Flood in Oregon, January 17 to 20, 2024

Area Forecast Discussion
National Weather Service Medford OR
406 PM PST Wed Jan 17 2024

Updated AVIATION Section

.SHORT TERM...Today through Thursday night...After receiving
reports of flooding in SW Curry County we issued a flood warning
for that area. As of 11 AM, when we issued the warning, 3-5 inches
had fallen, with 1 to 1.5 inches of additional rain possible. This
has caused widespread areal flooding and even caused some
roadways to be covered. Please do not drive through flooded roads
because they could be washed out underneath! It would not be
surprising if this rainfall was causing rockfall in the mountains,
too. Rain showers will ease, but not end, around 7 PM tonight,
and we will reevaluate then if we need to extend the warning. The
flooding of the Coquille at Coquille remains ongoing and is
expected to last at least into Saturday.

There have also been some gusty winds in the region as the base
of a trough passed through the area. Sites at the coast reported
gusts at 30-50 mph, and east of the Cascades at speeds of 40-70
mph. Winds will be on the decline now that we are transitioning
into being in the trough's wake, which is dominated by northwest
flow. Expectations are for winds to return to more normal strength
in six to twelve hours time, and so we have re-issued the wind
advisory for portions of Klamath and Lake County through 10 PM
tonight to cover any stray gusts that may still occur.

Meanwhile, the northwest flow will continue to support showers
moving into the area, maintaining a 50-90% chance for rain/snow
showers from the Cascades west in Oregon. Areas in NorCal and east
of the Cascades will maintain a small chance (10-30%), but
generally the action will be winding down until Thursday morning.
New snow amounts in the Cascades should remain below a foot in
the highest areas tonight. But, there will still be enough snow on
the passes (1-4 inches) that we continued the winter weather
advisory for now.

Thursday morning a small ridge of high pressure will
force any remaining moisture up and over the Cascades. This will
bring a 50-70% chance for rain/snow that morning, with drier
conditions west of the Cascades. The high pressure will continue
to move east throughout the day, quickly restoring onshore flow.
Moderate chances for rain return to the coast in the afternoon,
but the front will be weak and not much inland movement is
expected. However, Thursday is still expected to be fairly
widespread and limited sun is expected to break through.
Temperatures will return to normal, to about 5 degrees above
normal.
-Miles

.LONG TERM...Friday into Thursday

Warm moist southerly flow, which can be considered an atmospheric
river, persists on Friday as a low moves into the region from the
southwest.  The chance of precipitation increases significantly
along the coast by Friday evening with nearly 100% chance for rain
and relatively lower chances farther to the east. With a southerly
track,  snow levels will be higher and pushing north of 6000 feet
across the entire forecast area.  The focus of the snow accumulation
will be in northern California and in the Trinity Mountains, but
impacts should be minor with no major roads above 6000 feet open
during winter.

This low eventually pushes into Oregon around mid day Saturday. Some
ensemble members have the low remaining over the outer waters,
although those seem to be the outliers.

The atmospheric river(AR) essentially persists through most of the
extended forecast period with integrated water vapor transport
remaining above 250.  There appears to be a drop off around Tuesday
according to the ensembles, so this AR is essentially lasting for
over 7 days between now and next Thursday, which is on the longer
side for an AR event.  However,  the IVT values don't exceed 250 by
much, so one could consider this a weaker AR given the lack of
dynamics in this moist persistent flow.

River flooding seems unlikely based on the latest hydrographs from
the river forecast center and the relatively light precipitation in
the extended forecast.

No major impacts anticipated in the extended forecast, although
elevations above 6000 feet in northern California should see some
good snow due to this favorable southwest flow. One could also
consider the rain falling so close to freezing east of the Cascades
brings a risk of freezing drizzle around Monday morning. We'll keep
an eye out for this risk in future forecast cycles.

-Smith

&&

.AVIATION...18/00Z TAFs...Moist onshore flow continues to bring some
light precipitation from the coast to the Cascades this afternoon
with freezing levels in the 5000 to 6000 foot range. IFR is
prevalent along the coast with MVFR and higher terrain obscurations
over many inland areas. Interior west side valleys like the Umpqua
Basin, Illinois and Rogue Valley all have VFR currently.
Precipitation is spotty or non-existent over the mountains in NorCal
and east of the Cascades, but SSW winds aloft are still fairly
strong with mountain wave turbulence likely. Winds are also
surfacing at some of the usual spots east of the Cascades (Summer
Lake) with gusts to 50-55kt. At Klamath Falls, gusts may reach 25 kt
for a few more hours before easing by around 03Z. Brief breezy
conditions at Medford/Roseburg for the next couple of hours should
die off by 02Z. A warm front will move in with a another shot at
some light rain/snow later this evening/overnight. Ceilings will
continue to be mostly in the MVFR category, though local IFR is
likely as well into Thursday morning. Freezing levels will
eventually rise to 7000 to 8000 feet during this time frame.
Conditions should improve for most areas Thursday afternoon.
-Spilde

&&

.MARINE...Updated 200 PM Wednesday January 17, 2024...High and very
steep, hazardous swell will persist into this evening, but gradually
subside overnight as southwest winds ease. However, south winds
restrengthen to gales over the outer waters on Thursday, building
very steep and hazardous seas once again. Winds and seas will be
slightly lower within 10nm from shore, but still hazardous to small
craft through Friday. Additional fronts are likely to maintain
unsettled weather into this weekend. -Spilde

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Wind Advisory until 10 PM PST this evening for ORZ030-031.

     Winter Weather Advisory until 8 PM PST this evening for ORZ027-
     028.

CA...Wind Advisory until 10 PM PST this evening for CAZ085.

PACIFIC COASTAL WATERS...Hazardous Seas Warning until 10 PM PST this
     evening for PZZ350-356.

     Small Craft Advisory from 10 PM this evening to 4 AM PST Friday
     for PZZ350-356.

     Hazardous Seas Warning until 4 AM PST Friday for PZZ370-376.

&&