EF0 tornado, Canyon City, Oregon, November 5, 2023

Area Forecast Discussion
National Weather Service Pendleton OR
1030 AM PST Sun Nov 5 2023

.Updated aviation discussion...

.AVIATION...18Z TAFs...Dense fog with VLIFR conditions continue
at KYKM though it has lifted over most of the rest of the area.
Anticipate the fog burning off at KYKM in the next couple of
hours with improvement to VFR. VFR conditions are expected
elsewhere through this afternoon. The next system, which can be
seen on satellite and radar offshore, will be arriving this
evening with rain and a mix of VFR/MVFR conditions after 23Z at
KDLS, KRDM and KBDN and after 02Z at other TAF sites, though KYKM
will have a mix of MVFR/IFR conditions. Winds will remain under
10 kts through the next 24 hours. Perry/83

&&

.PREV DISCUSSION... /issued 921 AM PST Sun Nov 5 2023/

UPDATE...Current radar and visible satellite imagery showing
pockets of light returns moving northeast along the Blue Mountain
foothills and across Central Oregon under cloudy skies.
Visibilities at the Yakima Airport have recently improved, but
there continues to be areas of dense fog across the Yakima Valley
when viewing local webcams. Thus, the Dense Fog Advisory will
stay active until 10AM, unless more widespread improvement to
visibilities is observed beforehand.

Rain chances will increase from the south through the early
afternoon before increasing through the Gorge and into the
Columbia Basin during the late afternoon hours as rain extends
overnight and into Monday associated with an incoming upper level
system. Conditions support a slight chance (15%) of thunderstorms
over the Eastern Mountains and the John Day-Ochoco Highlands
between 1PM-7PM later today, as marginal MUCAPE and steep mid-
level lapse rates are present. High temperatures will reach into
the upper 50s to low 60s across lower elevations of the Basin,
which is 5-10 degrees above normal. There will also be breezy,
southwest winds expected to occur this afternoon across Central
Oregon, with gusts of up to 20 mph possible. Rain continues
through much of the day on Monday and early Tuesday as this broad
system slowly pushes onshore and through the Pacific Northwest.
75

PREV DISCUSSION... /issued 423 AM PST Sun Nov 5 2023/

SHORT TERM...Today through Tuesday night...Current satellite
imagery shows yet another low pressure system churning offshore,
west of the Oregon coast with its center at roughly 43.5N, 140W.
Over the forecast area, radar is picking up lingering showers over
the eastern mountains of Oregon and some lighter activity up along
the Cascade crest. While the recent precipitation has led to
rises in area rivers and streams, no flooding is anticipated based
on latest forecasts. As far as current highlights, dense fog has
developed in portions of the Yakima Valley and lower Columbia
Basin of Washington so have kept the dense fog advisory for Yakima
Valley in effect through 10 AM this morning. More patchy coverage
is present in the vicinity of Hanford so have opted for forgo an
advisory and issue a special weather statement for the lower
Columbia Basin of Washington. Fog has dissipated in the Kittitas
Valley and eastern Columbia River Gorge so the dense fog
advisories that were in place earlier this morning have been
cancelled.

Looking forward, ensemble guidance is in very good, though not
excellent, agreement on the 500 mb height pattern today into
Monday. The upper-level low is forecast to remain offshore
through today with another round of showers for the region,
primarily the eastern mountains, before swinging into the Pacific
Northwest on Monday. Regarding the track of the low, there are
some differences in ensemble mean 500 mb height patterns which
does reduce forecast confidence in details for Monday and Tuesday.
Tuesday into Tuesday night, ensembles swing the trough through
the Pacific Northwest with a ridge building overnight in its wake.

Delving into the details for today, CAMs show showers across the
mountains today, though thunderstorm chances appear low (10-15%)
for the eastern mountains. Marginally supportive soundings with
some meager MUCAPE (<500 J/kg) and decently steep mid-level lapse
rates (6.5-7.5 C/km) have led to the inclusion of a mention of
thunder for the eastern mountains this afternoon and evening, but,
again, confidence is low.

Monday is when the main forecast challenges arise. As the upper
low swings onshore, guidance is in good agreement in breezy to
windy conditions for much of the forecast area, especially central
Oregon and east into the Columbia Basin and along the Blue
Mountain foothills. High-end wind potential appears to be 50-60
mph based on a 850-700 mb jet ranging from 40-65 kts in
deterministic soundings setting up somewhere between central
Oregon and south-central Washington and east into the Columbia
Basin on Monday afternoon and evening into Monday night. There is
still variance in the track of the low and the low-level jet so
confidence is just low-medium in needing a wind advisory (30-60%
chance) for central Oregon and the foothills of the Blue
Mountains. The other forecast challenge for Monday is thunder
potential as mid-level lapse rates steepen to 7- 7.5 C/km in
forecast soundings. Combined with more supportive MUCAPE (~500
J/kg) and the cold core aloft, thinking is isolated thunder will
have a low chance (15%) of developing over the eastern mountains
and along the Blue Mountain foothills Monday afternoon and
evening.

