Small Craft Advisory 27, NWS Medford, OR

Area Forecast Discussion
National Weather Service Medford OR
1115 AM PDT Sun Mar 11 2018

.Updated aviation discussion.

.DISCUSSION...Made some minor updates this morning to winds for
this afternoon. Near term, high resolution models are showing east
to southeast flow increasing this afternoon over the Cascades and
Siskiyous and this will funnel into the southern end of the Rogue
Valley. Winds won't be anything troublesome, but it will be a
breezy day for the northwest-southeast oriented valleys and the
higher terrain. So have brought wind grids more in line with these
models.

Fog and low clouds have filled the interior valleys this morning and
coastal stratus hugs the coast north of Cape Blanco. These low
clouds will burn off later this morning, and once skies clear,
temperatures will climb leaving a warm, dry spring-like day for
the area.

The main focus for the forecast package this afternoon will be on
the upcoming transition to cooler, active weather starting with a
system approaching the coast Monday afternoon. For more details
on this system, see the previous discussion below. /BR-y

&&

.AVIATION...11/18Z TAF CYCLE...Enough changes should occur between
now and tomorrow morning so that fog and low clouds should not have
as much coverage or duration tonight compared to last night.
Sunshine and warmth should dry up the soil some, and winds aloft
will begin to increase significantly. Thus, we expect most locations
to remain VFR through tomorrow. Only moderate confidence exists in
the TAFs at North Bend and Roseburg, where we are forecast VFR or
MVFR, but conditions could degrade to IFR late tonight. We have
higher confidence that Medford and Klamath Falls will remain VFR
through tomorrow. SK

&&

.MARINE...Updated 730 AM PDT Sunday 11 Mar 2018...Relatively
calm winds and seas will continue today, then start changing early
Monday as a strong low pressure system shifts toward the coast from
the southwest. Winds and seas will approach small craft advisory
conditions in the southern outer waters Monday afternoon...then by
midnight, expect southerly gales to have spread throughout the
marine waters. There is some potential for a period of storm force
winds Monday night. Gales and very steep seas will persist Tuesday
afternoon, then winds will start diminishing. Seas will remain very
steep and hazardous from fresh swell and a steep west swell train
through Tuesday night. Wednesday, seas will subside but likely
remain hazardous to small craft until Friday due to persisting swell
and unsettled weather in the region. -MSC/SK

&&

.PREV DISCUSSION... /Issued 500 AM PDT Sun Mar 11 2018/

SHORT TERM...Minor changes were made this morning, mainly to
tweak wind and precipitation forecasts for the front Monday night
into Tuesday.

As expected, showers have ended and skies are clearing from the
south this morning. Boundary layer moisture remains abundant in
the valleys, and this will lead to another round of fog for many
areas. Fog has already formed in the Umpqua Basin and along
portions of the coast, but based on cameras, it doesn't look
terribly dense just yet. Although there isn't any sign of fog yet
outside the door here in Medford, there isn't much reason to think
we won't see some, as we often do, by dawn. It remains to be seen
how dense the fog will be, so we haven't put out an advisory just
yet. We'll keep a close eye on that, and issue one if dense fog
becomes imminent.

Once this fog burns off, today will be mostly sunny with spring-
like temperatures as an upper ridge briefly develops overhead.
We're still on track to see temperatures 5-10 degrees above normal
today - that is, upper 60s on the west side and upper 50s east of
the Cascades. Tonight will be quiet and seasonably cool under
mostly clear skies, at least early on. We could easily see fog
again tonight, but the coverage will hinge on how much, if at all,
the sun is able to dry the airmass.

Things start to change tomorrow as a strong frontal system
approaches from the west. We'll probably see increasing high
clouds tomorrow, but the most noticeable change may be the
increasing south-southeast winds during the afternoon. The airmass
remains warm tomorrow, and this will produce another warm day,
especially where winds downslope (such as the Rogue Valley). The
flip side of that is that winds upslope in some areas (such as
around Mt. Shasta), and this will produce increasing chances for
rain through the day.

The front approaches the coast Monday night and then moves onshore
Tuesday morning. At the same time, a large upper trough remains
offshore with spokes of energy and associated surface lows
rotating around it through Tuesday night into Wednesday morning.
This complicates the wind forecast, because timing and strength of
these lows will be important, and models still show differences in
all that.

Wind will probably blow all Monday night through the Shasta
Valley and into at least the southern end of the Rogue Valley
around Ashland, and this will hold temperatures up. Often times,
Medford itself decouples and winds die down at night, but even
light winds will probably result in a warmer than normal night. As
far as wind speed is concerned, this certainly looks like it will
be worthy of a high wind warning in the Shasta Valley, and a high
wind watch remains in effect there. Elsewhere, widespread
advisory level winds are probable, and high winds may be realized
in some locations like over the mountains and portions of the east
side. Speaking of the east side, strong winds will be possible
from Monday afternoon all the way through Tuesday evening.
Confidence in these details is not high, so we've held off on
issuing any watches at this time. These details should come into
focus soon.

The important thing for residents and travelers to remember is
that breezy to windy conditions are possible from Monday afternoon
through Tuesday afternoon just about everywhere in our forecast
area.

Lastly, this does not look like a prolonged atmospheric river
event for coastal areas. There will be periods of heavy rain, for
sure, but it doesn't seem to linger too long in any one location.
The exception may be in central Siskiyou County where prolonged,
moist, upslope flow is possible for much of Monday night and
Tuesday. The good news there is that snow levels will be
relatively high which limits snow impacts to the highest
elevations, well above cities and most roads. -Wright

LONG TERM...Wednesday through next Saturday. Wednesday morning
starts with the GFS showing a deep closed upper level low out
beyond 130W while the ECMWF is showing a upper level trough at
this time. The GFS operational run with the closed low is the
outlier with only a couple of GFS ensemble plots indication this,
while the ECMWF plots are in much better agreement with the
ensemble means of both the GFS and ECMWF and so am leaning towards
the ECMWF at this time in this time frame. The main difference is
that the winds are 180 degrees out in the coastal waters with the
GFS wrapping winds wrapping around a very deep low directly
beneath the upper level low that is already indicated as an
outlier, and have leaned towards the ECMWF solution. The wrap
around the low will see moisture brought up from the south into
the cooler air mass being brought down aloft bringing the snow
level down to bring good round of snow to the mountains in extreme
SW Oregon and Siskiyou county Wednesday and Thursday.

The operation models come into agreement with each other and the
ensemble means by Thursday afternoon, bringing a closed low down
inside of 130W off of Cape Mendocino. The EC is showing a
secondary low over eastern Washington, with the upper divergence
to the NE of the closed low being distorted by the low to the NE
and the GFS placing the upper level divergence over our forecast
area. With this uncertainty have gone with ensemble MOS pops in
the Thursday- Friday period. Friday is seeing the GFS brings the
center of the closed low just off of Cape Mendocino while the GFS
brings the center of the low just inside of 130W off of the bay
area. The model ensemble plots become very "messy" on the tail end
of the extended, with differing solutions on path, locations and
depth of the lows that wobble slowly southward, and have gone with
the ensemble MOS low pops for the tail end of the extended. Sven

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...None.

CA...High Wind Watch from Monday evening through Tuesday morning for
     CAZ081.

Pacific Coastal Waters...Gale Watch from Monday afternoon through
Tuesday afternoon for      PZZ350-356-370-376.

MNF/TRW/MSC/SK