Area Forecast Discussion
Issued by NWS Medford, OR

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FXUS66 KMFR 202233
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
233 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 Cascadeas 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.

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.

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

&&

.AVIATION...20/18Z TAF CYCLE...Mostly VFR conditions will prevail
into this evening...but there will be some MVFR cigs in showers with
higher terrain obscured. Winds will be gusty from the Cascades
east...but they will diminish later today.  Areas of MVFR cigs with
higher terrain obscured will develop in rain this evening and
persist into Tuesday morning. Winds will also increase...and
widesprad turbulence is expected...but it won`t be quite as windy
at the coast as it was Monday morning.

&&

.MARINE...Updated 830 AM PST Monday 20 February 2017...The second
storm moves through the waters from the southwest tonight and
Tuesday morning, and very steep hazardous seas are likely before
conditions slowly improve on Tuesday as equally dominant swells
(one fresh) will be 90 degrees apart from each other. After
Tuesday, conditions should improve through the remainder of the
week.

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...High Wind Warning until 7 PM PST Tuesday for ORZ030-031.
     Winter Weather Advisory from 10 PM this evening to 4 PM PST
     Tuesday for ORZ030.
     Wind Advisory until 7 PM PST Tuesday for ORZ029>031.
     Winter Storm Warning from 10 PM this evening to 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 from 10 PM this evening to 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 through late tonight 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...Hazardous Seas Warning until 10 PM PST Tuesday for
     PZZ350-356-370-376.

$$

NSK/BMS/JRS



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