Area Forecast Discussion
Issued by NWS Medford, OR

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000
FXUS66 KMFR 181205
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
405 AM PST Mon Dec 18 2017

.DISCUSSION...

Fog and freezing fog has developed over the usual places this
morning in the Umpqua valley and near the Illinois valley in
Grants Pass. Clouds are making it really difficult for
visibilities to come crashing down in Medford, but we`re currently
looking at 6SM here at the airport. Thinking that will continue
through the morning as the the clouds passing through should
mess with the inversion meeting complete saturation.

The other topic for this morning is the winter storm system for
Tuesday evening into Wednesday. The biggest concerns are...

* Low level snow with snow levels roughly around 2000 feet ahead
  and behind of frontal passage.

* Travel issues over the higher passes, specifically the Cascade
  passes.

* Possibly some minor travel issues over I-5. With snow levels
  around 2000 feet, some snow should fall over the lower passes.

The bulk of the snow fall should occur along the front around 3z
Wednesday to about 12 Z Wednesday. The sustained west flow up onto
the Cascades later in the morning will keep the snow falling in
moderate amounts along the Cascades post frontal passage. Storm
total snow accumulations will be approaching up to 16 inches near
Crater Lake over a 24 hour time period ending at 0Z Thursday.
With a negatively tilted trough, this system will remain very
efficient at pulling in moisture ahead of the front. This will
result in higher than normal snow fall east of the Cascades. Snow
totals east of the Cascades will be roughly 4 to 8 inches with
localized areas reaching 10 inches near Chemult and Crescent along
Highway 97.

After that front passes, snow levels fall down close to the valley
floor in the Rogue Valley. With temperatures forecasted near
freezing in Medford, it seems reasonable that we`ll see some post
frontal snow fall during the morning hours after 12z. With ground
temperatures above freezing, it`s hard to see anything really
sticking to the roadways. However, accumulating snow on grassy
surfaces seems probable.

Wind is the other issue with this system moving through. 700 mb
wind flow increases ahead of the system around 00z Wednesday with
the models showing 65 to 80 knot winds on the east side. The
pressure gradient ahead of the front is suggesting a 9mb pressure
gradient between Redding and Medford. Therefore, we`ll probably
see advisory level winds in the Shasta valley with gusts up to 40
mph there. East of the Cascades near Summer Lake, we`re expecting
gusts up to 50 mph.  -Smith

&&

.AVIATION...18/12Z TAF CYCLE...Along the coast and over the coastal
waters...Areas of MVFR cigs will clear to VFR by this afternoon.
Areas of MVFR will return and rain will develop late this evening as
a strong cold front approaches the coast. West of the
Cascades...areas of MVFR/IFR cigs/vsbys in low clouds and fog in
valleys will clear to VFR by late morning. Low clouds and fog are
likely to form again during the evening and continue into early
Tuesday morning. The remainder of the area will remain VFR through
tonight. From the Cascades east...VFR conditions will prevail
through tonight. -DW

&&

.MARINE...Updated 300 AM PST Monday 18 Dec 2017...Weak high
pressure today will be followed by increasing southerly winds
tonight ahead of a strong cold front. The front will move through
the waters Tuesday with gales and high and very steep seas. High and
steep swell with gusty westerly winds will follow Tuesday night into
Wednesday night. Winds will turn north Wednesday night and persist
into the weekend with gradually diminishing seas. -DW

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Air Stagnation Advisory until 8 AM PST Tuesday for ORZ029>031.
     High Wind Watch from Tuesday morning through late Tuesday night
     for ORZ030-031.
     Winter Storm Watch from Tuesday afternoon through Wednesday
     morning for ORZ029-030.
     Air Stagnation Advisory until 8 AM PST Tuesday for
     ORZ023-024-026.
     Winter Storm Watch from Tuesday afternoon through Wednesday
     morning for ORZ023>026.
     Winter Storm Watch from Tuesday afternoon through Wednesday
     morning for ORZ027-028.

CA...High Wind Watch from Tuesday morning through late Tuesday night
     for CAZ085.
     Winter Storm Watch from Tuesday afternoon through Wednesday
     morning for CAZ080>083.

Pacific Coastal Waters...Small Craft Advisory from 4 AM to 4 PM PST Tuesday for
     PZZ350-356-376.
     Gale Warning from 7 AM to 4 PM PST Tuesday for
     PZZ350-356-370-376.
     Small Craft Advisory from 1 AM to 7 AM PST Tuesday for PZZ370.

$$

CZS/NSK/DW



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