High Wind in Oregon, December 26 to 27, 2023

Area Forecast Discussion
National Weather Service Pendleton OR
1011 PM PST Tue Dec 26 2023

Updated Aviation Discussion

.AVIATION...06Z TAFs...Fog is already socked into PDT, with fog
also creeping into ALW. Expecting both sites to remain LIFR
conditions overnight until southeasterly winds increase and help
to clear out tomorrow morning. MVFR to IFR ceilings for
DLS/YKM/PSC overnight but expecting the stratus layer to remain
above the ground, but also cannot rule out the chance for lowered
conditions if the stratus lowers. Winds expected to increase at
RDM/BDN overnight with gusty conditions between 20-30 kt possible.
An initial wave of rain may impact DLS overnight, but most likely
rain moves in after 18Z tomorrow. Goatley/87

&&

.PREV DISCUSSION... /issued 844 PM PST Tue Dec 26 2023/

UPDATE...Per surface observations, winds have been increasing in
the Grande Ronde Valley with sustained winds of 25 to 35 mph with
gusts to around 50 mph at the base of Ladd and Pyles canyons. Over
the northern areas of the Grande Ronde Valley, winds have
increased to 15 to 25 mph with gusts to 35 mph. Pressure gradients
between Baker City and Meacham (BKE & MEH) have increased to 4-5
mb, while the rule of thumb threshold for Wind Advisory wind
speeds is 5 mb between BKE and MEH. Forecast pressure difference
between BKE and MEH by the NAM40 model is forecast to increase to
over 6 mb by 10 PM PST this evening, and then over 7 mb by
midnight. The GFS40 model is weaker, with forecast pressure
differences to increase to over 5 mb by 1 AM PDT Wednesday, and
over 6 mb by 7 AM PDT Wednesday, and then over 7 mb for only a
short time period around 10 AM PDT late Wednesday morning. While
the GFS40 is showing weaker gradients than the NAM40, both are
still showing pressure differences that support High Wind Warning
wind speeds in the southern Grande Ronde Valley, for at least a
short period of time. In addition, the HREF ensemble maximum is
forecasting wind gusts of over 65 mph over the Southern Grande
Ronde Valley as compared to the HREF ensemble mean. However, the
HREF ensemble mean is even forecasting wind gusts up to around 60
mph for a short time. These are in line with the deterministic
NAM40 and GFS40 wind speeds derived by the pressure gradients.
Therefore there is high confidence (at least 80-90%) for High Wind
Warning winds during the morning hours into the afternoon
Wednesday over the southern Grande Ronde Valley in and at the base
of Ladd and Pyles Canyons. Confidence is only moderate (around
50-60%) for High Wind Warning winds in the northern portion of
the valley. However, winds there are likely to reach advisory
level wind speeds of 25-35 mph with gusts to 50-55 mph. The NBM is
showing lower probabilities of winds this strong, and believe the
NBM is underdone, given the high forecast pressure gradients.

Elsewhere, strong winds are also likely across the northern Blue
Mountains of OR and along the northwest slopes to a small area
along the base of the northern Blue Mountains of OR, in the
Foothills. There are some large discrepancies between the HREF and
NBM forecast wind speeds along the northwest slopes of the
northern Blues down to the base of the northern Blues in the
Foothills (mainly from Cabbage Hill northeast to Weston, OR, where
typically winds are the strongest in these areas with this type
of weather pattern). The HREF ensemble is forecasting much higher
wind speeds than the NBM. Due to these discrepancies and
uncertainty, confidence of High Wind Warning winds in these areas
is only moderate (50-60%). However, confidence is high (80-90%)
for Wind Advisory speeds of 25-35 mph with gusts to 50-55 mph.
However, would not be surprised if there are localized gusts to
around 60 mph, but not widespread enough to warrant a High Wind
Warning.

A cross barrier wind maximum at 850 mb is forecast by the NAM12
to reach 45-50 kts over the crest of the northern Blue Mountains
of OR. These winds are likely to be accelerated due to the stable
air mass, over the crest of the Blues, down along the NW slopes
to the base, as these winds are squeezed by the terrain barrier
and the stable air mass. However, am not expecting winds to
extend too much further west-northwest across the western portions
of the Foothills of the northern Blues of OR. Winds further west,
such as over Pendleton, will become breezy, with wind speeds of
10-20 mph with gusts to 30-40 mph at most.

It is even possible that winds that far away from the base of the
Blues do not increase enough to mix out the fog and low clouds
with the strong surface based inversions now in place over KPDT up
through KALW and westward into the Lower Columbia Basin.
Visibilities have dropped to a quarter mile in fog at KPDT since
just before sunset. Currently have a dense fog advisory in effect
for the Foothills of the northern Blue Mountains until 07Z.
However, this may very well need to be extended through the night.
If winds increase enough on Wednesday, the inversions over the
Foothills of OR will have a better chance of mixing out for a
while. However, the Foothills of the Blues in WA, in areas such as
KALW and Dayton are not likely to mix out.

