967
FXUS66 KMFR 132234
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
234 PM PST Thu Dec 13 2018

.DISCUSSION...

.Short Term...Tonight through Monday... Currently, the forecast
area remains under ridging aloft, with mid to high level clouds
and a few areas of fog in sheltered valleys. Expect most valleys
will fog in again tonight, but some improvement is likely by
sunrise due to incoming clouds and precipitation with the next
front, which as satellite imagery shows, is lurking just offshore.

Model guidance has backed off considerably on the strength of this
upcoming system, especially the higher resolution models, which
showed a significant wind storm just 24 hours ago. Now, all suites
have come into very good agreement, resolving a more typical, but
fast moving, frontal passage. Winds will be gusty, especially
along coastal headlands, in the southern Rogue Valley, the Shasta
Valley, and across the East Side, but they do not appear as strong
as we were seeing yesterday. Have upgraded the High Wind Watches
for the Shasta Valley and the coast to a Warning, and the Watch
for the East Side to an Advisory. Also, have issued a Wind
Advisory for the southern Rogue Valley near Ashland. See PDXNPWMFR
for more details.

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. Snow levels should remain well above 4000 feet
for much of the event, and as a result, a few inches of snow is
expected over the higher passes during the day Friday. Showers
will continue into Saturday morning as the trough passes overhead,
but the remainder of the day should be dry.

After a brief break Saturday, another front will push onshore
Saturday night into Sunday. Winds will be a concern once again,
although models are trending slightly weaker with the system, and
widespread headlines may not be necessary. This front should move
through a bit slower and is expected to have more moist inflow
than the previous one, and therefore rain fall amounts are
expected to be higher. Meanwhile, snow levels are expected to jump
up to above 5500 feet, leaving all but the higher peaks out of the
threat of heavy snow.

Snow levels will drop Monday morning as the upper level trough
swings through, but by then, precipitation will be more showery,
and amounts will be light. High pressure will attempt to build in
briefly during the day Monday, but scattered showers are likely
to continue as moist onshore flow persists.

Also of note, with the ongoing line of systems in the Pacific,
numerous swell trains are being generated that will create very
hazardous beach conditions for the next week or more. High Surf
advisories continue, with the likely chance of upgraded warnings.
Those attempting to view the waves and conditions at the coast
should be sure to 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
easily result in serious injury or death. -BPN

.LONG TERM...Monday night through Thursday...The active pattern
continues with models showing an atmospheric river setting up over
the region with a very moist warm front moving into the Pacific
Northwest Monday night into Tuesday, then gradually lifting north
Wednesday into Wednesday night. However, the brunt of this heavy
rainfall may remain north of the area. Ensemble forecasts indicate
that the heavy rain with this front will occur mainly north of
the CWA, from around Reedsport northward. So expect mainly
moderate rain along the coast and light to moderate precipitation
inland. However if the front settles further south on Tuesday,
portions of the coast and coastal mountains could see heavy rain.
Conversely, if the front shifts further north, this would result
in less rain. So, confidence is low in the details of
precipitation forecast, given that small changes in the track of
this front will yield a large change in the amount of
precipitation over southern Oregon and into northern California.
So will continue to monitor this portion of the forecast with
future models runs. Of note, snow levels are expected to rise as
this front moves into the area and to increase to 7000 to 8000 ft
elevation during the day Tuesday.

Strong south winds are possible across the coastal waters with this
frontal system. Current models and ensembles show a period of
extended gale force winds Monday evening through Tuesday night. For
inland locations expect light to moderate south winds with some
gusty winds at the coast and over the mountains.

Wednesday into Wednesday night, models and ensembles indicate a high
pressure ridge building over the area which will allow the front to
shift northward. So expect decreasing chances for precipitation
during this period with the possibility for dry weather. This break
is brief though as models show a cold front moving into the region
on Thursday. Confidence is low though on the timing and strength of
this system. -CC

&&

.AVIATION...For the 13/18Z TAFs...Shallow fog exist in the Rogue
Valley, Including Medford and Grans Pass, and the Umpqua Basin,
including Roseburg this morning. These locations should see
improving conditions around 20z, but it`s possible this could be
delayed by an hour. The TAF`s at Roseburg and Medford show
conditions improving at 20z, so this will need to be monitored. Also
low clouds and fog exist in the Illinois Valley.

Once the fog and low clouds clear out, VFR conditions will prevail
for the rest of the TAF period. Elsewhere, VFR conditions will
continue for the next 24 hours. However it`s possible ceilings could
lower below VFR at North Bend if the incoming from arrives sooner.

Low level wind shear will be a concern at North Bend and Roseburg
this evening into Friday morning and tonight into Friday morning
at Medford and Klamath Falls as a strong front approaches.
-Petrucelli

&&

.MARINE...Updated 200 PM PST Thursday 13 December 2018...Steep to
very steep, mainly swell driven, seas continue this afternoon with
slight improvement this evening. Southeast winds are already over
the waters and will only increase over time as a strong front
approaches. Winds reach gale force tonight and continue into Friday
as the front moves through. The NAM is now in close agreement with
the others in terms of the timing and location of the surface low
which is northwest of the waters. This will lessen the potential for
storm force winds, but solid gales are still likely with steep wind
driven seas late tonight through at least Friday morning. The
general consensus is for the winds to shift southwest and gradually
decrease late Friday morning with winds diminishing below gales
Friday afternoon. For now, we`ll leave end time of the gales alone
for now, in the event the front is slower to move onshore.

Whatever the case turns out to be with the winds, this will be
followed quickly by a very powerful west swell, and the combination
of this swell and residual fresh swell (or wind wave, depending on
timing) will drive combined seas to as high as 25 feet by late
Friday morning into Friday afternoon.

The next front is expected late Saturday into Sunday, and this is
likely to be AT LEAST gales again. Guidance shows this front to be
slower than the first, and this may produce a more prolonged wind
event over the waters, which means the potential to whip up very
large wind waves. This front will be followed by a large swell, and
guidance still shows the potential for this to be one of the highest
swell events of the season. Needless to say, Sunday could feature
extremely high combined seas. Expect this active pattern to continue
well into next week, so be prepared for additional heavy swell
trains and periods of strong winds with heavy rain. -Petrucelli


&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...High Surf Advisory until 4 AM PST Sunday for ORZ021-022.
     High Wind Watch from 1 AM PST Friday through Friday afternoon
     for ORZ021-022.
     Wind Advisory from 7 AM to 7 PM PST Friday for ORZ030-031.
     Wind Advisory from 10 PM this evening to 1 PM PST Friday for
     ORZ026.

CA...Wind Advisory from 7 AM to 7 PM PST Friday for CAZ085.
     High Wind Warning from 10 PM this evening to 1 PM PST Friday for
     CAZ081.

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

$$

NWS MFR Office Area Forecast Discussion