Heavy Snow in Oregon, February 14 to 16, 2024

Area Forecast Discussion...UPDATED AVIATION.
National Weather Service Pendleton OR
359 AM PST Wed Feb 14 2024

.SHORT TERM...Wednesday through Friday...Messy and complex setup
for the period will lead to everything from heavy mountain snow to
light rain showers. The combination of cool NW flow aloft and an
oncoming central Pacific low will make for a tricky forecast as
precip amounts and type vary widely across relatively short
distances. Confidence, however, is increasing as we approach the
onset of precip.

Latest overnight satellite imagery shows the central Pacific low
making its way toward our neck of the woods, as the cloud shield
leading the system begins to enter central Oregon. Latest CAMs
suggest precip will arrive by around sunrise for central Oregon,
before encompassing the entirety of the forecast area by the early
evening hours to around sunset.

Precip is expected to begin as snow in central Oregon, before warm
air advection accompanying the oncoming low leads to a transition to
rain, at least for elevations below 3500 feet. The toughest part
about the forecast, however, is just how strong that warm air
advection actually shapes up to be, as conventional wisdom around
here suggests that systems coming from the SW will produce rain for
everywhere outside the mountains. However, a combination of the NW
flow overhead, along with the early morning timing of precip, makes
the ptype forecast more difficult. Current thinking is that temps
will hover around to just below freezing for the start of precip for
our population centers, before rising closer to the upper 30s and
lower 40s by the evening, allowing for more of a mix to all rain. As
this low merges with the NW flow, however, a broader trough
structure looks to form, resulting in more cold air advection
heading into the evening hours. Thus, the most efficient wintry
precip (at least outside of our high mountain zones) is not
anticipated to take place until late evening Wednesday overnight
into Thursday morning. However, anticipation is that the lower Basin
and foothills of the Blues will see enough warm air to preclude
anything more than just a mix of rain and snow with little to no
accumulation.

This oncoming low has resulted in multiple winter weather headlines
in effect through Thursday (central Oregon) and Friday (everywhere
else). Current forecast suggests that the Oregon Cascades, immediate
eastern foothills, and Klickitat County stand to see the heaviest
snow amounts. QPF drops off fairly significantly heading north into
the central Washington Cascades and east into the Basin, before
amounts pick back up across the eastern mountains. This wide
gradient in QPF and ptype is most evident across the Gorge, where
White Salmon could receive heavy snow up to 8 inches, while the
eastern end of the Gorge (Biggs Junction, Arlington) receives mostly
rain and little to no snow. Please refer to the text products of the
winter weather advisories for more information, as the public should
note that they may not receive headline-worthy snow even if their
area is currently under an advisory of some kind.

Guidance suggests precip ends across the forecast area by daybreak
Friday as transitory ridging builds in over the PacNW. Relatively
cold temps will prevail behind this system, with highs in the upper
30s to low 40s across our population centers Friday. Evans/74

.LONG TERM...Saturday through Wednesday. The primary sensible
weather highlights include 1) a gradual warming trend Sunday
through midweek with a general gain of 8 to 10 degrees across
south central WA, Lower Basin, and central OR by Wednesday and 2)
an unsettled weather pattern with multiple opportunities for
precipitation.

The large scale pattern early Saturday will be characterized by a
deep, anomalous upper-level trough over the eastern North Pacific
and an amplified ridge across the Four Corners that extends north
into the Yukon. Ensemble guidance is in good agreement with the
upper ridge deamplifying and weakening through Monday as broad
cyclonic flow dominants the western CONUS with multiple shortwave
troughs projected to eject eastward. The initial shortwave trough
is anticipated to eject from northern CA to the Northern
Intermountain West Sunday followed by another shortwave trough
ejecting from the base offshore CA to the Interior NW Monday.

While fair-good agreement is present with the synoptic pattern
based on the spread amongst 500 mb member forecasts through the
weekend, much of the uncertainty that exists appears tied to the
evolution of the upstream ridge and aforementioned upper trough
rather than the evolution of these shortwave troughs that eject
east. Confidence in the large scale pattern/interior NW forecast
is moderate (50-70%). While these disturbances will provide broad
large scale forcing for ascent, moisture will be a limiting
factor and will limit mountain snow potential and the overall QPF.
The greatest ensemble mean IVT and the best chances for IVT > 150
kg/m (>60%) are mainly south of the forecast area focused across
the central and northern CA coast. Trajectories are also largely
meridional in nature and will further limit the inland transport
of moisture through Monday. Nonetheless, periodic chances plague
the forecast this weekend through Monday. Thereafter ensemble
guidance are in modest agreement with the upper trough moving
near/onto the west coast Tuesday. Thereafter ensemble means start
to show largely zonal flow to be overhead Wednesday that could be
a consequence of increasing spread in guidance. Low-moderate
(30-50%) confidence in place by Wednesday with unsettled
conditions still persisting mid week. Of which, clustering
scenarios highlight the potential for weak troughing to still be
in place possible (60-65% chance) across the west coast to weak
ridging/zonal flow (35-40% chance). Sensible weather-wise, a
warming trend still looks to be favored over time early next week
through midweek.

&&

.AVIATION...12Z TAFS...Increasing aviation concerns 6-24 hrs as a
winter storm develops today. Latest satellite imagery shows low
clouds across south central WA into the western Lower Basin; stratus
also apparent underneath the cloud shield associated with this
upcoming weather systems. Generally VFR currently area-wide, except
LIFE across the central OR terminals.

Increased confidence (80%) in precip spreading across central OR
near 18Z into south central WA and the northern Blues by 3-6Z
tonight. However, p-type and changeover to snow will be a challenge
with greater spread in guidance surrounding timing and surface
temperatures (confidence 60-80%) at terminals. Current thinking is
central OR will see changeover first near 00Z with changeover closer
to 06Z for KPDT/KALW. Lower confidence (50-60%) in all
snow/changeover at KPSC and KDLS terminals late tonight. Increased
confidence in sub-VFR ceilings after 00Z, excluding central OR
terminals that will be MVFR-IFR today/tonight. Otherwise, light
winds will prevail through the TAF period (less than or equal to 10
kts).

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  38  30  36  26 /  10  80  70  50
ALW  41  32  38  28 /   0  80  80  50
PSC  45  33  39  29 /   0  60  60  40
YKM  40  30  38  25 /  10  70  60  30
HRI  42  32  39  28 /  10  80  60  50
ELN  38  27  37  24 /   0  50  50  20
RDM  36  26  41  26 /  90  80  40  40
LGD  41  29  43  27 /  10  90  70  60
GCD  43  31  45  29 /  50  90  50  50
DLS  41  33  39  32 /  50  90  80  60

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...Winter Weather Advisory from 10 AM this morning to 4 AM PST
     Friday for ORZ041-502.

     Winter Storm Warning from 10 AM this morning to 4 AM PST Friday
     for ORZ508-510.

     Winter Storm Warning until 4 AM PST Friday for ORZ509.

     Winter Weather Advisory until 10 AM PST Thursday for ORZ511.

WA...Winter Weather Advisory from 10 AM this morning to 4 AM PST
     Friday for WAZ024-030-521>523.

&&

SHORT TERM...74
LONG TERM....80
AVIATION...80