902
FXUS66 KMFR 092354
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
354 PM PST Sat Nov 9 2024

...Corrected AVIATION discussion...

.SHORT TERM...Through early Tuesday morning, Nov 12, 2024...

Satellite imagery shows mostly mid and high level cloudiness over
the area this afternoon in advance of an incoming trough offshore.
The trough is supporting a weak frontal boundary, which is attached
to a swirling low about 275 miles due west of Cape Mendocino. Radar
is picking up on some precipitation over the coastal waters, and
even some light echoes west of the Cascades at times, but a dry
layer below 12,000 feet has prevented precip from reaching the
ground. The trough offshore will move onshore tonight into Sunday
morning with the surface front getting strung out and weakening over
the area. This will moisten the air mass and lead to some rain
chances across the area, highest along the coast and over to the
Cascades with diminishing probabilities farther south and east. But,
rain amounts remain meager, generally a hundredth or two and largely
less than a 0.10 of an inch. One area though, the SW coast and coast
ranges, might get a little help with some orographic lift, which
could enhance precip for a time Sunday. There might be 0.25 of an
inch or so in some of those locations. We might get a few drips here
in Medford, but right now it looks like we`ll be lucky to get
anything measurable. Overall, high temperatures Sunday will be at
least a few degrees above normal (in the upper 50s to low 60s for
the west side valleys; mid 50s over the east side), but with the
cloud cover, it may not feel like it.

Meanwhile, a Pacific jet stream disturbance coming through the
southern reaches of a deep closed low over the Gulf of Alaska will
energize another front which will move onshore Sunday night into
Monday. Model depictions of Integrated Water Vapor Transport (IVT)
show a healthy, but narrow atmospheric river (~500 kg/m/sec)
associated with this front, with PW values around or just over 1.00
inch. Model wind fields in advance of the front show speeds of
45-55 kt at 700 mb traversing the area during this time. Gradients
support peak wind gusts along the coast of 40-50 mph, but well
shy of the criteria (>58 mph) we`d issue a warning on. Expect
S-SE winds to pick up in the Shasta Valley Sunday afternoon, then
especially Sunday evening/night. Similarly, we expect gusty S-SW
winds to develop over the higher terrain of the east side and also
near Summer Lake. These locations have been included in a Wind
Advisory for wind gusts up to 50 mph. The band of moderate to
heavy precipitation associated with the front will move onshore
Sunday evening, then push to the Cascades by Monday morning. Rain
amounts at the coast look to be in the 1-2 inch range for most
locations with up to or perhaps even a little more than 3 inches
in the Curry coast ranges. Here in the Rogue Valley, the most
likely amount is 0.25-0.50 of an inch. Most other areas east of
the Cascades and in NE Cal should also receive a wetting rainfall
with this front, though the Christmas Valley and northern Lake
County appear to get the least with 0.10 of an inch or less. In
terms of snow, snow levels up around 7000-7500 feet will drop
quickly to around 5000 feet Monday morning, so we do expect some
winter hazards over the Cascades. We have issued a Winter Weather
Advisory from 4 am PST Monday through 10 am PST Tuesday. Snowfall
rates could reach 1"/hour at times (most likely between 6-10 am
PST Monday) up at Crater Lake as the front moves through. While
snowfall will also occur farther south, including at Mt. Ashland,
impacts should be less since the main thrust of post-frontal
showers will be in the Cascades from around Highway 140 northward
Monday night into Tuesday morning as snow levels drop a bit more
to around 4500 feet. This could bring a couple of inches of snow
along Highway 97 north of Chiloquin. -Spilde


.LONG TERM...Tuesday through Saturday, Nov 12-16...

A warm front will pass through the area during Tuesday, producing
spotty precip, mainly along and west of the Cascades with snow
levels rising again. Generally speaking, winds will be a bit
breezy out of the south due to an increasing north/south pressure
gradient, and this will result in some increased winds in the
Shasta Valley and across the East Side, but nothing too out of the
ordinary. A brief period of dry weather and near normal
temperatures is expected late Tuesday afternoon into the evening
as the warm front lifts to the north, then conditions deteriorate
Tuesday night as the next storm system approaches the area.

The first concern with this system is rain. The leading edge of
precipitation should pass over the coast overnight Tuesday night,
with rain spreading west to the Cascades by midday, then to the East
Side through the afternoon. With IVT values of between 500 and 750,
moisture inflow will be strong, however, much of that is due to the
stronger winds associated with the system (more on that below). What
is important is the direction of the prevailing flow, which in this
case, will be out of the southwest. This will provide good upslope
mechanics along the coastal ranges, the Cascades, and the Siskiyous,
and especially in the Brookings area. These are where the heaviest
rainfall amounts will be, although totals will not be altogether
unusual for systems like these and during this time of year.
Forecast rain amounts were bumped up above NBM/WPC in these areas,
especially along the south coast near Brookings.  That wind
direction will, however, also produce downslope flow in the Rogue
Valley and east of the Cascades. Rainfall totals here were reduced
below NBM/WPC amounts, but we still expect a good precipitation
event for all of the forecast area. Overall, rainfall amounts will
be healthy, but given the low river levels, no major flooding is
expected. Nuisance flooding, such as small creeks and streams,
blocked ditches, or urban areas are possible.

