Area Forecast Discussion
Issued by NWS Medford, OR

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352
FXUS66 KMFR 250440
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
840 PM PST Sat Feb 24 2018

.UPDATE...We are in the process of making some updates this
evening. First, pops will be lowered and dependent grids will be
adjusted. Things have calmed down over southern Oregon and NorCal
this evening. Radar is devoid of meaningful echoes, and other than
the isolated shower along the coast and patchy light snow in the
Douglas Cascades, there hasn`t been any precipitation at any of
the obs sites for several hours. Based satellite and high
resolution model guidance, this will continue to be the case
through at least dawn tomorrow.

The other change will be to the timing of the winter storm
headlines. Latest guidance shows snow developing later than
previously thought in the Cascades from Highway 140 south and the
Siskiyous. Snow will also be delayed across the mountains of the
east side. As a result, the start of the winter storm warnings in
these areas will be pushed back until 00Z Monday (4 PM LST
tomorrow). Expect these updates to be sent by 10 PM. -Wright

&&

.PREV DISCUSSION... /Issued 425 PM PST Sat Feb 24 2018/

SHORT TERM...Through Tuesday...
Winter weather and wind headlines for this afternoon and evening
have been expired since the current weather system and resultant
impacts have ended. Snow levels rose to between 2500 and 3500 feet
this afternoon and, with the higher sun angle warming the surface,
roads across most areas have become mostly wet. Those in
mountains and the east side that are still snow covered have seen
lesser accumulations and main thoroughfares have been treated by
road crews, so impacts should be minimal. Altogether, while we
did see the Oregon Cascades get some strong accumulations today,
the storm system appears to have pushed a bit stronger to the
south and east, into the Siskiyous, than model guidance had
previously indicated.

For Sunday through Monday we`ve issued the next round of headlines
as well as a weather story graphic with expected snow
accumulations. Model guidance has increased precipitation amounts
with this weather system, especially along and near the coast,
coastal mountains, into the Siskiyous, and across northern
California and the east side.

Precipitation will push into the area Sunday morning across Coos
and Douglas counties. Models have slowed the southeastward
progression of the frontal system on Sunday, with the bulk of the
snow impacts reaching areas south of the Umpqua Divide and Lake of
the Woods now expected to arrive Sunday evening and night. We`ll
need to continue to keep an eye out for how fast snow levels fall
and how much resultant snow accumulates in the Sexton and Canyon
Mountain Pass areas as well as in the West Side valleys Sunday
night into Monday morning. For now, we`ve issued advisories for
areas above 2kft.

The weather will settle down briefly for Monday afternoon into
Tuesday before becoming active again. BTL

LONG TERM...Wednesday through Saturday...
Models continue to show good agreement with active pattern over
the Pacific Northwest. An upper trough and low pressure system is
forecast to shift down the Pacific Northwest coast Wednesday and
Wednesday night. With the low moving into the Pacific Northwest,
there is the potential for a strong cold front to move inland and
impact the area late Wednesday into Wednesday night. This front
shifts east of the area on Thursday with an upper trough moving
over the area and bringing a showery pattern. Then a cold and
showery pattern continues Friday into Saturday.

On Wednesday, there is general agreement that a relatively strong
and moist frontal system will approach the Pacific Northwest coast
then move inland Wednesday night into Thursday morning. Models
indicate that this front may bring abundant moisture into the area
late Wednesday through early Thursday. Areas of moderate to heavy
rain and snow are possible, especially from the Cascades west. Snow
levels are expected around 3000 to 4000 feet with this system. So,
impacts from accumulating snow are possible on higher mountain
passes and as well as for the Mount Shasta area. East of the
Cascades, expect light to moderate precipitation, mostly falling as
snow. Also models indicate that a strong mid level 700 mb jet of
around 45 to 55 kt may move into the area ahead of the front. This
jet aloft combined with an increasing surface pressure gradient will
bring the potential for areas of strong gusty to develop, mainly in
the Shasta Valley and from the Cascades east.  However, due to
variability in the details on the strength and track of this low
pressure system, there is lower confidence on the exact details and
impacts with this storm. Will need to monitor this system
with future model runs.

