NWS Medford, OR wrote its last area forecast discussion before the first event at Dec 13, 8:00 PM PST, 10 hours 1 minute earlier.
Area forecast discussion, Dec 13, 8:00 PM PST, 10 hours 1 minute before
Area Forecast Discussion
National Weather Service Medford OR
800 PM PST Thu Dec 13 2018
.DISCUSSION...14/00Z NAM in. 14/00Z 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.
The axis of a strong long wave ridge has broken to the east of the
area, and southwest flow aloft has developed, opening the storm
door. The upstream trough will move in quickly, pushing a strong
front onshore Friday.
The models have differed on the timing of the front, but now the
13/12Z NAM/GFS/EC all go with moving the front onshore early
Friday afternoon. Another source of uncertainty has been 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 13/12Z NAM and GFS zeroed in on a maximum KACV-KOTH gradient
of just over 8 mb at 14/21Z. The 14/00Z NAM tops out at just over
7 mb at 14/21Z. The 14/00Z GFS goes with a max of -8.5, but in a
departure from previous trends, it does so at 14/15Z. It weakens
the front quite a bit as it moves onshore. In any case, this will
support gales over the coastal waters and high winds will only
impact typical problem areas like the Shasta Valley, the East
Side, and the immediate coastline. High Wind Warnings have been
issued for the Shasta Valley and coast, and Wind Advisories have
been issued for parts of the east side and Rogue Valley. The
13/12Z EC is much weaker with a maximum gradient of 3 to 4 mb, but
this solution looks anomalous at this time.
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 moves out, but
the remainder of the day should be dry.
Their won't be much ridging aloft Saturday as the pattern will
remain progressive and the next upstream trough will move in
Sunday. The associated front will push onshore 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 will move through a bit more slowly and it
will have more available moisture, so rainfall amounts will be
higher. Snow levels will 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 precipitation will be more showery by then and
amounts will be light. High pressure will build in briefly during
the day Monday, but scattered showers will likely 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.
Long term discussion from the Thursday afternoon AFD...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 14/00Z TAFs...VFR conditions will prevail across
the area through Friday morning. There may be some patchy fog in
interior west side valleys briefly late tonight, but not expecting
widespread fog. A strong front will approach the area overnight and
strong winds aloft will lead to LLWS tonight into Friday morning.
These strong winds aloft, along with high level cloud cover, should
limit widespread fog development. Conditions will deteriorate at the
coast first, with MVFR cigs lowering to IFR in heavy rain late
Friday morning. BR-y
&&
.MARINE...Updated 800 PM PST Thursday 13 December 2018...Seas will
remain elevated and steep to very steep tonight as a strong front
approaches. South to southeast winds will increase ahead of this
front, and seas will build further due to a combination of wind
waves and a building powerful west swell. Models are in better
agreement with the timing of the front and position of the
associated surface low. High end gales with occasional storm force
gusts are expected late tonight into Friday. The general consensus
is that winds will shift southwest and gradually decrease after
Friday morning and winds diminish below gales by Friday afternoon. A
very powerful west swell follows, and the combination of this swell
and residual fresh swell/wind wave will drive combined seas to as
high as 25 feet by late Friday morning and into Friday afternoon.
The next front is expected early Sunday, bringing another round of
strong winds, at least gales. 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 generate very large
wind waves. This front will be followed by another 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/BR-y
&&
.MFR WATCHES/WARNINGS/ADVISORIES...
OR...High Wind Warning from 1 AM to 1 PM PST Friday for ORZ021-022.
High Surf Advisory until 4 AM PST Sunday 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.
15/15/03