National Weather Service Raw Text Product

542 
FXUS66 KPDT 202345
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
345 PM PST Tue Feb 20 2024

.Updated Aviation Discussion.

.AVIATION...00Z TAFS...VFR conditions across all sites, with the 
exception of KDLS/KYKM due to reduced ceilings of OVC018-019 which 
is expected to improve to VFR within the next two hours. Conditions 
will be degrading for all sites except KBDN/KRDM, which will stay 
VFR through the period as light rain moves in Wednesday morning. 
KPSC will incur MVFR conditions through the overnight and early 
morning hours on Wednesday due to reduced ceilings of OVC015. Fog 
will be redeveloping overnight and extend through much of the 
morning at KPDT/KALW with visibilities dropping below 1/2SM and 
ceilings of OVC004. 75


&&

.PREV DISCUSSION... /issued 253 PM PST Tue Feb 20 2024/ 

SHORT TERM...This afternoon through Thursday night...Satellite 
reveals a closed low offshore with the forecast area under a deep, 
moist southwest flow. Radar shows ongoing shower activity across
the Blue Mountains and their foothills as well as along the 
Cascade crest of Washington. Looking to the surface, patchy fog, 
locally dense, persists in fog-prone areas, most evident from 
Madras north to north-central Oregon and along the foothills of 
the Blue Mountains. In Washington, most fog appears limited to 
ridgetop/summit locations of Kittitas and Yakima counties. RGB 
imagery (day cloud phase distinction) shows the extent of the 
lingering cold pool and stratus/fog to be roughly below 2200-2800 
ft MSL in the Columbia and Deschutes plateaus with partial 
clearing elsewhere.

This afternoon, the focus has turned to convective potential. 12Z 
HREF members suggested that modest (250-750 J/kg) MUCAPE would 
develop along preferentially heated slopes of the Ochocos north to
the southern and northern Blue Mountains this afternoon. RAP-
based mesoanalysis fields indicate an even more modest 100-500 
J/kg (highest in far southeast Deschutes County and the majority 
of Grant County) is present as of 2:30 PM. Satellite does show 
some showers developing in Deschutes, Crook, and Grant counties 
where clearing, surface heating, and low-level(8-9 C/km) and mid-
level (7-8 C/km) lapse rates were most supportive of early 
initiation. Confidence has increased in isolated lightning 
activity with showers as the most robust cells in far southern 
Deschutes and northern Lake counties have produced lightning, so 
have maintained and expanded the "slight chance" mention of 
thunder for most of the Blue Mountains and their foothills. While 
not favored, 12Z HREF soundings do show weak low- level SRH 
(<50-100 m2/s2) and localized effective shear of 25-35 kts over 
the region so cannot rule out an isolated organized, weakly 
rotating cell tracking off the Blues into the foothills, 
especially if something manages to initiate along the remnant cold
pool boundary near Condon and Heppner where observed surface 
heating, CAM- advertised MUCAPE, and low-level SRH are maximized. 
That said, no severe threat is anticipated (<5% chance in a non-
supercell tornado).

Looking to tonight, some fog may redevelop with a moist air mass 
still in place and snow cover aiding near-surface cooling and 
moistening in portions of central and north-central Oregon. Have 
opted to forgo any preemptive dense fog advisories as confidence
(10-40% chance of areas of <0.5 mile visibility per 12Z HREF 
probabilities) is not sufficient in coverage to warrant issuing in
advance of development.

Wednesday, the closed low is forecast to weaken and track
northeast towards Vancouver Island with another round of weak 
shortwaves wrapping around and over the PacNW. Showers are 
forecast again, especially during the afternoon. While CAMs are 
loosely suggesting some instability for the Blue Mountains and 
adjacent foothills again, confidence is too low (10-15% chance) to
include "slight chance" mention of thunder in the forecast.

