Area Forecast Discussion
Issued by NWS Medford, OR

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000
FXUS66 KMFR 210409
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
809 PM PST Mon Feb 20 2017

.UPDATE...A broad area of moderate to sometimes heavy rain and
snow has moved into the area this evening ahead of a compact
surface low set on making its way onshore tonight. The low will
also bring gusty winds to much of the area, and several wind
headlines remain in place. The forecast appears on track with this
scenario, and not much has changed in the latest runs of the
models, so no major updates have been made to the forecast.

For more details, see the previous discussion below. -BPN


&&

.AVIATION...21/00z TAF CYCLE...Mostly VFR conditions will prevail
into this evening...with some MVFR cigs in showers with higher
terrain obscured. MVFR cigs will become more widespread later
tonight, as showers increase. Expect IFR vis in heavier showers,
especially along the coast and in the Umpqua Valley. Winds have
mostly calmed down for the night... but gusty south to southwest
winds will pick back up early tomorrow morning as a low moves
inland. Freezing levels across the area will drop by 1000 to 2000
feet tomorrow morning. -MSC

.MARINE...Updated 800 PM PST Monday 20 February 2017...A compact
low pressure center will bring high winds, meeting small craft
criteria, or perhaps even low end gales in the southwestern part of
the marine area. Expect moderate to heavy rain tonight through
tomorrow morning. Winds will veer to westerly tomorrow morning, with
gusts hitting at least small craft criteria in most areas. Steep
chaotic seas will persist in the southeastern waters into tomorrow,
with steep wind waves and multiple swell trains. After a surface
trough passes over tomorrow afternoon, precipitation will diminish
to scattered showers, with a slight chance of thunderstorms in the
southern waters. Winds will diminish, but seas will remain chaotic
through tomorrow night. Winds and seas will come down somewhat
Wednesday, and remain moderate through the weekend, as several
systems pass through the area. -MSC

&&

.PREV DISCUSSION... /Issued 420 PM PST Mon Feb 20 2017/

DISCUSSION...We`ll transition to a relatively wet, windy, and
warm weather pattern to a colder, showery pattern over the next 24
to 48 hours. One more strong low pressure system in the recent
series of storms will traverse the coastal waters tonight into
Tuesday morning, bringing with it another round of moderate to
strong winds across the forecast area. Latest model data suggests
less coastal winds than the low that moved through early this
morning (sub- warning level winds), but we still expect noteworthy
40 to 50 mph gusts from Brookings to near Port Orford as the low
moves toward the coast early Tuesday morning.

Further inland, winds will have more of an impact. The southwesterly
winds in the low levels will bring gusts to 45 mph to portions of
Josephine, Jackson, and Douglas Counties. Isolated power outages are
possible, and a Wind Advisory has been issued for this wind threat.
The alignment of the winds does not suggest suggest strong winds for
the Rogue Valley, moreso in valleys that are oriented southwest
to northeast like the Illinois, portions of the Applegate, and
areas further north near Azalea. Even further east, the strongest
winds are expected. A mix of High Wind Warnings and Wind
Advisories continue for areas east of the Cascades and in the
Shasta Valley. Winds will ramp up most places late tonight and
peak Tuesday morning before diminishing Tuesday afternoon.

Moderate to heavy rain is still expected to result in river and
stream rises, especially in portions of Siskiyou, western Josephine,
and Curry Counties. Models have trended lower on overall
precipitation, but we are maintaining Areal Flood Watches for the
heavy rain threat that should end between Tuesday morning and
afternoon. In the higher elevations, generally above 5000 feet,
moderate to occasionally heavy snow will bring 10 to 20 inches of
snow to the High Cascades and Siskiyous through Tuesday. Further
south, another 6 to 12 inches of snow is expected in Siskiyou
County high terrain above 5000 feet, with higher amounts above
6000 feet. Further east, snow is expected in the high terrain and
also in the Chemult area where we have issued a Winter Weather
Advisory for tonight into Tuesday.

