Area Forecast Discussion
Issued by NWS Medford, OR

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000
FXUS66 KMFR 210533
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
933 PM PST Sat Jan 20 2018

.DISCUSSION...The 00z models (NAM/GFS) have arrived and most of
the going forecast remains on par with the guidance. Winds will
ramp up at the coast overnight into Sunday morning ahead of the
front and with 65-75kt 850mb winds, this should be enough to
cause high wind gusts (>58 mph) along the coast from about Cape
Arago southward. As the front heads inland Sunday, expect strong,
gusty winds at the upper slopes/ridges. In the Shasta and Rogue
Valleys, the forecasted MFR-RDD pressure gradient is significant
(-8 to -9 mb). However, since the associated upper trough is not
negatively tilted, the mid-level flow is more westerly. This is
important, because this will likely prevent an all-out wind storm
here. Even so, it will be breezy to windy with peak gusts to 55-60
mph in the Shasta Valley (esp. south end near Weed) and 35-40 mph
in the southern part of the Rogue Valley near Ashland. East side
areas will also have gusty winds tomorrow, strongest near Summer
Lake and over the higher terrain. After all that, we have left the
current headlines intact and they can be viewed at NPWMFR.

With respect to precipitation, models continue to show
precipitation overspreading the area Sunday with the heaviest
expected along the coast south of Cape Blanco and into portions of
southern and western Siskiyou County. This isn`t a classic upslope
snowstorm for Mount Shasta City, primarily due to the mid-level
flow being more westerly, but with snow levels in the 2500-3500
foot range with heavier precipitation, there should still be some
good snow there and especially just above the city (Snowman
/Black Butte Summits). The GFS has shifted its track slightly
farther south, but the NAM/ECMWF continue to produce several
inches of snow there. Even though snow is expected to pick up
Sunday afternoon, the greatest impacts on I-5 are likely to be
during the evening around sunset. There will also be some snow
over the passes at Siskiyou Summit, Lake of the Woods and Crater
Lake. All the warnings/advisories can be seen at WSWMFR.

This system will exit the area to the south and east Sunday night
and it will dry out on Monday. -Spilde

&&

.AVIATION...22/00Z TAF CYCLE... VFR/MVFR conditions are occurring
everywhere with partial terrain obscuration in any MVFR
areas...scattered showers will be possible along the coast. Winds
along the coast and west of the Cascades increase ahead of a front.
We have added low level wind shear for OTH RBG and MFR as surface
winds are forecast to remain below 10 knots. MVFR conditions an
tonight with rain causing terrain obscurations from west to east
Sunday morning. -FB

&&

.MARINE...Updated 800 PM PST Saturday 20 Jan 2018...
A strong cold front will move through the waters tomorrow morning
bringing steep seas to the region.  Storm force winds are expected
ahead of the front Sunday morning north of Gold Beach and up to 30
nm away from shore.  Gale force winds can expected elsewhere late
Saturday night into late Sunday morning.  Steep seas will move in
behind the front bringing hazardous seas conditions to the majority
of the outer waters.  Eventually, conditions will improve Sunday
night into Monday morning as the storm system moves eastward. The
weather will deteriorate again when the next front approaches
Wednesday evening. This storm system is not expected to be as strong
as the one this Sunday, but should still produce some hazardous
weather in the waters.

-Smith

&&

.PREV DISCUSSION... /Issued 345 PM PST Sat Jan 20 2018/

DISCUSSION...Models continue on track with the upcoming strong
winter storm moving into the region tonight. Winds will pick up on
the coast by sunrise Sunday, and even though there is a slightly
offshore gradient, the 850 mb winds are 65-75 knots from the
southwest, which will tuck the winds into the coast, and the high
winds warning at the coast still looks on track. Inland the
gradient up the Shasta valley is still strong Sunday, but the 700
mb winds are more westerly that would normally produce warning
strength winds, and an advisory is still in order. East of the
Cascades 700 mb winds 55 to 65 knots still indicate a wind
advisory. With the more westerly 700 mb winds snow levels look to
remain lower than if there was strong more southerly winds like
the usual strong wind events, and snow levels should be in the
3500-4000 foot range in Oregon and 2500 ft in the Mt. Shasta area
to 3000 feet in Western Siskiyou county with the heavier
precipitation rates. with temperatures hovering around freezing,
snow may be wet and heavy at times, and with the gusty winds may
collect on trees with some branches breaking and even possibly
some trees toppling in the Mt. Shasta area and the Cascades,
possibly resulting in localized power outages. Models are
indicating greater amounts east of the Cascades than previously,
and have issued an advisory in that area as well.

