High Wind in Oregon, December 25 to 26, 2024

Area Forecast Discussion
National Weather Service Pendleton OR
952 PM PST Wed Dec 25 2024

.UPDATED FOR AVIATION...

.AVIATION...
06Z TAFS...Conditions are VFR or MVFR, for the most part across
the region. However, YKM has IFR CIGS. All sites that are not
already MVFR are expected to deteriorate overnight as _RA continue
over the area. Conditions should return to VFR on THursday
afternoon as the rain ends.

Additionally, winds will increase and become gusty later tonight
or towards morning, with gusts generally in the 25 to 30 kt
range. Winds should decrease to 10 kts or less late Thursday
afternoon or Thursday evening.

&&

.PREV DISCUSSION... /issued 400 PM PST Wed Dec 25 2024/

SHORT TERM...Tonight through Friday Night. Active short term
period with multiple wind and winter headlines in effect. The main
weather stories include:
- Moderate-heavy mountain snow across the Cascades and northern
  Blue mountains, heaviest along the Cascade crest, tonight-
  tomorrow;
- Windy conditions late tonight across central OR and north
  central OR to along the foothills of the Blues and the Grande
  Ronde Valley late tonight-tomorrow; and
- More mountain snow chances early Friday morning into Saturday
  morning with chances of 6” or more eclipsing 50-60% above 4-4.5
  kft.

The latest regional radar imagery is starting to show increasing
weak returns with a few trace or very light precip reports
starting to be seen along the eastern slopes of the Cascades and
parts of the eastern mountains. Latest water vapor imagery shows
an impressive moisture plume directed WSW into OR and southern WA.
The latest MIMIC-TPW shows precipitable water values around 1.5”
starting to move onshore with an impressive plume stretching back
to north of Hawaii. The system’s attendant warm front and AR
appear about to move onshore into coastal OR over the next couple
hours. The warm front is anticipated to hung up over the
Cascades/southern OR overnight. This is forecast to be followed
by a stronger shortwave trough and its cold front that expected
to move east into the Cascades and the area early tomorrow. This
will set the stage for moderate-heavy mountain snow over the next
24-30 hours.

Generally, precipitation and snowfall forecasts look largely on
track with latest CAMs, HREF, and other guidance but made some
adjustments to reflect latest trends. A solid ~12-18 hr window or
so looks present of AR conditions, greater than 250 kg/m/s, more
likely than not (>80% chance in both GEFS and EPS) directed out
of the WSW across the Cascades starting between now and 00 UTC
tonight. Confidence is further buttressed by EPS mean IVT
forecasts exceeding the 90th percentile 00-12Z tonight with mean
values in excess of 300 kg/m/s. Of note, snow levels will be
rising through tonight based on the character and trajectory of
the AR before decreasing tomorrow with the aforementioned cold
front. This will impact the snow and snow ratios when it comes to
mountain snow. Current thinking is the winter storm warning for
the upper/lower slopes of the WA Cascades will be low-end warning
criteria (the upper slopes starting at 12”) with the
heaviest/warning snow more likely than not above Snoqualmie pass
level (i.e., above 3,500 ft). Elsewhere, OR Cascades and northern
Blues look highly likely to see 6” or more above 4,000 ft over
24-hours through tomorrow (ending 00Z Fri). Elsewhere, lowlands
will see rain with much of the area (outside central OR) highly
likely to see at least a quarter inch of rain (chances greater
than 80%).

Otherwise, windy conditions will spread overnight in tandem with a
synoptically imposed, tight surface pressure gradient. A strong
low-level jet stream as depicted by the EPS will be directed
across central OR into southern WA early tomorrow too. Mean 700
mb and 850 mb wind speed forecast eclipse the 90th and 97.5th
percentiles, the later seen with over 40 kts at 850 mb. As such, a
decent signal is seen with the EPS and its extreme forecast index
with wind gusts values eclipsing 0.7 for an atypical wind event
tomorrow. Current thinking is mixing will be best across central
OR to tap into these winds in terms of momentum transfer. Overall,
high confidence in these winds across central OR, north central
OR, Grande Ronde Valley, and foothills of the Blues owing to the
agreement amongst deterministic guidance and chances for exceeding
45 mph seen in ensemble guidance like the HREF and NBM (>80%).

Lastly, a second embedded shortwave trough in the progressive flow
will be ejected into the Pacific NW early Friday that will promote
another period of unsettled weather with increasing odds for
moderate mountain snow with chances of 6” or more eclipsing 50-60%
above 4-4.5 kft. That said, snow levels will be elevated with this
system thus there is some uncertainty on the need for any winter
weather headlines for the WA/OR Cascades and Northern Blue
Mountains. Of which, the OR Cascades looking the warmest/having
highest snow levels and the least confidence in seeing any
headlines.

LONG TERM...Saturday through Wednesday...A progressive weather
pattern will bring a string of upper troughs across the area through
the long term period. This will bring rain and mountain snow to the
area. Despite the active weather, the Extreme Forecast Index
indicates mostly weather that is not too usual for late December.
The main highlight is QPF for Saturday night and Sunday with a value
of 0.89 centered over our area. It also has a shift of tails over
most of Oregon and the Washington Columbia Basin which reaches as
high as 1 over the southern Blue Mountains and the Ochoco-John Day
Highlands. This indicates that some ensemble members have much
higher QPF values, so there is a possibility of a more extreme event
that the majority of the ensemble members are indicating. The EFI
also has overnight low temperature values of 0.85 to 0.90 Saturday
night and Sunday night, most likely due to a south to southwesterly
flow aloft and heavy cloud cover over the region.

