Area Forecast Discussion
Issued by NWS Medford, OR

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000
FXUS66 KMFR 200444
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
844 PM PST Sun Feb 19 2017

.UPDATE...After the 00Z models have come in, it does appear more
likely that high winds will impact the coast by early morning,
both in BL wind fields and due to the strong surface pressure
gradient. A limiting factor may be that the surface low will be
filling, but given the other parameters, enough wind momentum
should still be in place to produce strong winds. Therefore, have
upgraded the High Wind Watch there to a High Wind Warning.

Winds elsewhere have also trended upwards, not only with this
first system, but also with the second punch expected to arrive
Tuesday morning. While the time between strong winds in the
Shasta valley will be between 12 and 15 hours, across the East
Side, there may only be a break of 6 to 9 hours Monday evening
before gusty winds return. For the sake of simplicity, have
expanded the current Wind headlines on the East Side across the
gap and through the next event as well. While confidence is good
concerning the winds during the second event, there remains the
possibility that the low will track slightly more to the south or
north, and this could lead to changes in timing and strength for
any given area. Will address this if it arises with further
updates over the next few forecast cycles.

Otherwise, the current forecast appears to be on track, so no
other updates were necessary. For more details, see the previous
discussion below. -BPN




&&

.AVIATION...20/00Z TAF CYCLE...Areas of MVFR and VFR cigs with
higher terrain obscured will persist through tomorrow with
widespread showers, breezy south winds, and freezing levels 3000 to
4000 feet above sea level. Winds will increase at all the terminals
tonight after a brief easing ahead of a frontal passage. Low level
wind shear may develop at KOTH late tonight...it all depends on the
strength and trajectory of the offshore low.

&&

.MARINE...Updated 704 PM PST Sunday 19 February 2017...South
winds are decreasing somewhat faster than previously forecast and
have brought down winds and waves to match the ongoing buoy
observations. South winds will increase again tonight and will reach
gales. The GFS and NAM which both have been handling the winds
relatively well continue to indicate gales beginning early Monday
morning. No changes to any watches or warnings are necessary with
this update, but the new gale warning issued a few hours ago still
includes the southern waters a bit earlier to include bits of the
coastal jet beginning to form around 1 AM or so. In addition to the
stronger winds, high long period swell with nearly equally will
build over the waters tonight. The second storm by Monday night is
less certain, but there is still the potential for gales and very
steep seas before conditions will be slow to improve on Tuesday as
equally dominant swells (one fresh) will be 90 degrees apart from
each other. After Tuesday, conditions should improve through the
course of the remainder of the week. -Schaaf

&&

.PREV DISCUSSION... /Issued 410 PM PST Sun Feb 19 2017/

..Updated aviation discussion...

DISCUSSION...This evening through Monday night...Two areas of
low pressure and an atmospheric river will bring moderate to
strong winds and heavy precipitation to the forecast area through
Tuesday. Precipitation will increase over the forecast area
tonight into Monday, focused primarily over the coast and Northern
California. Snow levels will start out around 4500 feet this
evening then rise to 5000 to 6000 feet through Monday night, then
lower on Tuesday morning, with periods of heavy snow affecting
higher terrain of Siskiyou County. We have issued a Winter Storm
Warning above 5000 feet for the heavy snow impacts. Rain and
snowmelt will cause rapid river and stream rises in Northern
California and portions of Southwest Oregon, mainly from the
Illinois River Valley westward. Area Flood Watches have been
issued for the expected impacts from heavy rain. At this point
there are no mainstem river points that are forecast to reach
flood stage, but we will be monitoring closely the Coquille River
at Coquille, South Fork of the Coquille at Myrtle Point, Klamath
River at Fort Jones, Pit River at Canby, and Sprague River at
Beatty.

Additionally, moderate to strong winds will impact the Shasta
Valley and portions of the East Side, and we`ve maintained Wind
Advisories and High Wind Warnings for these areas (High Winds for
East Side). Models are still in enough disagreement for the exact
strength and track of the offshore low tonight that we have issued
a High Wind Watch for the coast. Latest data points towards
isolated areas seeing gusts to 60 mph on the coast, but will let
the next suite of model runs help increase confidence before
deciding whether a warning is appropriate. For the Rogue Valley,
we have increased winds some for tonight with a brief period of
advisory- level winds, but winds just above the surface aren`t
aligning well with the valley (moderately at H9 but not at H8),
and WRF2K output doesn`t look impressive, so have not issued an
advisory at this time.