A cold, unstable air mass moves in Tuesday behind the front, and
this brings the chance for thunderstorms to the coastal waters and
portions of the coast, mostly south of Cape Blanco. LI values of -2
means some of these storms could feature small hail or become strong
in nature. The unstable air mass also means that heavier showers
could bring snow levels sharply lower than earlier in the day, and
will bring some snowfall to Siskiyou Summit on Interstate 5. Snow
levels drop further Tuesday night, nearing the critical pass
levels along Interstate 5 near Sexton Pass, so we`ll be closely
monitoring how much precipitation accompanies the colder air. If
snow levels trend lower for Tuesday night, a couple inches of
snow on Sexton Pass is possible. -NSK

Wednesday 22 Feb through Thursday Night 24/25 Feb 2017. The
models agree quite well during this interval. All of them start
off Wednesday with a long wave upper level trough offshore just
off the west coast...which moves through the Pacific northwest
Wednesday and out to the east by late Wednesday night. Flow aloft
will shift from southwest aloft to northwest to north by late
Wednesday night.

There will likely be widespread shower activity over the area until
the trough clears out to the east.  After that...weak short waves
embedded in the flow will keep some light shower activity going.
Winds will be light to moderate during this interval.

Normally this would not be a high impact scenario...but it will be
cold and that will allow snow levels to drop to 1500-2000 feet
Wednesday morning and 500-1000 feet Thursday morning. This
will bring the snow levels near the valley floors in the Rogue and
Illinois valleys as well as parts of the Umpqua Valley. Amounts will
generally be a trace to a half inch or so in those areas.  It is
unusual to get appreciable snow there this late in the season, so it
will have impacts if it comes to pass.

The next feature of note is a short wave that will move over an
offshore long wave ridge to be just offshore late Thursday night.
Depending on the trajectory of this low...it may bring another round
of enhanced shower activity to the west side. Snow levels will be
quite low 5000-1000 feet again late Thursday night...so once again
there may be significant snow in many of the west side valleys.
-Jbird

Friday through Monday night...The long term period continues the
overall wet and cold pattern with another low dipping south from
offshore of Vancouver, B.C. The ECMWF and the GFS are in relative
agreement regarding this scenario, but a few key elements--
specifically how close to the coast this low tracks--will create
quite a bit of uncertainty regarding the type of precipitation and
just how much we may see. The GFS, per usual, is more progressive
and brings the low into California near Eureka. The ECMWF, on the
other hand, is slightly slower and wobbles a strong shortwave
northward into southern Oregon over the weekend. Have gone with a
blend of the GFS and ECMWF (with a slight lean toward the ECMWF)as
well as the previous forecast due to their striking similarities
in their solutions and that the blend looks more reasonable than
either individual model solution.

This means that temperatures will remain chilly as compared to the
warmer start to the forecast, and snow levels will be low enough to
cause low elevation snow showers across west-side valleys. Right
now, snow levels start off around 1000 feet, and rise to 2000 feet
during the weekend. That being said, snow levels are notoriously
difficult to forecast this far out; and confidence is relatively low
on whether or not the possibility for valley snow will continue.
Chances are, however, that the valleys will not see as much snow as
they did in early January. Regardless, since this forecast will be
prone to big changes (the addition of shortwaves or wobbles in the
track of this low); it will need to be closely monitored for updates
to the forecast. -Schaaf

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...High Wind Warning until 7 PM PST Tuesday for ORZ030-031.
     Winter Weather Advisory until 4 PM PST Tuesday for ORZ030.
     Wind Advisory until 7 PM PST Tuesday for ORZ029>031.
     Winter Storm Warning until 10 PM PST Tuesday for ORZ030-031.
     Flood Watch through Tuesday morning for ORZ021-022-024.
     Wind Advisory from midnight tonight to 10 AM PST Tuesday for
     ORZ023>026.
     Winter Weather Advisory until 10 PM PST Tuesday ABOVE 4000 FEET
     for for ORZ027-028.

CA...High Wind Warning until 7 PM PST Tuesday for CAZ085.
     Wind Advisory until 7 PM PST Tuesday for CAZ084-085.
     Winter Storm Warning until 10 AM PST Tuesday ABOVE 5000 FEET for
     for CAZ080.
     Flood Watch until 4 AM PST Tuesday for CAZ080>083.
     Winter Storm Warning until 10 AM PST Tuesday ABOVE 5000 FEET for
     for CAZ082-083.
     High Wind Warning from 1 AM to 4 PM PST Tuesday for CAZ081.
     Wind Advisory from 1 AM to 4 PM PST Tuesday for CAZ081.

Pacific Coastal Waters...Small Craft Advisory for hazardous seas until 10 PM PST Tuesday
     for PZZ350-356-370-376.
     Hazardous Seas Warning until 10 PM PST Tuesday for PZZ356-376.
     Hazardous Seas Warning until 10 PM PST this evening for
     PZZ350-370.

$$

BPN/NSK/BMS/MSC/JRS


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