Small Craft Advisory 130, NWS Medford, OR

Area Forecast Discussion
National Weather Service Medford OR
946 AM PST Tue Dec 13 2016

.AVIATION...13/18Z TAF CYCLE...Stratus in the west side valleys
will burn off to VFR by around noon today and that continues
into early evening. Elsewhere, VFR conditions will continue into
the evening. A strong warm front will bring light to moderate rain
to the west side terminals from south to north starting late this
evening and continuing through the remainder of the TAF period.
Cigs/Vis are expected to be generally MVFR although conditions may
worsen tomorrow morning as a cold front begins to push in as well.
On the east side, snow will spread in with similar timing. Flight
conditions are likely to be much worse as a result with IFR to
LIFR Cigs/Vis likely to develop after midnight, including at KLMT,
and significant snow accumulations by dawn Wednesday. Snow will
change to rain from south to north late morning tomorrow. -Wright

&&

.PREV DISCUSSION... /Issued 856 AM PST Tue Dec 13 2016/

DISCUSSION...A relatively quiet weather day today will precede
about two days of high-impact, fairly complex weather, arriving
tonight. A warm front will overrun lingering cold air tonight
and bring snow to most mountain and East Side areas tonight. A
changeover to rain is expected most areas Wednesday morning. Heavy
rain will impact much of the area on Wednesday as snow levels rise
and a plume of moisture, originating from the tropics, surges into
the forecast area. Winds will become strong too on Wednesday over
mountains, portions of Northern California, and the East Side. A
cold front will push south and east out of the forecast area
Thursday, bringing an end to the precipitation and ushering in a
colder, drier period of weather.

We'll be refining the forecast today, focusing on the high-impact
weather through Thursday night.

Please see the previous discussion below which highlights some
hazard products we have issued for the heavy snow, high wind, and
heavy rain.

MARINE...Updated 200 AM PST Tuesday 13 December 2016...Winds and
seas will remain below advisory criteria today. Our next attention
will be late tonight through Wednesday afternoon. A frontal
boundary is expected to move towards the area later this evening
bringing steady precipitation. The models are in pretty good
agreement showing a surface low developing along the frontal
boundary late tonight. However they differ with respect to the
strength and track of the surface low. This is especially
important because it could make a difference between having gale
force winds over the southern waters or nothing at all. The NAM
and ECMWF show a slightly stronger surface low while the GFS is
weaker. The 0z NAM run tracked the surface low far enough north
where the southern waters could get impacted by gales for at least
6 hours. Now the 6z run tracks the surface low further south, so
it remains to be seen if future runs will continue to show this.
The above reasoning leads to low confidence of upgrading the gale
watch to a warning. So for now we'll leave the watch as is and let
the day shift have another look at this. Taking the 6z NAM at face
value would put borderline gale conditions from Brookings south.
Any south winds will quickly shift north as the surface low moves
inland Wednesday afternoon and night, but should remain below
small craft. Note, even with a further south track of the surface
low, fresh south swell could still sneak into the southern waters
Wednesday evening and night. -Petrucelli

PREV DISCUSSION... /Issued 425 AM PST Tue Dec 13 2016/

SHORT TERM...A very moist warm front will approach the area from
the southwest today then gradually move inland tonight and
Wednesday. Moderate to heavy precipitation is expected with this
warm front. Heavy snow is possible from the Cascades east in
Southern Oregon. Near the coast and into western Siskiyou County,
heavy rain is expected tonight through Wednesday.  A surface low
will track into the extreme southwest Oregon coast by Wednesday
afternoon and will bring a cold front across the area Wednesday
night into Thursday with additional precipitation. Strong gusty
winds are also expected ahead of and with the cold front passage
for locations in the mountains and in the Summer Lake area
Wednesday and Wednesday night.

The 06z and 00z model runs have come into better agreement with
the track of the low and the timing of a warm front moving into
the area tonight and Wednesday, followed by a cold front moving
across the area Wednesday night into Thursday. Models show a plume
of moisture with precipitable water around 1.0 to 1.3 inches
moving into the Northern California and Curry County coasts
tonight into Wednesday. This warm and moist airmass will spread
inland inland tonight into Wednesday bringing overrunning
precipitation to the area. Ahead of this front, expect relatively
cold air in place with snow levels of around 2500 to 3500 ft
elevation in Southern Oregon and around 4000 to 5000 ft elevation
in Northern California. Snow levels are expected to rise
Wednesday morning and afternoon as the warm front lifts northward
into Southern Oregon. Snow levels will rise to 6000 to 7000+ feet
elevation on Wednesday but may remain lower over far northern
Klamath and Lake Counties.

