National Weather Service Raw Text Product

963 
FXUS66 KPDT 201807
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
1007 AM PST Tue Feb 20 2024

Updated Aviation Discussion

.MORNING UPDATE...Satellite reveals a closed low offshore with the
forecast area under a deep, moist southwest flow. In combination 
with 7-8 C/km mid-level lapse rates and modest upper-level lift 
this is facilitating ongoing shower activity across the forecast 
area. Looking to the surface, patchy fog and dense fog persist 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. Have opted to cancel the dense 
fog advisories early due to lack of coverage, but patchy fog, 
locally dense, will likely (70-100% chance) persist through the 
morning hours.

This afternoon, the focus turns to convective potential. 12Z HREF
members are in subjectively good agreement that modest (250-750
J/kg) MUCAPE will develop along preferentially heated slopes of
the Ochocos north to the southern and northern Blue Mountains.
While confidence is still low (10-20% chance) in any lightning
activity with showers, satellite does show areas of clearing, and
the aforementioned steepening mid-level lapse rates, developing 
surface-based instability in areas of clearing skies, modest 
synoptic lift, orographic lift, and weak shortwaves wrapping 
around the upper-level low has led to the inclusion of a "slight 
chance" mention of thunder for most of the eastern mountains of 
Oregon and southeast Washington down into the foothills. While not
favored, 12Z HREF soundings do show weak low- level SRH (<50-100 
m2/s2) so cannot rule out an isolated weakly rotating cell 
tracking off the Blues into the foothills. No severe threat is 
anticipated. Plunkett/86

&&

AVIATION...18Z TAFs...KDLS and KPDT are expected to be varying 
between MVFR and VFR due to mist and light precipitation, but KDLS 
is currently at LIFR with a low CIG at 17Z. However, CIGs for KDLS 
will improve through the day while the VSBYs fluctuate due to mist 
and showers within the vicinity until Wednesday morning when more 
light rain occurs. The CIGs and VSBYs for KPDT will be at VFR and 
with no precipitation. KALW is forecast to be at MVFR this morning 
with mist and showers but should improve tonight. Most sites are 
forecast to be at VFR by afternoon with winds below 10kts, with an 
exception of KDLS staying at MVFR. Feaster/97

&&

.PREV DISCUSSION... /issued 301 AM PST Tue Feb 20 2024/ 

SHORT TERM...Today through Thursday...A deep low continues to
spin away offshore the southwest Oregon coastline tonight while
pushing surges of moisture north across the PacNW. At the moment,
radar shows scattered showers moving across the PacNW, though the
heaviest showers are occurring outside of the forecast area.
Overhead, however, mid level cloud decks continue to move forecast
area while low stratus and areas of dense fog continue to shift
around the Blue Mountain foothills and central/north central OR.

Today, the deep low will weaken into an open trough as it makes
it's trek towards Vancouver Island, all the while pushing weak
plumes of moisture across the PacNW. This will result in light
rain and mountain snow showers moving across the forecast area
throughout today, with snow accumulations up to around 2 inches
possible above 4.5kft to 5kft in the mountains. Otherwise, any
rain accumulations in the mountains will be less than 0.2 inches,
while the lower elevations will see less than 0.05 inches. As for
fog concerns, areas of central and north central OR have cleared 
up tonight with the push of southerly winds, and with southerly
winds persisting through the remainder of the morning, the dense
fog advisories currently in effect will likely be ended early. As
for the Blue Mountain foothills, dense fog continues to be noted
from Pendleton to Walla Walla, with NBM guidance indicating that
this trend will continue late into the morning. Tonight, areas
that see precipitation throughout the day will see patchy fog
develop, especially in portions of the Columbia Basin, Gorge, and
adjacent valleys/foothills.