Wednesday night and Thursday, ensembles are in excellent agreement 
(>95% chance) that another closed low will take the place of the 
existing one offshore, with the remnants of today's low being 
absorbed into it. This leaves the forecast area under an upper-
level ridge with dry conditions forecast. Plunkett/86

LONG TERM...Friday through Tuesday...The extended period is 
characterized by weak upper level ridging giving way to an upper 
level low pressure system that pushes into the Pacific Northwest 
early in the week. This will bring sensible weather concerns of 
windy conditions and mountain snow across portions of the area. 
Elevated winds are expected to arrive late Sunday morning and
extend through the day on Monday. Widespread impacts are 
anticipated, with areas along the east slopes of the Washington 
Cascades, Simcoe Highlands, Eastern Gorge, Lower Columbia Basin, 
and the Blue Mountains and foothills of particular concern. 
Moderate confidence (50-60%) exists for advisory-level wind gusts 
of up to 47 mph or greater, as the GFS showcases a 10-15mb 
pressure difference between Spokane and Portland Sunday evening 
through Monday evening. The NBM also highlights a 40-95% chance of
47 mph wind gusts or greater during this timeframe, with the 
higher probabilities residing across the Simcoe Highlands, east 
slopes of the Washington Cascades, and the northern Blue Mountains 
of Washington. 

There is less confidence in the mountain snow concerns Sunday 
morning through Monday night, as guidance is not in full agreement  
with the overall synoptic pattern. However, recent model runs do 
increase confidence slightly (20-40%) that advisory-level snow 
amounts will be possible along the upper slopes of the Cascades 
and the northern Blue Mountains. This would bring 6-12 inches of 
snowfall along the Upper east slopes of the Washington Cascades, 
5-8 inches through the east slopes of the Oregon Cascades, and 
6-10 inches over the Northwest Blue Mountains of Washington above 
4000 feet. The NBM provides additional confidence in these snow 
amounts, as a 65-95% chance of 6 inches of snow or more is 
highlighted and drops to 21-86% chance of 12 inches of snow or 
more over the aforementioned areas during this timeframe. The 
higher bounds of this range is over the Oregon Cascades, and the 
lower bounds are focused along the Northwest Blue Mountains of 
Washington. 

An upper level ridge will build on Friday to allow for dry, mostly 
sunny, and 1-5 degrees above normal high temperatures to extend  
across the region. These conditions will change over the weekend
as the ridge weakens and retreats to the southeast as a deep, 
upper level low pressure begins its journey down the British 
Columbia coast from the Gulf of Alaska. This will tighten isobars 
between the incoming system and the departing ridge to provide 
breezy wind gusts of up to 20 mph across the east slopes of the 
Washington Cascades, Eastern Gorge, and the Blue Mountains and 
foothills on Saturday. High temperatures will bump up about 5 
degrees from Friday as rain/snow chances extend south along the 
Cascades overnight into Sunday. The low and associated trough will
move onshore along the southern Washington coast Sunday evening 
to provide widespread rain (15-35%) and mountain snow (55-80%) 
chances. Rain amounts will be lacking across the Basin and the 
Yakima/Kittitas Valleys, with up to 0.25" possible along the Blue 
Mountain foothills. The incoming system will further tighten 
pressure gradients to elevate winds across the area beginning 
Sunday and extending through the day on Monday. Northwest flow 
will attribute to high temperatures dropping 5-10 degrees on 
Monday as low temperatures will struggle to get above freezing 
across lower elevations Monday night into Tuesday morning. High 
temperatures look to stay 2-6 degrees below normal on Tuesday as 
northwest flow aloft persists to keep highs in the low- 40s to 
low-50s across lower elevations. 

Guidance is in good overall agreement with a departing ridge and 
incoming low pressure system early in the week, but differences 
arise regarding the strength of the incoming system as a 35-40% 
variance is present between ensembles. This is much improved with 
recent model runs, as now the ECMWF is hinting at a deeper and 
stronger incoming system than the GFS (which was the opposite just
12 hours ago). The cluster phase space and 500mb cluster heights 
also show a trend toward more consistency between ensemble members
and their deterministic solution. It is expected that the ECMWF 
will weaken the system slightly over future model runs with the 
GFS likely strengthening the system slightly. Thus, the NBM was 
utilized through the extended period to provide an applicably 
weighted scenario. 75


&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  35  51  32  52 /  30  40  20   0 
ALW  36  52  36  53 /  30  40  20  10 
PSC  36  54  35  53 /  10  20   0   0 
YKM  34  50  31  52 /  30  30  10   0 
HRI  35  54  34  54 /  20  30  10   0 
ELN  34  46  32  49 /  30  30  10   0 
RDM  34  48  28  51 /  20  10   0   0 
LGD  34  49  33  51 /  30  50  40   0 
GCD  32  49  29  51 /  20  30  10   0 
DLS  37  49  36  52 /  40  40  10   0 

&&

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

$$

SHORT TERM...86
LONG TERM....75
AVIATION...75