Area Forecast Discussion
Issued by NWS Medford, OR

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508
FXUS66 KMFR 220514
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
914 PM PST Sat Jan 21 2017

.SHORT TERM...Bombogenesis is occuring with a low that is rapidly
deepening as it moves northward along 130 West off the southern
Oregon coast. The latest 00z NAM run shows the central pressure
in the surface low at approximately 973 mb this evening. This low
is forecast to deepen to around 966 mb as it moves northward
along and just inside of 130 West tonight. As it does so,
southerly winds will rapidly increase over the coastal waters
as well as over inland areas this evening into Sunday. Heavy snow
is also expected over southern and eastern portions of the CWA
with light to moderate rain and snow elsewhere. Models are on
track with the expected winds and snow conditions.

Inland winds have increased in the Shasta Valley with gusts
reported to 45 mph this evening and will continue to strengthen
tonight. Overall, south winds will increase tonight into Sunday
over much of the area with very strong winds expected to develop
along the coast, in the Shasta Valley, in the southern Rogue
Valley. Winds in these areas are expected to gust to 60 mph or
higher. Gusts up to 80 mph are possible in the Shasta Valley and
along the coast. East of the Cascades and for higher terrain in
Douglas, Josephine and Curry county, also expect strong gusty
winds as well.

In addition to winds, this very strong cyclone will bring very
heavy snow and extremely dangerous travel conditions to the Mount
Shasta area. Roads may become impassable and local white out
conditions are possible tonight into Sunday morning in the Mount
Shasta area. Areas in western Siskiyou and into the Siskiyou
mountains, southern Oregon Cascades and higher terrain in Modoc
county will also see heavy snow. Moderate snow is expected in the
South central Oregon Cascades near Crater Lake and for higher
terrain in eastern Klamath and Lake counties as well as in the
valleys in Modoc county.

Please see the NPWMFR and WSWMFR for details.

Lastly, of note, models continue to support instability over the
coastal waters with this system and weak instability and CAPE
inland. This brings the potential for thunderstorms, especially to
the coastal waters, but also with a slight chance inland over
western portions of the area. Of note, snow levels generally
around 2500 to 3000 feet elevation tonight and Sunday are
expected to locally lower down to 2000 feet in areas of
convective showers.


&&

.AVIATION...22/00Z TAF Cycle...Mainly VFR this evening with just
isolated showers and snow showers causing local MVFR and terrain
obscuration. The next strong front will move into the coast around
06Z tonight and spread inland overnight through Sunday morning. This
will bring gusty southerly winds with low level wind shear
developing over the area, including KOTH, KRBG, and KMFR. Winds will
likely surface at KMFR and KLMT overnight and gusts to 35 kt are
possible. MVFR cigs will move in with rain, and the higher
elevations will see IFR conditions in snow. LIFR conditions will
develop in the Mount Shasta area. The lower conditions will continue
into Sunday morning, but VFR should return Sunday afternoon. There
is a slight chance of thunderstorms over the coastal waters and
along the immediate coast late tonight through Sunday night. -Spilde


&&

.MARINE...Updated 300 PM PST Saturday 21 January 2017...Very high
and very steep seas will persist over the waters through Monday
evening. Long period west swell 20 to 24 feet at 18 seconds this
evening will gradually lower overnight. However, another strong
storm will deepen to sub-970 mb along 130 West and move northward
tonight into Sunday. Storm force south winds will develop offshore
and spread into the coastal waters late this evening, then should
reach all the way to the capes south of Cape Arago very late
tonight. Mariners should consider remaining in port until these
dangerous winds pass. Gale force winds will continue through the
balance of Sunday into Sunday evening. We do expect winds and seas
to gradually subside Monday night into Wednesday. The remainder of
the week looks to be relatively quiet, except for some moderate
westerly swell Wednesday night through Thursday night.
-Spilde


&&

.PREV DISCUSSION... /Issued 433 PM PST Sat Jan 21 2017/

Short Term...Through Monday...
A major winter storm will affect portions of Interstate 5 this
evening through Sunday morning with very heavy snow and strong
winds. 12 to 18 inches of snow are expected in the Mount Shasta
area from Dunsmuir to Weed. While it is difficult to nail down
exactly where the snow and high wind transition zone will be in
the Shasta Valley, it is safe to say this is very likely to be a
major snowfall event that will wreak havoc with travel in the 10
pm to 4 am time frame. Snowfall rate of 2-3 inches per hour are
expected during that time period.