That`s the first low.  Another low will follow on the first`s heels
Monday night and features an overall stronger windfield, with 700mb
southwest winds of 75kt Tuesday morning and a tighter south to north
pressure gradient. GEFS members show even more variance in the
position and strength of the low center Tuesday.  We have issued a
High Wind Watch for the Shasta Valley for Tuesday morning, but we
may need watches for the coast and East Side as the GEFS
member solutions become tighter.

Tuesday 21 Feb through Friday Night 24/25 Feb 2017. The models
agree quite well during the first part of the period. All of them
start off Tuesday with a long wave upper level trough offshore
around 135W and southwest flow aloft along the west coast. All of
them move the trough onshore Wednesday with an upper level ridge
edging into the area. There will likely be widespread shower
activity over the area until the trough clears out to the east.

The next feature of note is a short wave that will move over the
ridge Tuesday night and then dig a trough offshore Wednesday night
into Thursday. The models have been coming more into agreement on
the location of the closed upper low in this trough, moving it well
offshore west of 130W Thursday and then moving it toward the
northern California coast Friday night...weakening as it approaches.
However, the GFS keeps a closed upper low intact while the EC
solution weakens into an open trough. This affects the surface
reflection of the closed low/trough, as the GFS solution moves a
closed surface low north offshore on Friday, while the EC solutions
maintains light northerly winds initially and then brings in a
weaker low Friday night. Will go with the more consistent EC
solution for now and keep offshore winds northerly into Friday.

Given that the flow aloft will remain northwest to north in this
scenario...there is support for a drier weather regime over the area
for Thursday and Friday as compared to Tuesday and Wednesday. It
won`t be completely dry unless the flow aloft comes around to
northeast. It will be quite cool though. Snow levels will drop to
near the west side valley floors, but there won`t be much in the way
of precipitation by that time.

Saturday through Monday... Models agree on slight ridging
building on Saturday, drying conditions out during the day.
Northeast flow will maintain below normal surface temperatures.
The dry weather appears to be short lived... the GFS develops a
closed upper low near Vancouver Island, then swings it into the
PacNW coast and brings precipitation to southern Oregon and
northern California on Sunday and Monday. The EC shows a similar,
but less strong system. With low surface temperatures lingering,
any precipitation over the mountains and east side will be snow...
but at lower elevations precipitation type will be a challenge.
It is possible that valley floors will see snow, or even freezing
rain... however this forecast is a week out, so confidence is low
by default. But, a snow dance never hurts. -MSC

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...High Wind Warning from 4 AM Monday to 7 PM PST Tuesday for
     ORZ030-031.
     Wind Advisory until 7 PM PST Tuesday for ORZ030-031.
     High Wind Warning from 1 AM to 1 PM PST Monday for ORZ021-022.
     Flood Watch through late Monday night for ORZ021>024.
     Wind Advisory from 4 AM Monday to 7 PM PST Tuesday for ORZ029.
     Flood Watch through late Monday night for ORZ028.

CA...High Wind Warning from 4 AM Monday to 7 PM PST Tuesday for
     CAZ085.
     Wind Advisory until 7 PM PST Tuesday for CAZ085.
     Wind Advisory from 4 AM Monday to 7 PM PST Tuesday for CAZ084.
     Flood Watch through late Monday night for CAZ080>083.
     Winter Storm Warning until 10 AM PST Tuesday ABOVE 5000 FEET for
     for CAZ080.
     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 until 10 AM PST Monday for CAZ081.

Pacific Coastal Waters...High Wind Warning from 1 AM to 1 PM PST Monday for PZZ356.
     Small Craft Advisory until 7 AM PST Monday for
     PZZ350-356-370-376.
     Gale Warning from 1 AM to 4 PM PST Monday for
     PZZ350-356-370-376.
     Hazardous Seas Warning from 7 AM to 4 PM PST Monday for
     PZZ350-356.

$$

BPN/BMS/NSK/MSC



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