Wednesday, the open trough will push east across the PacNW with 
scattered showers across the region. The Cascades will see the 
brunt of the precipitation accumulations through Wednesday, with 
liquid equivalent accumulations between 0.25 to 0.5 inches, and 
snow amounts between 5 to 7 inches along the high ridges and 
crest. It's a different story across the eastern mountains, Blues,
and Wallowas as rain shadowing from the Cascades will only result
in QPF amounts up to 0.15 inches, with snow amounts along the 
high ridges between 0.5 to 2 inches. While the lower elevations 
may see an isolated shower or two move across that will result in 
a few hundreths of an inch of rain, except along the Blue Mountain
foothills where 0.05 to 0.1 inches are forecast.

Wednesday night, the open trough will exit to the east and allow
an upper ridge to build over the PacNW. This will result in dry
and quiet conditions developing Thursday. Lawhorn/82 

LONG TERM...Friday through Tuesday...The long term will start off
on the quiet side as high pressure ridging will sit over the
region and bring seasonably warm conditions, but the next system 
to bring winter back will arrive by the end of the weekend into 
next week, with lower snow levels and seasonably normal to below 
average temperatures in its wake.

Friday starts under high pressure ridging leaving the region quiet
overall. Some passing shortwaves and moisture through Sunday
afternoon may bring a passing shower or snowfall in the Cascades
or Northern Blues, but confidence is low (<30%) in any activity.
On Sunday an upper level system will dive down from the Gulf of 
Alaska and follow the Canadian coastline before moving inland just
to our north across the border. This deep low will bring cooler
temperatures, a surge of moisture, and gusty winds to begin next 
week. The NBM shows a 30-80% chance of wind gusts in excess of 30 
mph, and it's likely this will become more likely in the coming 
days as we get into higher resolution data. The GFS is already 
presenting pressure gradient differences of around 10-12 mb from 
Spokane to Portland, which would be favorable of at minimum wind 
advisory (45+ mph) gusts.

On the side of precipitation, the forecast here is a bit more
complex. The NBM is favoring fairly stout snowfall in the
mountains with snow ratios back up into the 10+ inch range and
48-hour probabilities of 25-90% at the Blues, Wallowas, and
Cascades seeing 10+ inches. But with the potential for onshore
flow which would be a bit warmer than the forecast is allowing
combined with high temperatures only a day or two ago in the mid
30's to low 40's for most of these mountain zones, the NBM may be
overdoing the expected snowfall. For now, going to allow the
forecast to play out as the NBM has produced, but expecting
snowfall amounts to see a decrease in the coming days unless the
cold temperatures truly hold as forecast confidence increases.

Overall, there is moderate confidence (40-50%) in the expected
conditions. The first few days of the forecast have generally good
agreement in ensembles, but in the last few as the system is
impacting us, we start to see guidance agreement fall apart in 
the strength of the troughing. If the troughing isn't quite as 
strong, we may get more zonal flow which would be warmer 
temperatures than forecast, and 40% of European members favor this
solution; whereas, 53% members of the GFS expect a more 
northwesterly flow which would bring cooler temperatures, and 
overall comparing ensemble highs from both models shows the ECMWF 
running several degrees warmer both this weekend and next week. 
Still, we should see a fairly spring-like weekend, followed by a 
wintry return in the mountains with the lower elevations cooler 
but not quite an Arctic return. Goatley/87

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  52  35  50  33 /  20  40  30  20 
ALW  52  36  50  35 /  40  30  30  20 
PSC  51  36  52  35 /  20  10  20   0 
YKM  47  34  48  31 /  20  30  20  10 
HRI  51  35  52  34 /  20  20  20  10 
ELN  45  34  43  31 /  30  30  30  10 
RDM  49  34  48  29 /  10  10  10  10 
LGD  49  34  48  32 /  40  20  40  40 
GCD  49  32  48  29 /  20  10  30  20 
DLS  47  37  48  35 /  40  40  40  10 

&&

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

$$

MORNING UPDATE...86
SHORT TERM...82
LONG TERM....87
AVIATION...97