Thursday into Friday, an upper trough is forecast to gradually move
inland over the region and bring widespread showers. Additional
disturbances moving around the trough and inland may bring enhanced
showers during this period. Snow levels will lower late Thursday
into Friday and are expected to lower from 2500 to 3500 feet down to
1000 to 2000 feet on Friday.

Friday night into Saturday, the trough will gradually shift eastward
and a shortwave ridge will build into the area resulting in
decreasing showers. There is significant variability on the  timing
of this ridge building into the area and decreasing shower activity.
May continue to see showers across the area on Saturday with
diminishing activity Saturday evening/night. However, given the
upper ridge is weak and models show another low pressure system
developing off the coast, expect only a brief break in the active
pattern next weekend. -CC

AVIATION...25/00Z TAF CYCLE...Scattered showers with areas of MVFR
cigs and gusty winds will linger into early this evening  then
expect showers to diminish with conditions becoming mainly VFR
across inland areas. This evening and tonight, another front will
approach the area then shift inland on Sunday. As precipitation
spreads into the coast late this evening and tonight, expect MVFR
cigs/vis to develop along coastal areas between 06-09z. Rain and
snow will spread inland with MVFR cigs/vis becoming widespread from
the coast into the Umpqua Valley around 10-13z. Elsewhere, west of
the Southern Oregon Cascades expect a mix of VFR with areas of MVFR
cigs to develop Sunday morning. Widespread mountain obscurations are
expected to develop late tonight into Sunday. East of the Cascades
and in Northern California expect mainly VFR conditions with local
MVFR cigs/vis developing on Sunday as precipitation spreads inland.
-CC

MARINE...Updated 226 PM PST Saturday 24 Feb 2018...West winds will
diminish tonight. At the same time, a west swell will build today,
pushing combined seas to small craft advisory levels. A stronger
front is expected to bring both stronger west winds and larger seas
to the area on Sunday. While winds diminish Monday, another even
larger swell will keep very steep seas going through Monday. Seas
remain steep Monday afternoon, but lengthening periods will allow
overall sea conditions to improve. Models are now indicating seas
building to 27 feet early Thursday, but this is more than ten feet
higher than indicated yesterday, and have gone midway between
yesterday`s and todays guidance for this event. -Sven

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Winter Weather Advisory from 4 PM Sunday to 4 AM PST Monday for
     ORZ029>031.
     Winter Storm Warning from 4 PM Sunday to 4 AM PST Monday for
     ORZ030.
     Winter Weather Advisory from 4 PM Sunday to 4 AM PST Monday
     above 2000 feet in the for ORZ023-024-026.
     Winter Storm Warning from 4 AM Sunday to 4 AM PST Monday above
     2500 feet in the for ORZ025.
     Winter Storm Warning from 4 PM Sunday to 4 AM PST Monday for
     ORZ028.
     Winter Weather Advisory from 4 AM Sunday to 4 AM PST Monday
     below 2500 feet in the for ORZ025.
     Winter Storm Warning from 10 AM Sunday to 4 AM PST Monday for
     ORZ027.
     Winter Storm Warning from 4 PM Sunday to 4 AM PST Monday for
     ORZ029.

CA...Winter Weather Advisory from 4 PM Sunday to 4 AM PST Monday for
     CAZ081>085.
     Winter Storm Warning from 4 PM Sunday to 4 AM PST Monday above
     2000 feet in the for CAZ080.

Pacific Coastal Waters...Small Craft Advisory until 4 AM PST Tuesday for
     PZZ350-356-370-376.
     Hazardous Seas Warning until 10 PM PST this evening for
     PZZ350-356-370-376.

$$



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