National Weather Service National Headquarters

• Home
• Site Map
• News
• Organization
• Search for: NWS All NOAA 

000
FXUS66 KPDT 231005
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
305 AM PDT Tue Apr 23 2024

.SHORT TERM...Today through Thursday Night...


Key Messages:

1. Chilly morning temperatures in the Basin today.

2. Breezy winds Wednesday afternoon and evening.


Current radar and infrared satellite imagery showing dry conditions
with high level clouds beginning to stream in from the west and
south as a weak upper level shortwave approaches the coast. This
slight increase in cloud cover overnight will allow radiational
cooling to not be as effective as previous nights, keeping morning
temperatures primarily above freezing. The only area in question of
potentially dropping below freezing is the northern Blue Mountain
foothills of Washington, as the cloud cover will reach this region
last and may lend enough time for temperatures to quickly drop
through the early morning hours. However, confidence is still low
(20-30%) of this occurring due to current observations and the NBM
keeping freezing probabilities below 10%, but will continue to
monitor as this is the greatest threat for a Freeze highlight.
Present high pressure, both at the surface and aloft, will also
allow cold air to pool at protected valley bottoms, but this will
not be widespread and only occur in isolated, steep valleys. The
weak shortwave will dissipate quickly as it moves onshore later this
morning, and will bring with it persistent southwest flow aloft
through the period. This will warm overnight low temperatures by 5
to 9 degrees Wednesday morning (low to mid-40s), and another
increase of 2 to 6 degrees Thursday morning (mid-to upper 40s). High
temperatures will trend slightly upward by 1-3 degrees today (upper
60s to low 70s) to Wednesday (low to mid-70s), but will drop 2 to 6
degrees Wednesday to Thursday (mid-to upper 60s) due to rain chances
returning.

A stronger shortwave and associated cold front will pass through the
Pacific Northwest Wednesday afternoon, which will set up a pressure
gradient along the Cascades to provide breezy conditions across the
Lower Columbia Basin, Simcoe Highlands, and the Kittitas Valley. The
GFS, SREF, and NAM all highlight a pressure gradient of between
6.8 to 7.6 mb from Spokane to Portland Wednesday afternoon, which
is indicative of elevated sub-advisory gusts. Winds will be
steadily increasing through the morning hours before peaking
through the afternoon as gusts between 25 and 30 mph will be
possible. Confidence in these wind values is high (80%) as the NBM
suggests a 70-90% of wind gusts of 25 mph or greater, but only a
20-40% chance of 30 mph wind gusts or greater. Winds will slowly
taper off after 7 PM as the gradient dissipates ahead of the next
system on Thursday.

A much stronger shortwave trough and associated cold front will
follow closely behind on Thursday to again elevated winds between 20
and 25 mph, but will bring with it more widespread rain chances as
it taps into ample moisture. Snow levels will hover between 5000-
6500 feet northwest to southeast, which will keep mountain snowfall
light (1-2 inches) and confined to higher elevations of the Cascades
and Elkhorns. The majority of precipitation will occur along the
Cascades, Blue Mountains, and and the Elkhorns (0.10-0.30 of an
inch), with light showers (0.01-0.05 of a inch) anticipated over the
northern Blue Mountain foothills, John-Day/Ochoco Basin, Eastern
Gorge, and lower elevations of Central Oregon. Precipitation chances
will increase through the day, encompassing lower elevations after 5
PM. 75


.LONG TERM...Friday through Thursday...Ensembles in broad agreement
on a more active, progressive pattern through the weekend, bringing
chances for precip and potentially mountain thunderstorms.
Confidence, however, is hindered as usual by these progressive
patterns, as timing differences frequently arise across ensemble
members due to the active nature of the synoptic pattern.

Ensemble clusters show good agreement on the region being
encompassed by broad troughing on Friday, which looks to deepen as
it progresses toward the Rockies. QPF is pretty so-so, as the
mountains show only around 0.1 of an inch across the NBM and only a
few ensemble members produce measurable precip for our population
centers. Being under the axis and thus dry pocket of the trough
looks to be the biggest factor in the mild precip forecast, however
such a pattern may produce sufficient lift for storms to develop
over the eastern mountains. Confidence is on the lower end, however
(15-20%), as global models produce little instability and shear is
weak, as expected under a trough axis. Pulselike, orographically
driven cells may form in the afternoon, but not anticipating
anything particularly strong.

Even though ensembles diverge in solutions a bit due to timing
issues brought about by the progressive synoptic pattern, there is
general agreement on the next system in the long term being a broad
Gulf of Alaska low moving into the area by early next week, ushering
in amplified SW flow aloft into the PacNW. Some ensembles stick
with more benign, zonal flow behind Friday`s system, but a majority
of ensemble members (around 75%) bring in this second low. The NBM
isn`t particularly keyed in on this system, and likely won`t be
until guidance consensus improves, and QPF ensembles are a bit too
far out to gauge severity, but such a pattern could produce area-
wide precip (confidence 40-50%) and potentially thunderstorms
(confidence 10-20%) by Monday or Tuesday.

Temps will remain seasonable to slightly below average as Friday`s
system pulls cooler air into the PacNW over the weekend. Should this
next system verify, however, expect temps to return back to more
seasonable values, to perhaps even slightly above average under
warmer SW flow aloft. Evans/74


&&

.AVIATION...12z TAFs...VFR conditions expected for the period, with
sct-bkn high clouds and light winds out of the north and east less
than 10 kts. Evans/74


&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  67  41  67  43 /   0   0   0   0
ALW  70  45  71  47 /   0   0   0   0
PSC  72  44  74  48 /   0   0   0   0
YKM  70  43  70  43 /   0   0   0   0
HRI  71  42  72  46 /   0   0   0   0
ELN  67  43  63  43 /   0   0   0   0
RDM  68  36  64  37 /   0   0   0   0
LGD  69  41  67  40 /   0   0   0   0
GCD  72  41  69  40 /   0   0   0   0
DLS  73  46  68  48 /   0   0   0   0

&&

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

$$

SHORT TERM...75
LONG TERM....74
AVIATION...74