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000
FXUS66 KMFR 010545
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
855 PM PST Thu Nov 30 2023

...Updated AVIATION discussion...

.DISCUSSION...The upper trough has shifted to the east of the
region. However, as the nose of an intensifying upper jet moves
in from the west and 700 mb winds increase from the northwest,
increased upper dynamics will combine with orographic lift to
bring elevated precipitation rates, especially to the coastal
ranges and Cascades late tonight into Friday morning. With the
cooler air mass slowly slipping eastward over the region snow
levels of around 3000 feet will slowly rise to above 4000 ft late
tomorrow morning over the Cascades during the heaviest of the
precipitation rates. Current forecast and the winter storm warning
looks on track and will not update this evening.

&&

.AVIATION...01/06Z TAFs...VFR conditions continue across northern
California and southern Oregon. In general, VFR levels will continue
for most areas through this evening. MVFR or IFR ceilings and
visibilities may develop in the Rogue Valley on Friday morning ahead
of the next round of activity.

The next wave of activity is expected to arrive at the Oregon coast
early Friday morning and move eastward, which could bring periods of
MVFR ceilings and visibilities to west side valleys. Widespread rain
showers are expected for areas west of the Cascades. Snow levels of
3500-4000 will limit any snow showers to higher terrain and the
Cascades themselves. Snow showers are widely expected across areas
east of the Cascades, but with higher amounts limited to higher
terrain. These rain and snow showers can locally lower ceilings and
visibility, and extra caution is encouraged. -TAD

&&

.MARINE...Updated 830 PM Thursday November 30, 2023...Active
weather will persist well into next week with multiple fronts
expected to pass through the region.

An approaching front will build very steep and hazardous seas in
waters north of Cape Blanco this evening, while steep seas will
continue in waters south of Cape Blanco. The steepest seas will be
north of Coos Bay, although these conditions do not require an
additional hazard level. These split conditions will last through
Friday evening. A Hazardous Seas Warning is in place for the
northern waters through Friday at 5 PM and a Small Craft Advisory
will continue for southern waters.

Northern waters will calm slightly Friday evening. A Small Craft
Advisory will cover all waters through Saturday at 5 PM.

A particularly strong system is expected to arrive Sunday morning,
bringing strong southwesterly winds. With westerly swell continuing
to move into the waters, steep or very steep and hazardous seas are
possible Sunday and could continue into the week. Additional
guidance and observations will help to determine the exact timing
and coverage of expected hazards. -TAD

&&

.PREV DISCUSSION... /Issued 255 PM PST Thu Nov 30 2023/

..Updated AVIATION discussion...

SHORT TERM...

The first round of snow is through the region with the cold front
pushing through the Cascades at this time. Precipitation mode has
become more showery along the coast and west of the Cascades. The
sun even broke out for a brief time here at the office. There is
cooler air moving in behind this front and the latest forecast has
snow levels down to about 3500 feet.

By Friday morning, we`ll see the next shot of rain and snow over
Oregon and northern California. The NBM is still predicting a
significant snow event over the higher Cascades with 1 foot of
snow falling over Crater Lake on Friday, although GFS and ECMWF
seem to have pivoted the moisture farther north than previous
runs. Friday is the most significant day as snow levels are still
relatively low around 4000 feet, so more roadways and locations
will be impacted by the snow. The overall dynamics with strong
westerly 45 knot flow around 4500 feet fit the pattern for a
Cascades Snow event. However, it`s entirely possible snow totals
come in lower than previously anticipated 24 hours ago.

In any case, a warm front is anticipated to push in sometime
Saturday morning, which will increase the snow levels around 4000
to 4500 feet up to 5500 to 6000 feet. A second warm front is
anticipated to arrive shortly after around Saturday evening as
snow levels shoot up to 7000 feet. The overall snowfall totals are
around 30 inches near Crater Lake with about 7 inches over Lake
of the Woods. The lower totals near Lake of the Woods show how
the rising snow levels really dampen values there. In addition,
the axis of all this moisture is really situated farther to the
north near Crater Lake and areas within Portland forecast region.
Also, with such strong west flow, the Siskiyous, Siskiyou Pass and
Mt. Ashland will really miss out on a lot of the snow as we`re
forecasting 4 to perhaps 6 inches in those locations. The warmer
air arriving Sunday could really dampen what ends up falling.

As for locations east of the Cascades, snow is really dampened
there too. The Warners will see some snow with Warner and Cedar
Pass seeing up to 4 inches over 48 hours. We ended up issuing a
winter storm warning for the northern sections of Klamath County
because that area along 97 always seems to see problems during
these heavier Cascade Snow events.

The potential for flooding in our region remains low between now
and early Sunday. The main reason for this is rivers are already
incredibly low and it seem areas to our north will really feel
the brunt of this atmospheric river and those to follow.

Lastly, wind was on our radar and we ended up issuing a wind
advisory east of the Cascades in zone 31. Summer Lake and much of
eastern Lake County have a good chance to see wind gusts around 45
to 50 mph around Friday afternoon to Saturday afternoon with
Saturday looking the better of th two. Winds around 700 mb are
solidly around 45 to 50 knots and the atmosphere looks fairly well
mixed east of the Cascades. We did consider a winter weather
advisory instead, although the areas impacted by snow are very
remote.