A strong south to southwest flow will develop behind a departing
upper ridge by morning, east of a strong upper low pressure
system over the NE Pacific. Precipitation with this system will be
light, and mostly over the Cascades. QPF amounts will be less
than a tenth of an inch over most of the CWA. However, they will
range from mostly a quarter to a half inch at most along the
Cascade crest, dropping off quickly the further eastward from the
Cascade crest you go.

Temperatures will be slightly warmer than normal with a slight
warming trend through Thursday, and then a slight cooling trend
after Thursday. Snow levels will be high enough to keep any
precipitation as snow only over the higher mountains, and rain in
the lower elevations. 88

PREV DISCUSSION... /issued 308 PM PST Tue Dec 26 2023/

SHORT TERM...Tonight through Thursday...Sensible weather concerns
revolve predominantly around windy conditions early tonight
through tomorrow. The main highlights include:
- breezy to windy conditions with gusts up to 65 mph in the
  southern Grande Ronde Valley and up to 55 mph across the
  foothills of the Northern Blues,
- near to slightly above average temperatures, warmest mainly
  across central OR and along the base of the Blue mountains, and
- rain and mountain snow showers developing across the small
  hours tonight through tomorrow night.

Latest satellite imagery shows low stratus across the Columbia
Basin, Gorge, north central OR and south central WA valleys with
high clouds starting to push across the Cascades. Latest water
vapor imagery shows a strong closed low well offshore the OR/CA
border with a decent plume of moisture streaming from the
southwest. The large scale pattern will be characterized by this
closed low staying offshore in the eastern North Pacific as it
weakens tomorrow and shifts north to be offshore British
Columbia. A decent belt of southerly mid-level winds associated
with this feature will move onshore and be positioned across
western OR/WA late tonight around 80-100 kts before these winds
weaken and overspread across eastern OR/WA. The shortwave trough
at its base will rotate and lift northeastward and move into
western WA and northwest OR tomorrow.

The main forecast challenges revolve around high wind potential
across the foothills of the Blues, northern Blue Mountains, and
Grande Ronde. Looking at the Northern Blues/foothills, the initial
wind field not as supportive for strong winds away from the base
of the northern Blues owing to cross-barrier flow not being
optimal and having a large v-component/generally south southwest
overnight-tomorrow. Meantime, there is still moderate static
stability with isentropes downstream only separating modestly
based on deterministic guidance at the base of the Northern Blues
Wednesday morning/early afternoon in conjunction with downward
vertical motion/subsidence along/vicinity of the barrier.
Further, this lack of a footprint in windy conditions could be
attributable to modest pressure differences between PDT and MEH
wherein differences are largely forecast 5 mb or less. Confidence
in winds greater than 45 mph is 80-90% or more. Latest raw HREF
probs show chances for 45 mph around 60-90% with a maxima just
east of Weston along the base of the Blues. Similar spatial
pattern in times of highest probabilities are seen in the NBM.
That said, would not be surprised a few isolated gusts in excess
of 55 mph occur at Cabbage Hill. Confidence in gusts 55 mph or
more around 50-60%. Of which, NBM chances for exceeding 55 mph or
more peaks under 50% across a small area along the base of the
northern Blues. Current thinking is winds will not extend far from
the mountains given the flow thus wind advisory is tied closely
to the terrain. May need to monitor in case winds depart from the
current forecast/develop to be higher than anticipated.

Elsewhere, better chances for high winds with gusts up to 65 mph
across the Grande Ronde Valley, especially Ladd and Pyles canyons
(80-90% confidence). Raw HREF probs have chance of 55 mph near 60%
but think this is underdone given pattern and local effects. A
decent pressure gradient is in place between BKE-MEH with pressure
differences in excess of 6 mb seen in the GFS and NAM forecasts,
situationally supportive of high wind-level gusts. That said, do
not think we will see these winds extend downstream of the Grande
Ronde with best chances for high winds near and south of the KLGD
terminal.

As the associated shortwave trough rotates northward into western
WA, decent southerly plume of moisture pushes north. While not a
favorable pathway for mountain snow or low elevation rain, this
will prompt increases chances that are highest across the Cascade
crests (75-100%). Low elevation chances remain low (15-30%). Precip
will be showery in nature with very light QPF, a few hundredths
across the low elevations. Mountain snow- wise, little snow is
expected across the Blues with 4-5.5 kft snow levels.

LONG TERM...Friday through Tuesday...The extended period is
characterized by an active pattern of parading upper level
shortwaves that dissipate as they move onshore due to the presence
of a strong upper level ridge over the Northern Rockies. This
will bring a daily dose precipitation over the Cascades, with
weekend chances (25-50%) across the northern Blue Mountains.
Slight rain chances (15-25%) will persist at lower elevations of
the Basin and Central Oregon, with slightly better chances
(25-30%) along the Blue Mountain foothills and along the east
slopes of the Cascades. The main weather concern through the
period is associated with breezy to windy conditions across the
northern Blue Mountains and the Grande Ronde Valley Friday as the
system approaches the coast to provide wind gusts up to 30 mph.
Temperatures will slowly increase through Sunday before a slight
cooling trend initiates and extends through the beginning of the
workweek.

