630
FXUS66 KPDT 212324
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
324 PM PST Thu Nov 21 2024

.SHORT TERM...Tonight through Saturday night...Active weather is
forecast to continue through the period as a broad and deep
closed upper-level low continues to churn offshore of the PacNW,
sending multiple shortwaves into the PacNW.

Ensemble NWP is in excellent agreement (>95% chance) that a
developing surface low and attendant frontal system off the
southwest OR coast will track northeastward towards the coast
this evening through Friday morning. Nearly all ensemble members
keep the low offshore, and send a warm front overhead tonight
through Friday morning. Frontal passage will be accompanied by
widespread precipitation (heaviest for the WA Cascade east slopes
through the Yakima and Kittitas valleys) and gusty southerly winds
along the Blue Mountain foothills and through the Grande Ronde
Valley.

Have opted to issue wind advisories, valid 2AM Friday through 3PM
Friday, for the base of the northern Blue Mountains of OR/WA and
the southern Grande Ronde Valley. While winds are not expected to
be as strong as Tuesday night/Wednesday`s wind event, a 40-55 kt
850 mb jet is advertised by the 12Z HREF. Should note that cross
sections, forecast soundings, and HREF surface wind fields all
exhibit more uncertainty in the magnitude and extent of winds
relative to the Tuesday night/Wednesday event, but confidence is
high (70-80%) in advisory-level winds.

Shifting westward, while the warm front will push snow levels
higher, CAMs continue to show potential for light freezing rain
(up to several hundredths of an inch) for sheltered spots along
the Cascade east slopes, primarily in WA. Additionally, some CAMs
keep the column cool enough tonight into Friday morning for
significant snowfall (5-10") along the WA Cascade east slopes and
higher elevations of the Simcoe Highlands. Confidence in this
solution is too low (<50%) to warrant an advisory at this time.

Friday afternoon, ensemble guidance is showing a band of
precipitation developing along and ahead of a surface cold front
from the eastern Columbia Basin through the Blue Mountains. Given
the ample moisture present from an inland-penetrating atmospheric
river (PWATs of 150-200 of normal) coupled with frontal and
orographic lift, probabilities of 0.5" or precipitation are
relatively high for our CWA: 40-70% for the lower elevations, with
70-95% for the northern Blues. Snowfall probabilities are not
quite supportive of any highlights for the northern Blues (20-50%
chance of 6" above 4500-5000 ft), and lower for Wallowa County
(up to 10% chance of 4") due to uncertainty in timing of the
changeover of rain to snow.

Lingering precipitation is forecast Saturday, especially for the
foothills of the Blues, Blue Mountains, and Cascade crest. Hints
of yet another surface low developing in the Pacific are present
in ensemble and deterministic guidance for Saturday night, but
more uncertainty is present in pattern details when looking at
ensemble solutions. Plunkett/86

.LONG TERM...Sunday through Thursday...Southwesterly flow will
prevail on Sunday and the region looks predominantly dry, except
for the possibility of some precipitation over the Cascades.

By Monday, an upper low/trough will approach the Pacific Northwest
coast and bring better precipitation chances, especially to the
mountains.

The main trough axis shifts to the south, over California, by
Tuesday and then moves eastward by Wednesday, which would bring a
more northerly flow over the northwest. Once again, most of the
precipitation chances are confined to the mountains. As the mean
trough moves eastward, the flow becomes more northwesterly, with
drier air moving in, and even some ridging by Thursday.

While there will be chances of rain through the extended period,
especially the early part, QPF amounts are generally fairly light.
SUnday into Monday will see some amounts up to 0.25 inches in the
Washington Cascades, and higher at the highest peaks, with only a
few hundredths elsewhere. Monday into Tuesday will see amounts up
to 0.25 inches in the Oregon Cascades, amount of 0.10 to 0.15 in
the higher elevations and much less elsewhere.

Model guidance is in reasonably good agreement through the period.
While the deterministic ECMWF is deeper with the trough moving
onshore at first, it eventually becomes very similar to the GFS by
later Tuesday, and given the natural variability of the forecast
at that time scale, the agreement is still good in both strength
and position.

Overall, heading into the busy holiday travel week, while it
doesn`t look to be very sunny, there doesn`t look to be any
significant storms. There could be some snow in the mountains, but
overall accumulations look to be on the lighter side at this
time.

&&

.AVIATION...00Z TAFS...Conditions range from VFR to LIFR. All
sites should experience some degradation to at least MVFR during
the evening or overnight. Gusty winds developing at BDN and RDM
will likely prevent long duration MVFR/IFR conditions there.

For the rest of the sites, high confidence in IFR/LIFR at ALW.
Confidence in IFR conditions is lower elsewhere, but can`t be
ruled out at PDT/PSC.

With the exception of BDN/RDM winds will generally be 10 kts or
less. At BDN and RDM expect winds to gust to around 25 kts later
tonight through much of Friday.

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  38  58  36  47 /  90  80  80  50
ALW  40  58  39  48 /  80 100  90  70
PSC  41  50  40  50 /  90  90  60  30
YKM  33  48  29  47 / 100 100  20  20
HRI  40  52  38  50 /  90  80  70  40
ELN  34  47  29  44 / 100  90  20  30
RDM  41  54  31  45 /  90  40  40  30
LGD  41  50  36  43 /  90  90  90  80
GCD  42  54  35  44 /  90  80  90  70
DLS  43  52  39  50 / 100 100  40  60

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...Wind Advisory from 2 AM to 3 PM PST Friday for ORZ049-507.

WA...Wind Advisory from 2 AM to 3 PM PST Friday for WAZ029.

&&

$$

SHORT TERM...86
LONG TERM....77
AVIATION...77