744
FXUS66 KPDT 311717
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
1016 AM PDT Fri May 31 2024

Updated aviation discussion

.SHORT TERM...Friday through Sunday...

Key Messages:

1. Breezy winds Saturday afternoon and evening.

2. Widespread rain showers return Sunday afternoon.

3. Warming temperatures through Saturday before cooling Sunday.

Current radar and infrared satellite imagery showing dry condition
as a few high level clouds pass over our northern zones. This is a
result of a transient upper level ridge that will be passing through
the Pacific Northwest today, keeping skies mostly clear to partly
cloudy north-to-south and high temperatures about 10 degrees warmer
than on Thursday. The ridge will exit to our east early Saturday as
an upper level shortwave follows closely in its wake, tightening
isobars between the two systems and allowing a weak pressure
gradient to develop along the Cascades. As a result, winds will
increase through Saturday morning before peaking in the afternoon
between 25-35 mph across the Simcoe Highlands, Kittitas Valley, and
along the Lower Columbia Basin of Oregon. Winds elsewhere are
expected to stay between 15-25 mph primarily out of the west-
northwest. Confidence in these wind values is moderate to high (70-
80%) as the HREF suggests a 60-80% chance over the Simcoe Highlands,
a 35-45% chance along the Lower Columbia Basin of Oregon, and a 10-
20% chance across the Kittitas Valley of 30 mph gusts or greater.
The NBM highlights more inflated probabilities of 35 mph or greater
as 70-80% over the Simcoe Highlands, 40-60% along the Lower Columbia
Basin of Oregon, and 20-40% over the Kittitas Valley. However, 850mb
winds of between 24-35 mph are advertised by the NAM and HRRR, as
the GFS, SREF, and NAM all keep the pressure gradient from Portland
to Spokane between 5.5-7.5 mb - well shy of the normal advisory
threshold (46 mph) of 12 mb. Thus, there is low probability (<10%)
of advisory-level winds Saturday afternoon and evening.

Saturday`s system is lacking in moisture as flow aloft is from the
west-northwest. This will lead to only higher level terrain over the
Cascades and Elkhorns receiving a chance (20-30%) of rain Saturday
afternoon and evening. The shortwave originated from a strong upper
level low pressure over the Gulf of Alaska, which will be
responsible for a much stronger upper level trough to begin dropping
along the British Columbia coast on Sunday. Flow aloft will shift
more from the southwest Sunday afternoon and evening, allowing
showers to begin to spill into the Basin and northern Blue Mountains
Sunday afternoon before becoming widespread across the region Sunday
evening and into the overnight as the system nears the coast. This
system is tapping into an atmospheric river (AR), which will provide
rain amounts Saturday afternoon and evening of 0.50-1.00" over the
Cascades, 0.30-0.50" through the east slopes of the Cascades and the
northern Blue Mountains, 0.15-0.30" along the northern Blue Mountain
foothills, and 0.02-0.10" for lower elevations of the Basin, Central
Oregon, Kittitas/Yakima Valleys, and the John Day-Ochoco Basin.
Confidence is high (80%) in regards to these rain amounts as the NBM
suggests a 90-100% chance over the Cascades, 60-80% chance over the
east slopes of the Cascades and northern Blue Mountains, 35-55%
chance along the northern Blue Mountain foothills, and a 10-30%
chance through lower elevations of the Basin, Central Oregon,
Kittitas/Yakima Valleys, and the John Day-Ochoco Basin of receiving
rain amounts of 0.25" or more 11 AM Sunday through 5 AM Monday.

The presence of the transient upper level ridge today will promote
mostly clear skies as afternoon temperatures reach near to slightly
above normal values, breaking into the upper 70s to low 80s across
lower elevations of the Basin. Clouds will be increasing late this
evening associated with the passing shortwave on Saturday, which
will also lead to a brief period of southwest flow aloft. These two
features will allow Saturday morning temperatures to be 7-11 degrees
warmer than on Friday to allow high temperatures to bump up 2-4
degrees on Saturday. Cloud cover will break up overnight into Sunday
ahead of the next, more robust system. Thus, overnight lows will be
1-4 degrees cooler than the previous night. These mostly clear skies
will be short-lived as the next system nears, which will increase
cloud cover rapidly through the morning and afternoon as high
temperatures stay about 5 degrees below normal on Sunday. 75


.LONG TERM...Monday through Friday...