With snow levels starting out low and moderate to heavy
precipitation associated with the front, expect significant
snowfall accumulations in portions of the Southern Oregon
Cascades and east side tonight and Wednesday. Snow is expected to
begin late this evening and tonight and is forecast to become
heavy by Wednesday morning. Snowfall rates up to 1 inch per hour
are possible in the Southern Oregon Cascades and across much of
Klamath and Lake counties Wednesday morning. This will bring
significant impacts these areas. Heavy snowfall may continue over
northern Klamath and northern Lake counties Wednesday afternoon.
However, expect snow levels to rise quickly Wednesday afternoon to
around 6000 feet or more over southern portions of Klamath and
Lake counties. Blowing snow and near whiteout conditions are
possible on Wednesday, especially in the Southern Oregon Cascades,
in the mountains above 6000 feet east of the Cascades and in the
Summer Lake area as strong gusty winds combine with snowfall. By
Wednesday evening expect most areas to transition to rain with
snow levels around 6000 to 7000 feet. However, areas near Fort
Rock and Chemult may see continued lower snow levels around 4500
to 5000 feet.

A winter storm watch is in effect for the Southern Oregon Cascades
and areas to the east in Klamath and Lake counties. See the WSWMFR
for details.

Wednesday night and Thursday, a cold front will gradually
move inland. Models are showing a strong 700 mb jet with southwest
winds of 65 to 75 kt moving across the area ahead of this front.
So expect south winds to increase and become strong and gusty
over the mountains and into portions of the east side, especially
near the Summer Lake area. A high wind watch has been issued for
late Wednesday afternoon and Wednesday night for these areas. See
the NPWMFR for details.

Also of concern will be the potential for heavy rainfall in
southwestern portions of the area, mainly in Curry and southern
Josephine counties as well as in western and southern Siskiyou
County. Current forecasts call for 3 to 4.5 inches of
precipitation tonight through Thursday over these areas. Heaviest
rain across Curry, Josephine and Western Siskiyou counties is
expected tonight through Wednesday night. In the Southern Siskiyou
county area near Mount Shasta moderate to heavy precipitation is
expected tonight through Thursday. Precipitation may begin as
snow, especially above 4500 feet tonight then transition to rain
as snow levels rise to 6000+ feet Wednesday morning. Heaviest
precipitation is expected in the Mount Shasta area from Wednesday
morning through at least Thursday morning...with moderate rain and
snow showers possible Thursday afternoon and evening.

As the front moves east Thursday afternoon and evening, expect
decreased precipitation for areas west of the Cascades with many
Areas transitioning to isolated or scattered showers. East of the
Cascades, precipitation is expected to decrease by Thursday
evening in Klamath county and by late Thursday night in Lake,
Modoc and eastern Siskiyou counties.

PREV LONG TERM DISCUSSION... /Issued 100 PM PST Mon Dec 12 2016/

Long term...Friday through Monday. The deterministic models and
associated ensembles are in pretty good agreement in the extended
period, and this leads to above average confidence in the overall
pattern, but relatively lower confidence in some specifics. The
period of Friday through Monday could generally be characterized
as chilly and more than likely dry, but with just enough potential
to keep an eye on.

The majority of the precipitation from the mid week system will be
off to our east by Friday. However, the upper trough axis will be
moving through early Friday, and the combination of low snow levels
and residual shower activity could lead to isolated impacts from
snow over the lower elevations. Given that this is 5 days off,
confidence in any specifics is low, but it's worth watching the
tail end of this departing system.

Snow levels will be down near sea level on Saturday, and models are
hinting at a weak shortwave moving through the flow. This could also
give us some snow showers at low elevations, but again, it's way to
far off to get specific. Sunday is the day most likely to be dry,
but there may be a system coming down the PacNW coast that could
bring some rain to the north coast and Umpqua basin by day's end.
Snow levels will be rising, so snow is less of a concern over lower
elevations by Sunday. Snow levels rise further Monday and really
jump up Monday night as a fairly juicy warm front arrives from the
west. -Wright

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...High Wind Watch from Wednesday afternoon through late Wednesday
     night for ORZ030-031.
     Winter Storm Watch from this evening through Wednesday afternoon
     for ORZ029>031.
     Winter Storm Watch from this evening through Wednesday afternoon
     for ORZ027-028.

CA...High Wind Watch from Wednesday afternoon through late Wednesday
     night for CAZ085.

Pacific Coastal Waters...Gale Watch from Wednesday afternoon through
     late Wednesday night for PZZ356-376.