An upper level trough begins to slowly approach the west coast as
the upper level ridge, that built over the area Thursday, begins
to slowly push east and depart our area. This will tighten the
pressure gradient between the two system to allow for breezy winds
to initiate along the northern Blue Mountains and the Grande
Ronde Valley. The NAM suggests a pressure gradient of 7-8mb
between Ontario and Meacham, and the SREF, GFS, and NAM show a
4-6mb difference between Baker City and Meacham on Friday. These
pressure gradient values correlate to wind gusts of up to 30 mph
out of the south through Friday afternoon and evening. Winds
through the northern Blue Mountain will also be heightened, with
gusts up to 25 mph possible out of the south-southeast. Confidence
in these wind values is moderate (50-60%) as the NBM highlights a
40-60% chance of wind gusts of 25 mph or higher over the Northern
Blue Mountains and through the Grande Ronde Valley. High
temperatures will be 3-7 degrees cooler on Friday than on
Thursday, with temperatures peaking into the low to mid 40s.
Precipitation will stay confined to higher elevations of the
Cascades on Friday, as snow levels hover between 4500-5500 feet.
The upper level trough pushes onshore along the California coast
Saturday morning, slowly pushing northeast and across our area
through the remainder of the weekend. This will allow for
precipitation chances (15-40%) to slowly propagate east into
Central Oregon Saturday afternoon before pushing into the Basin
through the evening and overnight. Higher precipitation chances
will reside at higher elevations of the Blues and Cascades, with
only slight chances (15-25%) at lower elevations of the Basin and
adjacent valleys. Up to an inch of snowfall is expected at the
highest elevations across the Cascades on Saturday, with up to
0.10 of an inch of rain likely. Lower elevations, including the
east slopes of the Cascades, Central Oregon, and the Basin are
likely to stay dry or at the most incur a trace of rainfall over
the weekend. Snow levels hover between 5000-6000 feet north-to-
south on Saturday. High temperatures drop 1-3 degrees Friday to
Saturday, keeping values slightly above normal for this time of
year. The trough continues to break up on Sunday as it encounters
the strong upper level ridge located over the northern Rockies,
allowing for precipitation chances (25-45%) to be confined along
the Cascade crest and the Blue Mountains on Sunday. Higher chances
will reside at higher elevations, with only a dusting of snow and
0.01-0.05 inches of rainfall expected. High temperatures will
increase another 3-5 degrees from Saturday, as Sunday is on tap to
be the warmest day in the extended period. A weak shortwave
follows closely behind the earlier, dissipating system to keep
precipitation chances (20-40%) at elevation over the Cascades and
Blue Mountains on Monday. However, amounts will be lacking as the
overall strength of the system is low and the presence of a
quickly building upper level ridge late Monday into Tuesday will
only lead to a trace of rainfall across the aforementioned areas.
Temperatures will begin to trend downward on Monday, with highs
dropping 2-4 degrees from Sunday. Cool conditions persist into
Tuesday as high temperatures continue to drop another 1-3 degrees
from Monday.

Guidance is in good agreement with the overall weather pattern of a
slowly moving upper level trough and a building upper level ridge
toward the tail end of the period, but differences in ridge/trough
strength and arrival of the early workweek ridge arise. These
differences are visualized via the 500mb EOF Patterns as 44-56%
ensemble variance is present Friday and Saturday associated with the
initial trough's strength. The ECMWF incurs a stronger trough and
ridge (located over the northern Rockies), which correlate to a
colder, less windy, and drier outcome with the ECMWF than the GFS.
The cluster 500mb heights and cluster phase space both provide
slightly more confidence in the ECMWF scenario, as ensembles are
more in-line with one another and the ensemble mean is better
represented by the deterministic solution. Thus, it is likely a
trend toward slightly cooler, drier, and less windy conditions will
ensue with future model runs as the NBM was utilized through the
extended period. 75

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  29  43  33  45 /  10  20  30   0
ALW  30  46  35  46 /   0  20  30   0
PSC  32  41  33  42 /   0  20  20   0
YKM  31  41  29  43 /  10  40  40   0
HRI  30  41  32  44 /   0  20  20   0
ELN  31  39  31  41 /  20  40  30   0
RDM  35  49  33  51 /  10  30  20   0
LGD  30  39  31  45 /   0  20  20   0
GCD  29  46  30  49 /   0  20  20   0
DLS  36  40  35  44 /  20  70  50  10

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...High Wind Warning until 10 PM PST Wednesday for ORZ049.

     Dense Fog Advisory until 7 AM PST Wednesday for ORZ507.

     Wind Advisory until 10 PM PST Wednesday for ORZ507.

     Freezing Fog Advisory until 7 AM PST Wednesday for ORZ510-511.

WA...None.
&&

EVENING UPDATE...88
SHORT TERM...80
LONG TERM....75
AVIATION...87