Area Forecast Discussion
Issued by NWS Medford, OR

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000
FXUS66 KMFR 130400
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
800 PM PST Wed Dec 12 2018

.DISCUSSION...13/00Z NAM in. 13/00Z GFS in through 48hr.

The northern hemispheric projection shows a progressive five wave
pattern around the globe, and that regime will not change over
the next 7 to 8 days or so.

Currently a strong long wave ridge is in place over the far west
U.S. with the ridge axis near the coast. Dry weather will prevail
through Thursday, except for possible light showers over the
outer coastal waters. Skies will clear later tonight, allowing for
cold low temperatures over some areas, along with late night into
morning low clouds and fog over some of the valleys.

The ridge will move slowly east, breaking to the east of the area
Thursday afternoon. Southwest flow aloft will develop, opening
the storm door. The upstream trough will move in quickly, pushing
a strong front onshore Friday.

The models differ a bit on the timing of the front, both between
models and run-to-run within the same model. However, the
variations are generally within a 6 hour window. The main source
of uncertainty is the trajectory and strength of a surface low
will will pass to the north. The models have been all over the
place with this one. The 12/12Z NAM solution had an ACV-OTH gradient
as high as 12 MB. The 13/00Z NAM run goes with a max of 7.3. Quite
a difference! If the higher gradient verifies, there will be storm
force winds at seas and widespread inland wind headlines. If the
gradient is near 8.0 mb, like the 13/00Z NAM and GFS, there will
be gales over the coastal waters and high winds will only impact
typical problem areas like the Shasta Valley, the East Side, and
perhaps the immediate coastline.

While there is some uncertainty in the strength and timing of the
front, confidence is high that Friday will be a wet and windy
day. A High Wind Watch is already in effect for the east side and
Shasta Valley. Wind Advisories will be needed later for other
areas. High winds are still a possibility for the coast as well,
but confidence is lower away from the capes.

Precipitation with this front will not be overly significant as it
will be a fast moving system, but periods of moderate to heavy
rain are possible. The Rogue Valley, a typical dry spot, will
likely experience downsloping winds, and rainfall totals will
probably be much less than surrounding areas. Snow levels will
remain well above 4000 feet for much of the event, and as a
result, there will be a few inches of snow over the higher passes
during the day Friday.

There won`t be much in the way of ridging behind the front Friday
night into Saturday, but showers will diminish into Saturday
morning, and the winds will decrease quite a bit as well.

A line of storms across the Pacific is generating numerous swell
trains that will create very hazardous beach conditions for the
next week or more. High Surf advisories continue, and they will
likely be upgraded to warnings later in the week. Those attempting
to view the waves and conditions at the coast should take the
proper precautions to keep safe, including staying off jetties and
exposed rocks, and remaining behind any safety barriers. Disregarding
these simple steps could result in serious injury or death.

Long term discussion from the Wednesday afternoon AFD...Saturday
night through Wednesday. After Saturday`s dry interlude another
strong front will bear down on the northwest Saturday night into
Sunday morning. Saturday night will experience mainly increasing
clouds and south winds - probably a Gale Force event for the
waters with a low-end high wind warning event for the south coast
and some advisories possible inland. The majority of precipitation
will hold off until Sunday as the associated upper trough splits
coming into the west, with most energy going north or south of our
warning area. So this should not be a major impact precipitation
event as the front moves through Sunday, plus snow levels will be
fairly high - around 6000 feet. Somewhat lower snow levels are
expected for the next system Monday which will probably remain
somewhat benign as well. Unlike the Sunday event, this system
should allow snow to fall 500 to a thousand feet lower and produce
higher accumulations in the Cascades around Crater Lake. -Stavish

&&

.AVIATION...For the 13/00Z TAFs...Upper level ridging has brought
the return of stable conditions, but a shortwave is moving over the
area, bringing some thick high level cloud cover and virga. VFR
conditions prevail and should do so for most of the area through
tonight, with the exception being the interior west side valleys.
Typically with stable conditions, especially in mid December, we`d
be fairly certain of fog development, however, this cloud cover is
pretty thick and some guidance is indicating it lingering around
until at least sunrise Thursday. This could delay or even prohibit
fog development. On the other hand, climatology and recent moisture
would ensure that fog develops regardless, so have backed off the
timing fog formation at KRBG and KMFR until a few hours before
sunrise. Confidence on the timing is low, however, as any break in
the high cloud cover would lead to fog developing quickly. /BR-y

&&

.MARINE...Updated 730 PM PST Wednesday 12 December 2018...
Conditions hazardous to small craft, primarily from seas, will
continue into Thursday. South winds increase Thursday afternoon as a
strong front approaches from the west, reaching gale force and
producing steep wind driven seas Thursday night into Friday morning.
For the most part, models are in fairly good agreement with the
timing of the front, however, the NAM continues to show a strong
surface low developing Thursday night into Friday morning. If this
comes true, this would delay the timing of the front and bring even
stronger south winds. For now, will go with model consensus and see
how future runs of the NAM handle the surface low. This strong front
will also coincide with the arrival of another very high and
powerful swell, the combination of which could produce extremely
high and dangerous seas up to 25 feet Friday.

Additional strong fronts and periods of powerful west to northwest
swell are expected this weekend into early next week. The next swell
event is expected Sunday into Monday, however, it`s not completely
clear on high high wave heights will get. There are still
discrepancies in the guidance which is leading to lower confidence
in peak wave heights. Even with the discrepancies, it still looks to
be one of the highest swell events of the season, so mariners should
be prepared for dangerous sea conditions during this time. Expect
this active pattern to continue well into next week, so be prepared
for additional heavy swell trains and periods of gusty winds.

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...High Surf Advisory until 4 AM PST Saturday for ORZ021-022.
     High Wind Watch from late Thursday night through Friday evening
        for ORZ030-031.

CA...High Wind Watch from late Thursday night through Friday evening
        for CAZ085.
     High Wind Watch from Thursday evening through Friday afternoon
        for CAZ081.

Pacific Coastal Waters...Small Craft Advisory until 4 AM PST Friday for
        PZZ350-356-370-376.
     Gale Warning from 10 PM Thursday to 1 PM PST Friday for
        PZZ350-356-370-376.

$$

15/15/03


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