Hazardous Seas Warning 3, NWS Medford, OR

Area Forecast Discussion
National Weather Service Medford OR
942 AM PST Sat Jan 13 2018

.AVIATION...13/18Z TAF CYCLE...Dense fog with LIFR visibilities
continues to affect inland valleys, including KRBG and KMFR. The
fog layer is only about 800 feet deep, but the morning sounding
showed that the temperature inversion is very strong. The latest
statistical guidance is very pessimistic about fog burnoff.
Nevertheless, fog typically gradually lifts and then breaks up, so
we still expect it to burn off today, but it is probably going to
take until mid- to late-afternoon (13/20Z at the earliest) until
it does so. -Wright

&&

.PREV DISCUSSION... /Issued 915 AM PST Sat Jan 13 2018/

DISCUSSION...There are areas of valley fog and freezing fog this
morning due to a shallow inversion with a sharp temperature
gradient in place this morning. A dense fog advisory remains in
effect through 10 AM. The shallowness will allow for slow
improvement with Siskiyou County, east side, and Jackson County
valleys likely to clear by late morning or early afternoon. There
is a likelihood of patches of fog persisting through the day near
Roseburg and Grants Pass. The forecast has been updated to reflect
a slower improvement of conditions. The sky condition is forecast
to behave similarly tonight into Sunday with mostly clear skies
except for areas of west side valley fog. East of the Cascades,
gradual drying of the air mass is expected to be sufficient to
prevent fog development.

MARINE...Updated 800 AM PST Saturday 13 Jan 2018...With weak high
pressure over the waters, winds will be relatively light through
tonight. West swell dominated seas will continue, but heights will
be just below advisory levels through tonight.

Conditions over the waters will become active again beginning
Sunday, and then as the door to the stormy Pacific swings open, it
will remain very active through the remainder of the forecast
period. South winds pick up Sunday as a weak front moves through,
and conditions will ramp up to small craft advisory for hazardous
seas as the first of many hefty swell trains arrives behind this
front. Monday looks particularly stormy as a much stronger
front moves through with near gale force south winds and wind driven
seas. Another powerful, medium period swell follows behind this
front Monday night and Tuesday.

Yet another front with south gales is possible some time Wednesday
or Wednesday night. Background swell will remain fairly powerful
during this frontal passage, and the combination of swell and wind
waves may make for very large seas. Behind this front on Thursday,
models are currently indicating an extremely powerful, long period
swell moving in. Latest guidance shows the possibility of west
swells between 30 and 35 feet with a period of 17 seconds Wednesday
night into Thursday. Given that this is still a week out, confidence
in the exact numbers is not high, but if this were to occur, it
would be the largest swell we've seen since December 10, 2015.
Swells that day peaked at 35 feet 16 seconds. The highest swells on
record for buoy 15 occurred on December 3, 2007 when swells peaked
at 40 feet with a dominant period of 14 seconds. Both of these
swells were preceded by gales and were probably very fresh, but the
length of their periods suggests they were indeed, swell.

Again, we are NOT saying that such an enormous swell is definitely
going to happen, but rather this is something to keep an close eye
on as we move through the week. The fronts (and associated
winds/seas) expected Sunday/Monday and again Wednesday are of more
immediate concern. -Wright

PREV DISCUSSION... /Issued 542 AM PST Sat Jan 13 2018/

UPDATE...Areas of dense fog have developed early this morning in
many western valleys, including portions of the Rogue, Illinois
and Umpqua valleys. Localized freezing fog is also possible in the
Rogue Valley, including in Medford. Visibilities of a quarter
mile or less are expected in dense this morning and will result in
difficult driving conditions. Area roadways will be affected by
dense fog including portions of Interstate 5 from Medford to
Roseburg, Highway 199 from Grants Pass to O'Brien and Highway 234
in the Sams Valley as well as other surrounding roadways. A dense
fog advisory is in effect this morning. Please see NPWMFR for
details. Drivers on area roadways should take extra precautions
when driving in fog including:

* Slow down and allow for plenty of room between you and other
  vehicles.

* Fog makes objects appear farther away than they are.

* Use low beams. High beams reflect back thick fog, making
  visibility worse for you and other drivers.

* Be prepared. Keep your headlights clean. That way, you're
  ready for low-visibility conditions

* Don't use cruise control. Most crashes occur because the
  driver is going too fast for weather conditions.

Given the strong subsidence inversion in place today, expect
dense fog to persist through mid morning with gradual improvement
in the late morning and early afternoon. Of note, with little
change in the ridge pattern over the next 36 hours, we may see
areas of dense fog redevelop again late tonight and Sunday morning
in western valleys and impact roadways once again early Sunday.

SHORT TERM...An upper level ridge is over the region and will
continue to strengthen today. This ridge is expected to persist
over the region through Sunday resulting in dry weather and areas
of morning valley fog. The ridge will weaken and begin to shift
eastward on Sunday night. A front will approach the area and
move into coastal areas Monday morning. Models show this front
weakening as it moves inland. Additional stronger fronts will
follow through late in the week as an active jet stream takes aim
at the area.