Tuesday, another low-level jet sets up as the trough swings
through and flow aloft turns northwesterly , though the jet is
weaker (30-45 kts in deterministic runs) over the Columbia Basin,
eastern Columbia River Gorge, and Kittitas Valley compared to
Monday. Confidence is still low (30-40% chance) in needing wind
advisories in these areas for Tuesday afternoon and evening with
the highest relative confidence in 24-hr maximum gusts (40-75%
chance) reaching advisory- level winds (45 mph) across the
foothills of the southern Blue Mountains, lower Columbia Basin of
Oregon, and western portion of the foothills of the northern Blue
Mountains of Oregon. Plunkett/86

LONG TERM...Wednesday through Sunday...An upper trough will be
departing the region on Wednesday, with an upper ridge poised to
build over the region. There continues to be some significant
differences between the deterministic models in both phase and
amplitude with this upper ridge. However, all of them, as well as
the ensembles are showing ridging building into the region.
Cluster analysis for Wednesday shows fairly decent agreement with
little variance, with all of the clusters showing this ridge over
the region on Wednesday, with only minor amplitude differences.
There will be some lingering light rain over the Cascades and
upper east slopes, as well as the northeast mountains. Elsewhere,
it will become mostly clear to partly cloudy by Wednesday
afternoon. Temperatures will be close to normal, perhaps just a
couple degrees below normal. Forecast confidence is Moderate to
High (60-80%) for Wednesday.

This upper ridge is expected to shift eastward, as a high
amplitude trough pushes into the PacNW on Thursday. However, the
deterministic models continue to show some significant
differences in the placement of the trough axis. The GFS and
Canadian keeps the trough well off the coast Thursday, and the
ECMWF has the trough moving inland, into the coastal areas.
Cluster analysis for Thursday continues to show good agreement
between the ensembles. In any case, will see rain increasing again
over the Cascades by late afternoon, and then rain spreading
eastward over the rest of the forecast area overnight.
Temperatures on Thursday cool down a couple degrees to about 2-4
degrees below normal. Snow levels drop to 3000-3500 ft MSL by
Thursday morning. As a result there will be snow instead of rain
over the mountains above these elevations. Only light snow
accumulations are expected with perhaps an inch or two at most
over the crest of the Cascades and Blues. Due to the increased
uncertainty though, forecast confidence decreases to only
moderate (50-70%).

Likely chances of rain and mountain snow will persist through the
day on Friday, with snow levels increasing to 4000-4500 ft MSL in
the afternoon. Then Friday night into Saturday the rain and
mountain snow will decrease as this upper trough moves out of the
area and is replaced by another upper ridge. However, due to
even greater differences in the deterministic models and
the ensembles now too, will go with the NBM forecast and leave a
slight chance to chance over mainly the mountains. There is also
much more variance shown in the cluster analysis for Friday and
Saturday. With the greater differences, confidence will decrease
to Low to Moderate (30-50%). Temperatures on Saturday will be a
couple degrees warmer than Friday, and will be closer to normal.

On Sunday, a southwest flow aloft will become stronger ahead of
yet another upper trough. There will be increasing chances of more
rain and higher mountain light snow on Sunday. However,
differences in the deterministic models and ensembles, as well as
greater variance in the cluster analysis for Sunday will result
in even lower confidence than on Saturday (20-40%). Temperatures
on Sunday will be close to normal.

Winds will be light through Wednesday night, and then it will
become locally breezy in southern areas of the CWA on Thursday
afternoon from the south to southeast, especially in the Grande
Ronde Valley. Wind speeds will increase to 10-20 mph in these
areas. Winds will then become light again on Friday, which will
persist through Saturday night. South to southeast winds will
then increase again by Sunday afternoon, especially in the Grande
Ronde Valley, with speeds of 10-20 mph. 88

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  59  45  59  42 /  20  80  90  60
ALW  60  46  60  44 /  20  80 100  70
PSC  60  47  61  47 /   0  70  80  40
YKM  55  42  55  38 /  10  90  80  60
HRI  60  46  62  46 /  20  80  80  50
ELN  54  40  52  37 /  20  90  90  60
RDM  58  41  55  35 /  50  90  80  40
LGD  56  41  55  36 /  40  90 100  60
GCD  59  42  56  35 /  60  90  90  30
DLS  59  46  60  46 /  70 100 100  80

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...Dense Fog Advisory until noon PST today for WAZ027.

&&

UPDATE...75
SHORT TERM...86
LONG TERM....88
AVIATION...83

Preliminary Local Storm Report
National Weather Service Pendleton OR
833 AM PST Sun Nov 5 2023

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0715 AM     Rain             4 NNW Island City       45.39N 118.07W
11/05/2023  M1.30 inch       Union              OR   Trained Spotter

&&

Preliminary Local Storm Report
National Weather Service Pendleton OR
927 AM PST Sun Nov 5 2023

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0630 AM     Rain             Elgin                   45.56N 117.92W
11/05/2023  M1.53 inch       Union              OR   Public

            From social media.

&&