Saturday will see a trough moving through British Columbia and
dragging a warm front through our area during the day and a cold
front Saturday night. This will give us snow levels at 3500-5000
feet. QPF amounts will be up to a quarter inch in the Columbia
basin, up to a half inch in central and north central Oregon and in
the mountains, up to an inch in the OR Cascades, up to a quarter
inch in the WA Cascades and up to an inch in the rest of the
mountains. NBM probabilities give the lower elevations a 40-50%
chance of a quarter inch of rain though chances are only 10-30%
for a half inch. Amounts will mostly come with the cold front
saturday night. Snow amounts will be up to 10 inches in the higher
mountains, but only an inch or two at pass levels, though higher
passes like White and Santiam Passes might get higher amounts.
Temperatures will be several degrees above normal with highs in
the upper 40s to mid 50s in the lower elevations and in the 40s in
the mountains.

Sunday, the next trough will move into the area and the central low
will be moving through our area instead of British Columbia like the
Saturday system. This will give us another round of rain and
mountain snow with heaviest amounts during the day on Sunday. Snow
levels will start out around 4000 feet Sunday morning and drop to
2500-3000 feet late Sunday night. QPF amounts will be lighter with
up to a quarter inch in the lower elevations and up to an inch in
the mountains. This will translate to 4-9 inches of snow in the
higher mountains though just an inch or two at mountain pass levels.
NBM probabilities give the lower elevations a 20-50% chance of a
quarter inch of rain. Temperatures will be a few degrees cooler than
Saturday.

Monday and Tuesday will be a break between systems and we will see a
drier northwest flow over the area. The lower elevations will be
mainly dry while the mountains will have a slight chance to chance
of snow. Snow levels will be at 1500-2500 feet before rising to 2500-
3500 feet Tuesday. The mountains will get 1-4 inches of snow through
that 48 hour period. The lower elevations will be generally dry and
the NBM probabilities of getting even a tenth of an inch of rain are
less than 10% everywhere. Temperatures will be in the mid to upper
40s Monday in the lower elevations and in the 30s in the mountains.
Temperatures will cool 3 or 4 degrees Tuesday.

Models have significant differences about the timing, strength and
location of the system arriving Tuesday night and Wednesday. The NBM
has the system bringing the heaviest rain and mountain snow late
Tuesday night and Wednesday morning with up to a tenth of an inch in
the lower elevations and 3-6 inches in the mountains. This seems
like a good compromise between competing solutions. Snow levels will
be 1500-2500 feet over most of the area rising to 2500 to 3500 feet
Wednesday afternoon. However, in central Oregon snow levels will be
3500-4500 feet. The low snow levels would bring an inch or two of
snow to area mountain passes. Temperatures Wednesday will be
slightly warmer with lower to mid 40s in the lower elevations and
30s in the mountains. Perry/83

AVIATION...18Z TAFs...An incoming system is bringing rain and
lowering cigs and vsby this afternoon. KYKM and KDLS are already
reporting precipitation with MVFR/IFR conditions. This should move
into all TAF sites by 04-06Z. Mainly RA with RA/SN at KYKM and
MVFR/IFR conditions will continue through 15Z-18Z at all TAF sites
though improving after 12Z. Winds will be increasing with KRDM and
KBDN having south to southwest winds at 15 to 25 kts with gusts to
35 kts tonight becoming southwest at 10 to 20 kts with gusts to 30
after 18Z. KDLS will have westerly winds at 10 to 20 kts with gusts
to 30 kts after 12Z. KALW and KPDT will have southerly winds at 10
to 20 kts with gusts to 30 kts after 06Z becoming southwest at 15 to
25 kts with gusts to 35 kts after 15Z-18Z. KPSC will have southwest
at 15 to 25 kts with gusts to 35 kts after 14Z. KYKM will have
southeasterly winds less than 10 kts becoming southwest at 10 to 20
kts with gusts to 25 kts after 18Z. Also kept southwesterly wind
shear of 35 to 45 kts at KRDM and KBDN from 02Z-18Z. Have also added
similar wind shear at KDLS, KPDT, KPSC and KALW 12Z-18Z. Perry/83

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  39  49  37  48 /  90  50  50  80
ALW  41  51  39  47 / 100  80  40  80
PSC  41  52  41  48 /  90  40  40  70
YKM  33  48  33  43 /  90  40  50  70
HRI  40  53  39  50 /  90  40  50  70
ELN  30  44  31  41 /  90  60  50  70
RDM  37  46  35  49 /  80  80  80  50
LGD  35  45  32  44 /  90  70  50  90
GCD  34  44  33  46 /  90  80  70  80
DLS  38  51  40  50 /  90  90  80  80

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...Wind Advisory until 10 AM PST Thursday for ORZ049-507-508-510-
     511.

     Winter Weather Advisory until 4 PM PST Thursday for ORZ502.

     Winter Weather Advisory until 4 PM PST Thursday for ORZ506.

     Winter Weather Advisory until 10 PM PST Thursday for ORZ509.

WA...Wind Advisory until 10 AM PST Thursday for WAZ029.

     Winter Weather Advisory until 4 PM PST Thursday for WAZ030.

     Winter Weather Advisory until 4 AM PST Thursday for WAZ026.

     Winter Storm Warning until 10 PM PST Thursday for WAZ522.

     Winter Storm Warning until 10 AM PST Thursday for WAZ523.

&&

SHORT TERM...80
LONG TERM....83
AVIATION...77