Area Forecast Discussion
Issued by NWS Pendleton, OR

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FXUS66 KPDT 271612
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
912 AM PDT Wed Mar 27 2024

.MORNING UPDATE...Soggy day expected today as a low pressure
system centered just off the Pacific coast pumps moisture into the
forecast area via SW flow aloft. Latest visible satellite imagery
shows quite the picturesque low slowly churning its way here,
before guidance cuts the low off and sends it southward towards
California. In the meantime, expect persistent rain and high
mountain snow through the day today, with some localized shower
activity possible at times this afternoon. SW flow will make for
elevated snow levels at 4500-5500 feet, so no real snow concerns
for our main mountain passes. Some early snow reports have come in
from the eastern mountains, however warm air advection is expected
to be robust this morning, quickly transitioning snow to rain, at
least at elevations below 5500 feet. Only other real concern is
downsloping winds thanks to the SW to S winds ahead of the low,
however latest guidance still suggests breezy, albeit sub-advisory
level winds, primarily across the Grande Ronde Valley and the
foothills of the Blues. For these areas, winds could gust up to 40
mph at times.

Main forecast challenge today will be monitoring the winds, as
well as QPF for amounts and trends. As far as the update goes,
changes were made mainly to PoPs, updating based on latest
guidance, as well as making some minor touch-ups to the winds
based on the early run of the 12z HREF. Evans/74

&&

.PREV DISCUSSION... /issued 427 AM PDT Wed Mar 27 2024/

SHORT TERM...Today through Friday night...Water vapor and
infrared satellite imagery shows a deep, closed upper-level low
offshore centered at roughly 47N, 138W. A weak warm front is
currently impinging upon the Cascades with increasing returns on
radar, though light precipitation reports this morning have been
confined to the Cascade crest and east slopes. Ensemble guidance
is in excellent agreement that the upper-level low will track
slowly eastward towards the coast through the day. This will place
the PacNW under a southwesterly flow aloft with increasing
precipitation chances through the day. A cold front will traverse
the forecast area this afternoon and evening, bringing the best
chances of precipitation with frontal passage. One area of
uncertainty in the forecast is the magnitude of precipitation
associated with the frontal band, and another is the exact
location of the band. Confidence is moderate to high (45-85%
chance) in a "wetting rain" (>0.10") for much of the foothills of
the Blue Mountains and Blue Mountains proper. Furthermore, 00Z
HREF ensemble maximum 3-hr QPF highlights the potential for
locally heavy showers within the band: a swath of 0.5-0.75" is
advertised along the foothills, base, and western slopes of the
Blues, and up to 0.5" for the rest of northeast Oregon and far
southeast Washington this afternoon and evening. While some very
weak instability (MUCAPE up to ~100 J/kg) is advertised by 00Z
HREF members, confidence is low (<10% chance) in thunder today.

Winds are forecast to be breezy out of the south today, especially
for the Grande Ronde Valley where NBM probabilities of 24-hr
maximum sustained winds >30 mph are 50-70% for the southern half
of the valley, and probabilities of 24-hr maximum gusts of >45
mph are 20-50%. Elsewhere, confidence is high (70-100% chance) of
winds remaining below advisory thresholds.

While snow is forecast today for the higher mountains (>4000-6000
feet, lowest for WA and highest for NE OR), confidence is low
(<25%) in reaching advisory criteria for any highway or interstate
passes.

Thursday, the upper-level low is expected to remain offshore with
another round of showers on tap, primarily during the afternoon
and evening. While 00Z HREF soundings are more supportive of
convection than today, given the shallow CAPE profiles, confidence
is still low (<15% chance) in more than an isolated rumble of
thunder so have omitted from the forecast. Breezy southwest winds
are forecast to mix down to the surface Thursday, especially for
central Oregon and the foothills of the southern Blue Mountains.

Friday, the upper-level low is forecast to weaken with showers
coming to and end by Friday night. Plunkett/86

LONG TERM...Saturday through Wednesday...The PacNW will be under
the influence of a split upper level trough Saturday. This places
a closed upper level low off the coast of central California and
a shortwave moving through Canada. The PacNW will essentially be
between these two systems with just some minor instability leading
to some showers mainly over the mountains. The overall trough
position shifts to the east on Sunday allowing an upper level
ridge over the eastern Pacific Ocean to begin moving into the
PacNW. A few mountain showers will remain possible near the Idaho
border but otherwise dry conditions. This ridge will gradually
pass over the region Monday and Tuesday with a return of warm
above normal temperatures. The ridge begins to be flattened by a
Gulf of Alaska trough moving into western Canada on Wednesday.
This will transition the flow over the PacNW to westerly and bring
back a chance of precipitation to the forecast area and cooler
temperatures.

AVIATION...12Z TAFS...A weather system moving across the region
today will produce occasional rain and ceilings BKN-OVC 030- 060
for all sites. Winds will be 5-15kts with RDM and BDN seeing gusts
around 25kts. Winds will decrease to less than 10kts by evening.
Rain will taper off to showers by late afternoon and evening.

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  58  36  56  35 / 100  70  20  20
ALW  58  39  57  37 / 100  90  60  40
PSC  60  39  62  39 /  90  30  20  10
YKM  55  32  57  33 /  90   0  10   0
HRI  61  36  61  37 / 100  40  20  10
ELN  50  31  54  31 /  90  10  20  10
RDM  54  31  50  31 /  90  30  20  10
LGD  50  34  52  32 /  80 100  50  50
GCD  53  31  50  31 /  90  90  50  30
DLS  56  38  57  39 / 100  30  50  20

&&

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

$$

SHORT TERM...74
LONG TERM....91
AVIATION...91


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