Bottom Line Up Front (BLUF)

1. Widespread rain and mountain thunderstorms overnight Sunday into
Monday.

2. Breezy to windy conditions Monday.

3. Drier conditions return midweek with warmer temperatures.

Models are in firm agreement with the upper level shortwave that is
being pushed into the region ahead of an upper level trough.
Guidance is showing a moderately strong atmospheric river (AR) is
being pulled into the synoptic flow which will lead to a increase in
precipitation Monday. Probabilities of the AR is hovering around 80-
90% and has been categorized as an AR 3 which is considered
moderate. That being said, widespread precipitation will grace the
majority of the region on Monday with over 60% of the ensembles
agreeing the lower elevations, such as the Basin and the Blue
Mountain foothills, will see greater than 0.25 inches of rain.
Over 60% of the ensembles are also in agreement the mountains will
see over 0.5 inches or more over a 24 hour period. Models remain
in firm agreement with the upper level shortwave weakening Monday
and flattening through Wednesday. Precipitation will remain mostly
over the Cascades with 60% of the ensembles showing only 0.10
inches of rain for Tuesday. With the instability associated with
the shortwave, model derived soundings are showing some slight
surfaced based CAPE values over 50 J/kg and lapse rates over 6.5
C/km with warm surface temperatures. This is enough to cause some
orographic uplifted thunderstorms over the higher terrain of the
Cascades and the Northern Blues. However, confidence is these
occurring is low (<20%) with guidance showing 15% probabilities of
thunder.

Not only will there be an increase with the precipitation, but also
winds will pick up again with the incoming upper level wave.
Guidance is showing a tightening of the gradients along the Cascades
with models showing a roughly 7-9 mb squeeze. This will bring back
the breezy to windy conditions across the majority of the region
with the key areas being the Cascade Gaps. 60-70% of the ensembles
are showing the Simcoe, Gorge, Horse Heaven Hills, Yakima and
Kittitas Valleys all seeing sustained winds over 30 mph with
gusts nearing or above 40 mph with isolated ridgelines seeing over
45mph.

Models remain in firm agreement with the upper level shortwave
weakening ahead of another upper level ridge Wednesday. Clusters are
in pretty good agreement with the solutions regarding the ridge,
however the clusters so show a slight variance with the amplitude.
This could cause the temperatures to vary slightly depending the
amplitude that follows through. Regardless, EFI is picking up on some
above average temperatures gracing the region. Raw ensembles are
showing 70% agreement that Pendleton and the surrounding area
will see temperatures in the upper 70s to low 80s, over 50% shows
the Columbia and John Day Basin, central and northern OR and
Kittitas Valley in the mid to upper 80s, while elsewhere will be
in the mid 70s. Temperatures will steadily increase through the
remainder of the period.


&&

.AVIATION...18Z TAFS...VFR conditions prevail this TAF
period with increasing high clouds late today and overnight becoming
BKN-OVC100-150 by Saturday morning. Winds will be light and less
than 10 kts today and overnight but expect winds to begin increasing
Saturday morning.


&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  75  50  76  49 /   0   0   0  10
ALW  77  53  78  54 /   0   0   0  10
PSC  79  55  82  55 /   0   0   0  10
YKM  78  53  78  47 /   0   0  10  10
HRI  79  52  81  54 /   0   0   0   0
ELN  75  52  70  48 /   0  10  10  10
RDM  76  46  71  44 /   0   0   0  10
LGD  72  46  72  46 /   0   0  10  10
GCD  76  47  74  46 /   0   0   0  10
DLS  82  56  76  55 /   0   0  10  10

&&

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

$$

SHORT TERM...75
LONG TERM....90
AVIATION...90