Winds will also be a concern. Forecast surface pressure gradients
along the coast between Arcata and North Bend are approaching -10mb,
and gradients between Redding and Medford of around -7mb, and model
solutions are suggesting 60 to 70 kt winds at 700 mb. With this in
mind, it is likely that Wind Advisories and Warnings will be
necessary, especially along the coast and across the East Side. Wind
headlines may also be necessary in the Shasta Valley and the Rogue
Valley near Ashland, but the prevailing southwest flow will not
align well with the valleys, so these areas will be mainly gradient
driven and therefore a bit weaker than otherwise. Winds at the coast
will peak Wednesday afternoon, with inland areas peaking into the
evening and overnight.

The final concern is snow. With the southwest flow tapping into
warmer air to the south of us, snow levels will be a bit on the high
side, beginning the event at around 6000 feet, then only dropping to
around 4500 to 5000 feet after the front passes, when precipitation
has likely transitioned to a more showery regime. This will keep
snow from being a major impact, but heavier showers, or the very
last gasp of the front itself, could produce just enough snow at the
higher passes to cause some travel difficulties.

With all of these factors, there is still time for the forecast to
change, so we will be keeping a sharp eye on the latest model runs
over the next several days. That being said, it is never to early to
begin preparations for impactful wind and rain.

Once the front passes through the area (exiting by Thursday
morning), the system slows, keeping upper level troughing overhead
through Friday afternoon. Temperatures will cool to about 5 to 10
degrees below normal as the cold air settles into the region. Along
with ongoing onshore flow, precipitation chances will continue
through this time, with snow levels hovering between 3000 and 4000
feet. There will not be continuous precipitation, as the NBM
suggests with near continual high PoPs, but instead we expect hit-or-
miss and off-again-on-again showers will be the rule Thursday and
Friday. Precipitation amounts will vary greatly from place to place,
and some showers may be strong enough to produce graupel or very
small hail.

Friday night into Saturday, the trough shifts just far enough to the
southeast to allow ridging to nudge in from the west. The NBM keeps
showers across the whole region during this time, but it is much
more likely that the area dries out Saturday as high pressure
settles in. -BPN


&&

.AVIATION...10/00Z TAFS...VFR levels continue across northern
Oregon and southern California ahead of a weak front approaching the
Oregon coast this evening. Scattered showers will be possible across
Oregon counties tonight and into Sunday morning. Inland shower
activity will decrease on Sunday afternoon while precipitation
chances along the Oregon coast increase ahead of an impactful cold
front. These elevated rainfall chances will move inland in future
TAF periods. Showers from either front can locally lower
visibilities and ceiling as well as obscure elevated terrain.

Guidance suggests low level wind shear will be present along the
Oregon coast and over higher terrain starting Sunday afternoon and
continuing through at least the end of this TAF period. South winds
at between 40 and 45 kt are possible at 2000 feet in this TAF period
and possibly beyond. North Bend looks to be the only terminal
directly under this possible wind shear, but guidance also shows
shear around the Rogue and Umpqua valleys, over the Cascades, and
scattered across areas east of the Cascades. -TAD

&&

.MARINE...Updated 200 PM PST Saturday, November 9, 2024...

Two fronts will move through this weekend, one later this
afternoon/evening and another Sunday night into Monday.

The first front, the weaker of the two, will bring hazardous to
small craft conditions for much of the outer waters through 10 PM
tonight north of Cape Blanco. Farther south, where winds will be
lighter, seas will be swell dominated and remain mostly below 10
feet. That front will stall and dissipate over the waters tonight.

The second front, the stronger one, moves through late Sunday into
Monday. This will likely bring a period of moderate to heavy rain,
gales, and very steep and chaotic seas. Latest forecast has wind
gusts up to 40 knots over the waters and perhaps 45 knots off the
coast of Port Orford. Models also continue to show this system being
followed by another large, long period swell, peaking around next
Tuesday. Early indications are showing peak swells 17-21 feet with a
period of 15-16 seconds. This would maintain very high and very
steep seas through midweek. -Smith/Hermansen

.BEACH HAZARDS...Updated 200 PM PST Friday, November 8, 2024...

Another large, long period swell is expected to arrive early next
week. Model guidance has been consistently showing NW swells
arriving following the front Sunday night into Monday. Seas briefly
subside immediately behind the front Monday (6 hours or so during
the morning), but then build significantly Monday afternoon and
Monday night, reaching a peak of 17-21 feet, 16 seconds by early
Tuesday morning. This will bring large breaking waves (23-27 feet)
to the beaches and coastline through Tuesday afternoon. Seas will
gradually subside Tuesday night. A high surf advisory has been
issued for 10 PM Monday through 10 PM Tuesday because of these
dangerous surf conditions.

Large breaking waves create hazardous conditions along and within
the surf zone, and could inundate beaches and low lying shorelines.
If you have plans to venture out to the beaches early-mid next week,
pay close attention to the ocean, stay away from the surf zone and
off of jetties. Also, keep checking back for forecast updates.
-BPN/Hermansen

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Wind Advisory from 7 PM Sunday to 7 AM PST Monday for ORZ030-
     031.

     Winter Weather Advisory from 4 AM Monday to 10 AM PST Tuesday
     for ORZ027-028.

CA...Wind Advisory from 7 PM Sunday to 7 AM PST Monday for CAZ081-
     085.

PACIFIC COASTAL WATERS...Gale Warning from 1 PM Sunday to 10 AM PST
     Monday for PZZ350-356-370-376.

     Small Craft Advisory until 10 PM PST this evening for PZZ370-
     376.

&&

$$

MAS/BPN/ANH