Much of the day was spent gearing up forecast elements and
headlines for the incoming storm, so please see your local point-
and-click forecast available from clicking the map on our web
page for further details.

In summary, a mix of high winds and heavy snow is expected across
the area tonight int Sunday, with the greatest wind and snow
impacts tonight. low pressure will rapidly deepen as it moves
toward us inside of 130 west longitude, and then will move
northward offshore. Most of the impacts will in Southern and
western portions of the area, with the most concern in areas
exposed to southeast and south flow. Thus, the Rogue and Shasta
Valleys and Siskiyous are areas likely to be most impacted.

Notable upgrades today were to go from a winter weather advisory
to a winter storm warning in siskiyous, and to upgrade from a wind
advisory to high wind warning for portions of the Bear Creek
portion of the Rogue Valley, to include Ashland, Talent, and East
Medford.

We`ll need to be on the lookout for thunderstorms along and near
the coast and gusty winds associated with any convection that
makes it inland with this system. additionally, snow levels are
likely to skirt the Sexton Pass area Sunday morning.

This storm series will then gradually wind down. BTL

Long Term...Monday through Saturday.
Monday begins with a deep low over the offshore waters moving
southeast inside of 130W longitude. The 12Z GFS is about 6 hours
faster than the 12Z ECMWF. The main impact from this difference
would be in the amount of precipitation at the coast into Josephine
and Siskiyou Counties on Monday afternoon, with the faster GFS
focused on the morning for higher QPF. The GFS ensemble members
support the operational GFS, but model bias and recent performance
favors the ECMWF. Snow levels on Monday morning are forecast to be
around 2000 feet and rise to around 3000 feet through the day.
Possible low end advisory level snowfall impacts in the ballpark of
1 to 4 inches include the Kalmiopsis Wilderness, western and
southern Siskiyou County, and to a lesser extent Hayes Hill and the
passes over the Umpqua Divide.

On Monday evening, the low is expected to be offshore near Pt. St.
George then continue south-southeastward down the northern
California coast. Showers and snow showers will quickly taper off in
southern Oregon with precipitation in our area lingering longest in
south central Siskiyou County where another 12 hours of low end
advisory level snowfall (amounts around 1 to 3 inches) is possible
above 2500 feet.

A narrow/high amplitude ridge is expected to follow and nudge toward
the coast on Tuesday then move inland on Wednesday. This looks to
produce a couple of dry days with areas of night and morning west
side valley low clouds and fog, and also very cold east side
temperatures...with highs in the mid 20s to lower 30s and lows
mainly in the single digits.  This also is likely to result in a
couple days of stagnant valley air.

Beyond Wednesday, model agreement diminishes. A weak or very weak
front is likely to approach the region from the west on Wednesday
night or Thursday. If the front is very weak it may merely improve
mixing of valley air a weak to moderate amount. The ECMWF solution
is stronger and produces very light precipitation amounts from the
Cascades westward Wednesday night into Thursday morning with a more
optimistic outlook for improved valley air quality.

High pressure is likely to rebuild behind the front and may persist
Friday through the weekend.  -DW

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Winter Weather Advisory until 10 AM PST Sunday for ORZ029>031.
     Winter Weather Advisory until 10 AM PST Sunday for ORZ027.
     Winter Storm Warning until 10 AM PST Sunday for ORZ028.
     High Wind Warning from 1 AM to 7 PM PST Sunday for ORZ021-022.
     Wind Advisory until 7 AM PST Sunday for ORZ023>025.
     High Surf Warning until 4 AM PST Sunday for ORZ021-022.
     High Wind Warning until 7 AM PST Sunday for ORZ026.
     Wind Advisory until 7 AM PST Sunday for ORZ026.

CA...Winter Storm Warning until 4 PM PST Sunday for CAZ082-083.
     Winter Storm Warning until 4 PM PST Sunday ABOVE 3000 FEET for
     for CAZ080.
     High Wind Warning until 9 AM PST Sunday for CAZ081.
     Winter Storm Warning until 4 PM PST Sunday for CAZ081.
     Winter Weather Advisory until 4 PM PST Sunday for CAZ085.
     Winter Storm Warning until 4 PM PST Sunday for CAZ085.

Pacific Coastal Waters...Storm Warning until 4 AM PST Sunday for PZZ350-356-370-376.
     Gale Warning until 10 PM PST Sunday for PZZ350-356-370.
     Hazardous Seas Warning until 9 PM PST this evening for
     PZZ350-356-370-376.
     Gale Warning from 4 AM to 10 PM PST Sunday for PZZ376.

$$

CC



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