-Smith

LONG TERM...Sunday (12/3) through Thursday (12/7)...The active
pattern will continue into the extended period. There is some wide
variation across the model suites regarding the timing and nature
of the individual systems. There has been a trend towards a
somewhat drier forecast for portions of the extended term within
the deterministic GFS/EC and many of the member runs. As typical,
it appears the NBM is lagging behind in this trend, so it still
maintains a hefty chance of rain throughout the extended. It is
difficult to illustrate this given the current state of the
forecast process, and therefore the forecast products still
suggest that there is a constant threat of precipitation. However,
we are fairly confident that there will be a roughly 24 hour
break in the action Monday. Hopefully the timing details will
become clearer over the next several forecast cycles.

The most likely scenario will be for us to see a brief lull early
Sunday morning as the Saturday system exits and the next system
approaches from offshore. We will still see a general westerly flow
aloft, and this typically gives us good upslope precipitation
production, especially along the coats and the western slopes of the
Cascades. This next system then moves onshore during the day Sunday,
but the model solutions are trending the bulk of the system farther
north, mainly due to the difficulty of the upper level trough to dig
into the larger ridge lying over and just offshore of California.
The trailing edge of the surface front should still bring us a
precipitation and winds, but the bulk of the moisture transport and
dynamics will keep to the north, more towards Portland and
Washington.

The trough exits and the ridge attempts to build back over southern
Oregon Sunday night into Monday. This is where things should trend
drier than previously thought, though the chance for showers does
not completely end (due to some overrunning along a warm front)
before the next system arrives Tuesday into Wednesday, with more
rain and wind.

Fortunately for travel concerns, snow levels are expected to remain
well above 5000 feet for the vast majority of the extended period,
so winter impacts will be minimal. We will have to keep an eye on
hydrological concerns, however, as despite the lack of any one
period of heavy rain, the quick successive storms could add up,
leading to rises on area rivers and streams. -BPN

AVIATION...01/00Z TAFs...Scattered showers continue to move
eastward across northern California and southern Oregon. Current
observations show VFR ceilings across most of the area and VFR
levels are expected to persist through the evening for most areas.
There is a chance of MVFR ceilings in the Umpqua Valley overnight
and into early Friday morning.

The next wave of activity is expected to arrive early Friday
morning, which could bring periods of MVFR ceilings and visibilities
to west side valleys. While widespread rain showers are expected for
areas west of the Cascades, snow levels of 3500-4000 will limit any
snow showers to higher terrain and the Cascades themselves. Snow
showers are widely expected across areas east of the Cascades, but
with higher amounts limited to higher terrain. These rain and snow
showers can locally lower ceilings and visibility, and extra caution
is encouraged. -TAD

MARINE...Updated 200 PM Thursday November 30, 2023...Active
weather will persist well into next week with multiple fronts
expected to pass through the region. Hazardous to small craft
conditions continue through this evening.

Seas will continue to build tonight with a steep northwest swell
between 11 and 12 feet across the waters. When a strong front moves
through Friday, gusty south winds will combine with the existing
swell to increase the steepness and chaotic-ness of the seas. This
will lead to a period of steep to very steep seas late tonight into
Friday afternoon. The steepest seas are expected north of Port
Orford where the strongest winds are expected. Gale force gusts are
possible north of Cape Blanco with the frontal passage Friday
afternoon.

The northwest swell will rebuild Saturday and grow through Sunday.
Meanwhile, another front will pass through the region bringing
another round of wind seas.

The strongest of these systems is likely to affect the waters early
Sunday through Monday, with gales possible north of Cape Blanco.
Seas will peak Sunday afternoon at around 14 to 17 ft, which would
make bar crossings increasingly more hazardous. Sea conditions are
likely to remain high and steep or very steep through Thursday.
Future observations will help to determine the timing and level of
additional hazards. -BR-y/Hermansen

BEACH HAZARDS...Active weather continues well into next week. A
series of fronts will move through the region during this time along
with multiple moderate swell trains. An extended period of sustained
wave action is expected. Breaking waves of 15 to 20 ft will coincide
with higher than normal ocean levels, leading to an increased risk
of beach erosion. Additionally, there is a minor risk of sneaker
waves as these swell trains build into the waters. Confidence is
low, however, considering wind wave interference is likely to limit,
but not completely negate the sneaker threat. If venturing to area
beaches during this time, use caution when visiting and remember to
never turn your back on the ocean! /BR-y/Hermansen

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Beach Hazards Statement from late Friday night through late
     Monday night for ORZ021-022.

     Winter Weather Advisory until 4 AM PST Friday above 3500 feet
     for ORZ025.

     Winter Storm Warning from 4 AM Friday to 4 AM PST Saturday above
     4000 feet for ORZ025.

     Winter Weather Advisory until 4 AM PST Friday above 4000 feet
     for ORZ027-028.

     Winter Storm Warning from 4 AM Friday to 10 PM PST Saturday
     above 5500 feet for ORZ027-028.

     Winter Weather Advisory from 4 AM Friday to 10 AM PST Saturday
     above 4000 feet for ORZ029.

     Winter Storm Warning from 4 AM Friday to 10 AM PST Saturday
     above 4500 feet for ORZ030.

     Wind Advisory from 10 AM Friday to 4 AM PST Sunday for ORZ031.

CA...None.

PACIFIC COASTAL WATERS...Small Craft Advisory until 4 PM PST Saturday
     for PZZ356-376.

     Small Craft Advisory until 10 PM PST this evening for PZZ350-
     370.

     Small Craft Advisory from 4 PM Friday to 4 PM PST Saturday for
     PZZ350-370.

     Hazardous Seas Warning until 4 PM PST Friday for PZZ350-370-376.

&&

$$

SBN/SBN/SBN