A weak upper level ridge builds in behind this system Monday,
bringing a break in the action Monday. The upper ridge then shifts
east ahead of another strong, but not quite as strong as Sunday`s
system into the region Monday night. However, this system will see
higher snow levels and passes in the Cascades will be primarily
affected. Sven

Long Term...Wednesday, Jan 24th through Saturday night, Jan
27th...Active weather will continue into next weekend as a series of
fronts move trough settles over the Pacific NW. A cold front will
move through our forecast area Wednesday. Snow levels will start out
in the 4500-5500 foot range during the afternoon then lower to
around 2000-2500 feet by Thursday. On Friday both the GFS and ECMWF
moves a weak ridge over the area with a surface warm front arriving
Friday evening. Confidence is high on precipitation at the coast and
into Josephine/Douglas Counties but not as high for Jackson County
eastward.

Snow levels will be rather tricky on Saturday in that trapped cold
air east of the Cascades could keep snow levels lower than what the
models forecast, whereas west of the Cascades could see snow levels
rising to around 5000 feet. The next cold front deepens offshore and
this could delay the start of next round of precipitation with
Sunday being dry. The GFS is quicker and bring precipitation back to
the area Sunday and Monday. /FB

AVIATION...21/18Z TAF CYCLE... VFR/MVFR conditions are occurring
everywhere with partial terrain obscuration in any MVFR
areas...scattered showers will be possible along the coast. Winds
will increase along the coast and east of the Cascades ahead of a
front that will bring MVFR conditions an tonight with rain causing
terrain obscurations from west to east tomorrow morning. -Schaaf

MARINE...Updated 200 PM PST Saturday 20 Jan 2018...Westerly swell
will gradually diminish this morning, decreasing briefly to 11 to 14
feet with small craft conditions expected through this evening.
Another strong front will move through the waters late Saturday
night into Sunday. Storm force winds are expected north of Gold
Beach early Sunday morning and gales are expected elsewhere late
Saturday night into Sunday afternoon. Wind driven seas will build
again Saturday night, peaking at 20 to 23 feet Sunday morning as a
fresh westerly swell moves in after the front. Conditions will
improve somewhat late Sunday night into Monday, but seas will remain
elevated and steep through the week. Another strong frontal system
is expected to come in late next week which could bring another
round of strong winds and steep seas. -Schaaf/Br-y

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...High Wind Warning from 7 AM to 7 PM PST Sunday for ORZ030-031.
     Winter Weather Advisory from 4 AM Sunday to 1 AM PST Monday for
     ORZ027>031.
     Wind Advisory from 7 AM to 7 PM PST Sunday for ORZ030-031.
     High Surf Advisory until 10 PM PST Sunday for ORZ021-022.
     High Wind Warning from 4 AM to 1 PM PST Sunday for ORZ021-022.

CA...Winter Weather Advisory from 4 AM Sunday to 1 AM PST Monday for
     CAZ084-085.
     Winter Storm Warning from 4 AM Sunday to 1 AM PST Monday for
     CAZ080>083.
     High Wind Warning from 7 AM to 7 PM PST Sunday for CAZ081.

Pacific Coastal Waters...Small Craft Advisory until 10 PM PST this evening for
     PZZ350-356-370-376.
     Storm Warning until 1 PM PST Sunday for PZZ350-356-370-376.
     High Surf Advisory until 10 PM PST Sunday for PZZ350-356.
     Gale Warning until 1 PM PST Sunday for PZZ350-356-376.
     Hazardous Seas Warning from 1 PM to 10 PM PST Sunday for
     PZZ350-356-370-376.

$$

MAS/SBN/FJB


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