FXUS66 KPDT 111806

Area Forecast Discussion
National Weather Service Pendleton OR
1006 AM PST Mon Dec 11 2023

Updated Aviation Discussion

.UPDATE...Condition trend calmer today into tonight as a ridge of
high pressure begins to build over the Pacific Northwest by
Tuesday. Headed through the remainder of the day, areas of dense
fog will be the primary weather related impact with current
traffic cameras and surface observations indicating heavily
degraded visibility along the foothills of the Blue Mountains and
in the Kittitas Valley - all of these areas are under a Dense Fog
Advisory until noon. It remains to be seen how quickly conditions
will improve in this regard, as high resolution guidance like the
HRRR and HREF don't depict too much improvement for the Blue
Mountain foothills, especially the mid to upper slopes between
roughly 1500-2500ft due to ample surface moisture and light
northerly upslope flow. It's possible the aforementioned Dense Fog
Advisory may need to be extended. Otherwise, a weak passing
upper-level shortwave produces light shower chances(15-45%) over
the eastern Mountains into portions north-central Oregon today
headed into the early evening hours before dry weather finally
returns overnight as the influence of the ridge takes hold. As a
result, overnight fog and low stratus formation will be the mainly
weather feature through Wednesday. Schuldt/99

.AVIATION...18Z TAFs...Variable conditions across the sites this
morning, with conditions as low as VLIFR expected through the
period. At site ALW, fog 1/2SM or lower with stratus less than
500ft AGL is expected to persist through the period, with low
confidence (20%) in prevailing improvements. Sites DLS/PDT/PSC
will continue to see IFR to LIFR CIG conditions, with vsby
improving to 3SM-6SM by this afternoon; periodic light misting
will be possible through the remainder of the morning at sites
DLS/PDT as well. Sites RDM/BDN/YKM will see some clearing CIGS
within their respective regions today, however, bkn-ovc CIGS AOA
1kft to 3kft AGL are expected to prevail through the afternoon,
then drop to less than 1kft AGL overnight. Vsby is also forecast
to drop to 3SM or less overnight at sites RDM/BDN/YKM. Winds will
remain light, less than 12kts, through the period. Lawhorn/82

PREV DISCUSSION... /issued 341 AM PST Mon Dec 11 2023/

.SHORT TERM...Today through Wednesday night...Current radar and
infrared satellite imagery showing light returns over the Blue
Mountains and foothills as upslope snow showers continue at
elevation over the Blues under mostly cloudy skies. This is in
response to a weak upper level shortwave that is approaching the
coast before slowly passing through the Pacific Northwest later
today. This will keep light snowfall occurring over the Blue
Mountains this morning before focusing more along the Southern
Blue Mountains through the late morning and early afternoon.
Minimal snow amounts are expected as moisture is lacking with this
passing shortwave. The main weather concern this morning will be
associated with areas of fog that developed overnight, which is a
result from the presence of low level moisture and a weak ridge
that is being eroded by the passing shortwave. A Dense Fog
Advisory has been issued for the Kittitas Valley and along the
Northern Blue Mountain foothills until noon as visibilities of one
quarter mile or less are occurring and will persist through the
morning hours. Confidence in these decreased visibilities
continuing through the morning is moderate to high (70-80%), as
the HREF highlights a 50-70% chance of visibilities of 1/2 mile or
less through 11AM Monday over the aforementioned areas. Morning
low temperatures should only drop into the upper 30s to low 40
along the Blue Mountain foothills, Eastern Gorge, Central Oregon,
and at lower elevations of the Basin. However, low temperatures
across the Kittitas Valley will drop to between 30-32 degrees so
there may be areas of freezing fog and slick roadways - so use
extra caution if traveling this morning.

Conditions will continue to dry through the day today as an upper
level ridge begins to build in from the west in the wake of the
passing system. This will again allow for areas of fog to develop
into Tuesday morning along the Northern Blue Mountain foothills
and the Kittitas Valley, as the HREF shows a 30-50% chance of
1/2 mile visibilities at 8AM Tuesday. Skies will slowly clear
through the day as the upper level ridge builds in from the west-
southwest. This will promote west-northwest flow aloft with high
temperatures warming 1-3 degrees from Monday, which is 2-5
degrees above normal for this time of year. Clear skies overnight
will attribute to much cooler temperatures Wednesday morning with
morning values in the upper 20s to low 30s across Central Oregon,
the Eastern Gorge, Lower Columbia Basin, Blue Mountain foothills,
and the Yakima/Kittitas Valleys. These cooler temperatures,
coupled with the possibility of developing overnight fog along the
foothills and east slopes of the Washington Cascades, will lead
to the potential for freezing fog to develop. This will need to be
closely monitored as slick roadways and surfaces may result.
Afternoon temperatures will be similar to Tuesday, if not a degree
or two cooler due to the presence of the infiltrating upper level
ridge. This ridge does begin to erode along the coast and broaden
to our east as a weak shortwave begins to ride the ridge and
approach the coast late Wednesday. The presence of the ridge and
its increasing influence across the area on Wednesday will
attribute to a pressure gradient developing across the Grande
Ronde Valley, allowing for winds to increase through the morning
and extend through the remainder of the day Wednesday. The GFS and
SREF are advertising a pressure gradient of 4-5mb between Baker
City and Meacham, which correlates to wind gusts of up to 45 mph
out of the south. Currently, these values are below wind advisory
criteria, but this will need to be monitored as wind highlights
may need to be considered. 75

.LONG TERM...Thursday through Sunday...Rather quiet weather is in
store during the extended period. On Thursday, a weak system will
bring precipitation mainly to the Washington Cascades. Models
agree decently on that. All of the deterministic models have
backed off even low pops further east, except for the NBM during
the day on Thursday.

The rest of the extended forecast is dry, with a ridge establishing
itself over the western CONUS. By Sunday, the axis of the ridge will
move over the Pacific Northwest.  Under this pattern, there will
likely be fog and/or stratus that will begin to affect portions of
the Columbia Basin and nearby areas.  Overall, the deterministic
models are in excellent agreement through the extended portion of
the forecast.

High temperatures on Thursday will range from the low 40s to around
50 degrees in central Oregon.  Highs on Friday will be a few degrees
cooler than Thursday. High temperatures on Saturday will range from
the low 40s to the low 50s in central Oregon.  Highs on Sunday will
range from the upper 30s to the upper 40s.  Overnight lows will
mainly be in the 20s.


PDT  43  35  42  27 /  30   0   0   0
ALW  41  36  42  31 /  30   0   0   0
PSC  43  37  43  33 /  10   0   0   0
YKM  43  29  42  30 /   0   0   0   0
HRI  43  36  44  31 /  20   0   0   0
ELN  41  29  39  30 /   0   0   0   0
RDM  47  33  45  28 /  20  10   0   0
LGD  41  30  41  28 /  40   0   0   0
GCD  45  32  45  27 /  40  10   0   0
DLS  48  40  46  38 /  20  10  10  10


OR...Dense Fog Advisory until noon PST today for ORZ507-508.

WA...Dense Fog Advisory until noon PST today for WAZ026-029.