Today through Sunday, expect dry weather across the area. Valley
fog has developed west of the Southern Oregon Cascades and in
western Siskiyou valleys and will persist through the morning.
Patchy valley fog is expected for areas east of the Cascades. The
timing of fog lifting and clearing is the main variable affecting
temperatures today. Expect gradual clearing of valley fog to
occur in the late morning and early afternoon hours. For areas
where fog or low clouds may into the afternoon, such as in
portions of the Rogue, Applegate, Illinois and Umpqua valleys,
have leaned towards the lower end of guidance for temperatures
today. Meanwhile, over the mountains and along the coast, expect
breezy offshore east to southeast flow today with relatively warm
temperatures and clear to mostly clear skies. Of note, some brief
morning fog is possible in some coastal areas, including near Coos
Bay but expect offshore flow to result in drying and clearing by
late this morning. With the ridge remaining in place, expect
similar conditions on Sunday.

On Monday, the concern shifts to a front approaching the coast
then gradually moving inland Monday afternoon and evening. Models
have trended slower with the front and have trended downward on
precipitation amounts. Overall, expect widespread light to
locally moderate precipitation along the coast beginning Monday
morning. Model solutions then support areas of light precipitation
spreading inland Monday afternoon and evening with the best chance
for rain from the Cascades west. Of note, breezy to gusty
southerly winds may limit precipitation in some western valleys,
especially the Shasta Valley and portions of the Rogue Valley.
Snow levels will remain high Monday with snow levels of around
6500 to 7500 feet. While this system on Monday will not be a heavy
rain producer, it will bring the potential for strong gusty winds
on Monday. Strong gusty south to southeast winds are expected
ahead of this front over the mountains and in the Shasta Valley.
Additionally gusty southerly winds are forecast to develop across
some east side areas, especially in the mountains and near Summer
Lake. Winds gusting to 40 to 50 mph are possible in the Shasta
Valley and Summer Lake areas as well as over the mountains as this
front pushes inland Monday. Also breezy southeast winds are
possible in the southern Rogue Valley.

In the extended forecast...additional stronger fronts are
expected to move into the area Monday night through Thursday. Each
front is expected to be progressively stronger and wetter. This
is followed by a cold and moist upper trough bringing widespread
rain and snow showers to the area late Thursday through Friday.
Will need to monitor this trough for the potential to bring
significant snow in the mountains. Periods of strong winds are
also possible through next week ahead of and with these frontal
systems. Also these storms may bring high surf conditions along
the Southern Oregon coast Monday night and Tuesday with increasing
chances for high surf late Wednesday through Friday.

PREV LONG TERM DISCUSSION... /Issued 315 PM PST Fri Jan 12 2018/

LONG TERM...Tuesday morning through Friday night...The extended
period will be a much more active period, especially when compared
to the meager winter we've experienced so far. The models are in
general agreement on the pattern albeit with the same differences
as previous model runs. The GFS remains the wetter and faster
solution compared to the EC and have maintained a blend of the EC
and GFS solutions with a lean towards the EC timing and
precipitation amounts. Overall there weren't many changes made to
the extended forecast.

Tuesday will likely start off as a rainy day, but transition to a
showery pattern as the upper level trough moves over the forecast
area Tuesday afternoon. Snow levels of around 6,000 feet Tuesday
morning will lower with the trough passage down to around 5,000 feet
Tuesday afternoon and down to around 4,000 feet Tuesday evening.
Showers will be diminishing Tuesday afternoon before snow levels
lower to 4,000 feet so any snow impacts should be minimal. There
will be a brief break in between systems overnight Tuesday into
Wednesday morning.

With the GFS being the faster solution with the next system, it
would have precipitation over the area by early Wednesday morning
while the EC holds off until late Wednesday afternoon. Either way,
both models are indicating that this system will be more impactful
than the previous system. Snow levels briefly come up to around
7,000 feet ahead of the incoming system, but will quickly drop back
to around 4500 feet by early Thursday morning as the upper level
trough moves through. Precipitation will continue with the trough
passage through Thursday night. While this equates to possible snow
impacts for higher passes, including Siskiyou Summit, it will be
much welcomed in terms of building snow pack and for those
interested in winter activities. Winds will be stronger with this
system. However, with model differences on the strength and timing
of these systems, it's difficult to be confident on the magnitude.
Currently it does look like at least advisory criteria will be
possible in the typical windy locations late Wednesday night into
Thursday.

Numerous showers will continue under swift northwest flow through
Friday with snow levels hovering around 3000 feet. Showers will
begin to taper off Friday night as a ridge begins to build in.

The question that naturally follows after mentioning another ridge
in winter is "will the active pattern continue?" The deterministic
solutions offer little guidance beyond this time frame as the GFS
would maintain the ridge through at least the following Monday while
the EC quickly pushes the ridge to the east, bringing another system
through by Sunday night. The ensembles however, support a more
progressive solution, which is also what the Climate Prediction
Center is indicating in their 8-14 day outlook (Jan 18th - Jan 25th)
with increased chances for above normal precipitation and below
normal temperatures. /BR-y

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Dense Fog Advisory until 10 AM PST this morning for
     ORZ021-023>026.

CA...None.

Pacific Coastal Waters...Hazardous Seas Watch from late Sunday night
     through Tuesday evening for PZZ350-356-370-376.
     Small Craft Advisory for hazardous seas from 10 AM Sunday to 4
     AM PST Monday for PZZ350-356-370-376.