073
FXUS66 KPDT 230250
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
650 PM PST Fri Nov 22 2024

.UPDATE...Latest radar data shows a band of rain extending across
eastern Oregon and southeast Washington with embedded heavier
convective showers. There have even been a few sporadic LTG
strikes this evening. Most of the lightning has been in Grant
county where the SPC Mesoanalysis page shows 100-250 J/kg of
MUCAPE. It doesn`t appear that t-storms will be a significant
threat tonight so will not add to the forecast. Otherwise added
some patchy fog for the Blue MTN Foothills of northern Oregon and
SE Washington for Saturday morning (extending the time period
from tonight into early Saturday) since at least some of the model
guidance suggests that fog could linger for a while into the
morning Saturday. Otherwise the current forecast is on track for
tonight and Saturday. 78

&&

.PREV DISCUSSION... /issued 352 PM PST Fri Nov 22 2024/

SHORT TERM...This afternoon through Sunday night...Multiple bands
of showers are developing in a weakly unstable air mass from the
Columbia Basin through the Blue Mountains. Given the ample
moisture present (ensemble-advertised PWATs of 150-200% of normal)
coupled with orographic and synoptic lift, 24-hr probabilities of
0.5" or precipitation ending 4PM Saturday are relatively high for
the eastern half of our CWA: 40-70% for the Blue Mountain
foothills and high mountain valleys, and 70-95% for the northern
Blues. Given high snow levels in the wake of this morning`s warm
frontal passage, snowfall probabilities are not quite supportive
of any highlights for the northern Blues (20-50% chance of 6"
above 5000 ft), and even lower for Wallowa County (up to 10%
chance of 4") due to uncertainty in timing of the changeover of
rain to snow as a cooler air mass moves overhead.

Fog, locally dense, may redevelop this evening and overnight.
Have included a mention of patchy fog in the forecast for many
fog-prone locations along the Blue Mountain foothills and portions
of the Columbia Basin, but confidence in dense fog was too low to
proactively issue any headlines.

Lingering precipitation is forecast Saturday and Sunday as a
couple shortwaves round the base of the broad upper low that
continues to churn offshore. Precipitation chances will be
highest for the Blue Mountains and adjoining foothills as well as
the Cascade crest. No highlights are anticipated at this time,
though ensemble and deterministic guidance are suggesting the
remnants of the surface low that impacted the PacNW
Tuesday/Wednesday will swing onshore late Sunday into Sunday
night, increasing cross-Blues pressure gradients and leading to
breezy southerly to southeasterly winds along the foothills. NBM
probabilities of advisory-level wind gusts (45 mph or greater) are
low (<20%). Plunkett/86

LONG TERM...Monday through Friday...A transition to a cool and
drier weather pattern will begin Monday and Tuesday as a weakening
upper trough moves ashore and crosses the area. Models then show
an upper ridge developing offshore giving our area a dry north to
northwesterly flow Wednesday through Friday. The situation looks
favorable for basin fog though models have just some spotty areas
of night and morning fog, so held off on mentioning it in the
forecast until we get closer to that timeframe. The Extreme
Forecast Index shows little in the way of unusual weather through
the period aside from temperatures several degrees below normal in
the latter part of the week.

On Monday, model clusters are in excellent agreement in having the
upper low and trough centered off the southern Oregon coast with a
southwesterly flow over our area. The NBM keeps a chance of rain
over the lower elevations with snow likely in the mountains with
snow levels at 3000-4000 feet. Amounts will be just a few
hundredths of an inch of rain and less than an inch of mountain
snow. Temperatures will be in the 40s with mid 30s to lower 40s in
the mountains. The NBM probability of reaching 50 is fairly low
with some pockets of 20-40 percent in the Blue Mountain Foothills
and John Day Basin.

Tuesday, model clusters begin to have some differences. 78 percent
of the model ensemble members have the trough moving overhead with
the upper low at the coast or just offshore while the remaining
members have the low over northern California and Nevada. They all
have varying ideas about the strength of the upper low. During
the day on Tuesday, a chance of rain showers in the lower
elevations will taper off in the afternoon and evening while
mountain snow will continue through the day and wind down
overnight. What little rain that falls will be mainly in the
morning and just a few hundredths of an inch. Mountains will get
up to an inch of snow mainly during the day. Temperatures will
drop to the lower to mid 40s with 30s in the mountains. NBM
probabilities have a 40-70 percent chance of reaching 45 in the
lower elevations but chances of reaching 50 are less than 15
percent even in the warmest locations.

Wednesday and Thanksgiving Day, model clusters show a ridge
building in the Gulf of Alaska sending a cold and dry
northwesterly flow over our area though about 20-25 percent of the
ensemble members (mostly from the GFS) have a northerly flow
which shouldn`t be significantly different in the impact on our
weather. Most of the area will be dry but the mountains will have
a slight chance of very light upslope snow. Despite the ridge
building, temperatures will change little from Tuesday, remaining
several degrees below normal. Thanksgiving Day will be in the
lower 40s in the lower elevations with mainly 30s in the
mountains. NBM probabilities give a 20-50 percent chance of
reaching 45 degrees in the lower elevations.

Friday looks to be similar to the previous two days with the ridge
remaining in control of the weather, though 23 percent of the
model clusters show the ridge retrograding west away from the west
coast and develops a more zonal westerly flow. The lower
elevations will remain dry while the mountains will have a slight
chance of snow flurries with little or no accumulation.
Temperatures will remain in the 30s and lower 40s across the area.
Perry/83

AVIATION...00Z TAFs...A trough over the Pacific Northwest will send
bands of precipitation into the area for the next 18 hours or so
with substantial rain, low stratus and fog possible at most TAF
sites. KALW and KPDT will be the most heavily impacted with periods
of moderate to heavy rain with LIFR/VLIFR conditions prevalent
through around 18Z tomorrow before improving to MVFR. KPSC, KYKM and
KDLS will have mainly MVFR conditions with occasional IFR cigs
possible. KRDM and KBDN will remain at VFR aside from possible MVFR
conditions with showers from 16Z-20Z. Conditions in general will be
improving after 18Z. KRDM and KBDN will have southwest winds at 10-
15 kts gusting to 25 kts through 03Z. Otherwise, winds will remain
below 12 kts for the next 24 hours. Perry/83

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  39  47  31  49 /  90  50  20  10
ALW  42  50  35  50 /  90  70  10  20
PSC  43  51  36  47 /  70  40  10  10
YKM  33  47  30  42 /  10  10  10  20
HRI  41  50  34  49 /  80  40  10  10
ELN  33  46  30  42 /  10  10  10  40
RDM  32  44  29  46 /  30  20  10  10
LGD  36  45  28  42 /  90  80  40  10
GCD  36  44  25  43 /  90  60  40  10
DLS  41  50  36  49 /  30  20  30  40

&&

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

$$

SHORT TERM...78
LONG TERM....83
AVIATION...83