Small Craft Advisory 10, NWS Medford, OR

Area Forecast Discussion
National Weather Service Medford OR
853 AM PST Mon Feb 13 2017

.DISCUSSION...The forecast has been updated to reflect more
extensive low clouds and fog in the vicinity of Grants Pass that
are likely to linger through the morning...and less extensive
coverage elsewhere due partially to a drying of the air mass and
partially due to the blanket of relatively thick high clouds.

Please see the previous discussion below. The focus for the
afternoon package will be on the details as our area returns to an
active pattern on Wednesday. The Wednesday into Thursday system
still appears to be the strongest/most impactful to our area in
this new series. The main effects will be strong southerly winds
and moderate to heavy rain. The following frontal system late
Friday also will likely be strong, but focused south of our
region.

&&

.AVIATION...13/12Z TAF CYCLE...IFR/LIFR conditions will continue in
the Umpqua Basin through the morning. Current TAF at Roseburg shows
improving conditions at 20z, but it could develop an hour earlier.
Skies are clear in Medford, but could not rule out patchy fog around
sunrise. Low clouds and patchy to areas of fog are possible again in
the Umpqua Basin late tonight, but confidence is low for lower
conditions to develop in Medford. Elsewhere, VFR conditions will
continue for the next 24 hours. -Petrucelli

&&

.MARINE...Updated 230 PM PST Monday 13 February 2017...Light winds
and seas will persist through today. A front will approach the
waters Tuesday...bringing increasing south winds. The front will
stall...but inch closer Tuesday evening into Wednesday with south
gales possible. Models show low pressure developing along the front
west of the waters Wednesday afternoon and moving northeast
Wednesday night. They differ with the position and strength of the
low, but the general consensus is the low could move inside of 130 W
and there is some evidence a coastal jet could form Wednesday
afternoon through Wednesday evening with 850mb winds between 70-80
kts. It's possible some of these stronger winds could mix down,
therefore have decided to issue a storm watch from 1 pm pst
Wednesday until 1 am pst Thursday. We also expanded the gale watch
to include all of the waters. Please see MWWMFR for more details.
Winds will gradually decrease Wednesday night, but how much and the
timing of when that will happen will depend on the timing of the
frontal passage. Stay tuned for the latest updates. -Petrucelli

&&

.PREV DISCUSSION... /Issued 608 AM PST Mon Feb 13 2017/

DISCUSSION...There will be little change in sensible weather
through early Tuesday. Strong valley inversions will keep air
still and trap pollutants. While some morning fog and stratus is
possible this morning and again tomorrow morning, increasing high
clouds makes it less likely than previous mornings. The air
stagnation advisory has been extended out through tomorrow
morning, but no changes were made to the short term otherwise.

The weather becomes much more active again Wednesday and remains
active all the way into next week. Models are in good agreement
with the first system, and it will be both a wet and windy one.

First the wind. A deepening sub 990 mb surface low is shown by
all models to move either along or just inside 130W on Wednesday
with a front trailing south along the Oregon/NW California coast.
There will almost certainly be a coastal jet as the front nears
the coast Wednesday evening, and all guidance shows corresponding
storm force winds over the waters as well as high winds over
portions of the coastline from north of Brookings to around
Bandon. At the same time, pressure gradients over land increase
dramatically. In fact, the GFS has a Redding-Medford gradient of
11.3 mb Wednesday afternoon, and this is plenty strong enough to
produce high winds through the Shasta Valley and possibly even the
Rogue Valley. The high winds then develop east of the Cascades,
where models show 700 mb winds of 70-80 knots Wednesday night,
which is also plenty strong for high winds on the east side. High
wind watches have been issued for all of these locations except
for the Rogue Valley where confidence is a little lower.

As mentioned, this system will also be a wet one. An atmospheric
river (AR) impacts the southwest Oregon and northwest California
coast as early as Wednesday morning, and then moves slowly inland
through Thursday morning. Total rainfall from this AR is expected
to be in the 1-2 inch range along the Curry coast and 2-4 inches
over the adjacent coastal ranges and into western Siskiyou County.
Inland areas will see much less rain, especially in the valleys
due to strong, downsloping winds, but our entire area should see
at least some rain from this system. Snow levels will be very high
(upwards of 10,000 feet at the onset), so snow melt is likely to
contribute quite a bit to runoff. For this reason, flooding is a
distinct possibility. We've decided not to issue any flood
watches and let the day shift coordinate with the neighbors and
RFCs about that potential.

By Thursday, snow levels come way down as the AR moves inland and
cooler air moves in. Precipitation also diminishes, so significant
snow impacts are not expected behind this system. There will be
several more systems to come in the days to follow. Confidence in
the details of subsequent systems is lower, but suffice it to say
that our weather will remain active for a while. -Wright

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Air Stagnation Advisory until 10 AM PST Tuesday for
     ORZ023-024-026-029>031.
     High Wind Watch from Wednesday afternoon through late Wednesday
     night for ORZ030-031.
     High Wind Watch from Wednesday afternoon through late Wednesday
     night for ORZ021-022.

CA...High Wind Watch from Wednesday morning through Wednesday evening
     for CAZ081.

Pacific Coastal Waters...Gale Watch from Tuesday afternoon through
     Wednesday afternoon for PZZ350-356-370-376.
     Storm Watch from Wednesday afternoon through late Wednesday
     night for PZZ350-356-370-376.
     High Wind Watch from Wednesday afternoon through late Wednesday
     night for PZZ350-356.

DW/SBN/MAP