditions to the area on
Friday. The incoming shortwave and associated weak cold front will
provide rain chances along the Cascades (30-45%) and the
Blues/Elkhorns (15-25%) this afternoon. The weak cold front will
also allow a pressure gradient to develop along the Cascades to
enhance winds through the Simcoe Highlands, Kittitas Valley,
northern Blue Mountain foothills, and the Lower Columbia basin of
Oregon, as gusts of 25-35 mph will be possible. Confidence in
these wind values is high (80%) as the HREF suggests a 40-60%
chance of wind gusts of 30 mph over the aforementioned areas,
dropping to a 10-20% when increased to 35 mph. Confidence in winds
reaching advisory level is low (10%) as the GFS and SREF showcase
a pressure gradient of 6-7mb between Portland and Spokane, well
below the normal advisory criteria of 12 mb. However, this will
not be the case with the next system on Monday, as it is much more
robust and may warrant the issuance of wind advisories over the
Simcoe Highlands, Kittitas Valley, northern Blue Mountain
foothills, and the Lower Columbia Basin of Oregon. The upper level
trough and associated cold front will tighten isobars on its
approach, and lead to a more enhanced gradient along the Cascades
as advertised by the GFS of 11-12 mb between Portland and
Spokane. Thus, gusts of 40-50 mph are anticipated to occur over
the aforementioned areas peaking between 11 AM and 8 PM Monday.
Confidence in these wind gusts reaching advisory level is moderate
to high (70-80%) as the NBM suggests a 70-95% chance of gusts
reaching or exceeding 47 mph over the earlier mentioned areas,
with the highest chances (85-95%) across the Simcoe Highlands.

The secondary concern resides with substantial rainfall associated
with an incoming upper level trough Sunday afternoon through
Monday. The moisture accompanying this system is supplied by an
atmospheric river (AR), inflating precipitable water values to
between 0.75"-1.25" (150-225% above normal). The AR is rather
stout in nature, as the ECMWF and GFS both showcase an Integrated
Vapor Transport (IVT) of between 600-800 kg/m*s, with the GFS
ensemble members showing high confidence (90-100%) in an IVT of
500 kg/m*s or greater. The timing of peak rainfall has trended
about 6 hours earlier with guidance over the last 24 hours, which
is now expected to occur between 11 AM Sunday through 11 AM
Monday. Rain amounts during this timeframe are as follows:
1.00-1.50 inch over the Cascades, Elkhorns, and northern Blue
Mountains; 0.45-0.85 of an inch across the northern Blue Mountain
foothills, east slopes of the Cascades, and Wallowa County;
0.20-0.40 of an inch over Central Oregon and the John Day-Ochoco
Basin; and 0.10-0.20 across the Yakima/Kittitas Valleys and over
lower elevations of the Columbia Basin. Confidence in these rain
amounts is moderate to high (60-80%) as the NBM showcases a 70-90%
chance of 1 inch or greater rainfall over the Cascades and
northern Blue Mountains, a 55-75% chance of 0.50 of an inch of
rainfall over the northern Blue Mountain foothills and Wallowa
County, a 40-55% chance of 0.25 of an inch over Central Oregon and
John Day-Ochoco Highlands, and a 40-50% of 0.25 of an inch or more
over the Yakima/Kittitas Valleys and lower elevations of the
Columbia Basin. These rain amounts may lead to times of moderate
to heavy rainfall, especially over mountainous terrain. This may
promote a rise in area rivers, which is currently only forecast to
bring the Naches River near Cliffdell and near Naches to action
(bankfull) stage Monday morning. Currently, forecasts only briefly
peak a couple inches above action stage, but this will continue to
be monitored as the event nears.

The final concern associated with the passing system Sunday and
Monday reside with the potential for thunderstorms to develop
across the Blue Mountains and east Monday afternoon and evening.
The earlier rain associated with the warm front on Sunday will
help to prime the atmosphere, which should reach convective
temperatures earlier ahead of the cold front on Monday. Favorable
surface based CAPE values of 500-900 J/kg and shear of 30-45 kts,
as advertised by the GEM and GFS respectively, provide ample
parameters for isolated thunderstorms to begin to develop by late
Monday morning. The main concern would reside with gusty winds,
hail, and frequent lightning. This will be monitored closely over
the next 48 hours as convection-allowing models (CAMs) become
available. 75



.LONG TERM...Tuesday through Saturday...Models continue to struggle
with the pattern over the PacNW. This struggle is reflected in the
EOFs varying 45-55% through the period. The main culprit of this
divergence is the Gulf of Alaska upper level low. Its position and
strength determines the strength and position of the amplifying
upper level ridge over the area. The cluster analysis shows
approximately 50-60 percent of the members build the ridge over the
PacNW while 40-50 percent favor the Continental Divide. With the
ridge building over the PacNW, it will be warmer and drier. If the
ridge builds over the Continental Divide it will not be quite as
warm with daily afternoon/evening thunderstorms over the mountains,
especially Thursday onward. The ensembles show a similar pattern
with the GFS favoring the Continental Divide solution while the
ECMWF favors the PacNW solution. In general terms, the ECMWF has a
better track record and most members (just barely) do favor the
PacNW so will lean towards its solution. If the GFS pans out,
daytime temperatures will be about 10 degrees cooler, but still
above normal. In summary, confidence is moderate-high (60-80%) an
upper level ridge will impact our weather late next week, but low
(20-30%) on final temperatures and precipitation chances over the
mountains given discrepancy in its position.

Daytime highs will begin the period near normal but warm each day
peaking Friday or Saturday 10-15 degrees above normal.

As noted earlier, if the GFS pans out the mountains will see daily
afternoon/evening convection beginning Thursday and continuing into
the weekend. At this time, confidence is low-moderate (20-30%) that
the GFS solution will pan out. Earle/81


&&

.AVIATION...06Z TAFS...VFR conditions are favorable during TAF
period with current high clouds at BKN-OVC 100-250. Gusty winds will
be at 15-30 kts with sustained winds at 10-20 kts around Saturday
morning for KDLS/KPDT/KRDM/KBDN until late evening. Feaster/97


&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  75  51  69  54 /  10  10  60  80
ALW  78  55  71  57 /  10  10  60  90
PSC  81  57  71  59 /  10  10  60  80
YKM  77  50  68  50 /  10  10  50  70
HRI  80  55  71  56 /  10  10  50  70
ELN  71  48  65  50 /  10  10  60  70
RDM  72  46  68  53 /  10  10  20  70
LGD  72  47  68  54 /  20  10  50  80
GCD  74  46  69  55 /  10  10  50  80
DLS  77  55  67  58 /  10  10  60  80

&&

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

$$

SHORT TERM...75
LONG TERM....81
AVIATION...97