Area Forecast Discussion
Issued by NWS Medford, OR

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FXUS66 KMFR 180625
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
1025 PM PST Fri Nov 17 2017

Updated AVIATION Section

.DISCUSSION...No updates are planned this evening. High pressure
building in over the area has ended any threat for showers, and
now we turn our attention to the cold temperatures and fog
tonight. Frost advisories and freeze warnings remain in effect for
much of the Umpqua Basin and the coastal zones inland from the
coast for late tonight and tomorrow morning, and this still looks
on track. It will be even colder elsewhere tonight, but we are
beyond the growing season in other locations. Fog is already
starting to develop over some of the valleys and along the coast,
and this is only going to become thicker and more widespread as
the night goes on. Pretty much all our valley locations are
expected to have fog by tomorrow morning, including the east side
valleys. With warm air building in aloft, there is also some
question as to how soon (or if) fog clears out tomorrow. Guidance
still shows fog decreasing by late morning, and we`ll stick with
that for now. However, there is a chance that some valleys, the
Umpqua, for example, clear very late or not at all. -Wright

&&

.AVIATION...18/06Z TAF CYCLE...Expect MVFR/IFR ceilings/visibility
in stratus and fog in the valleys along the coast and north of the
Umpqua Divide through Saturday morning. These will expand over the
west side valleys and also in the valleys of western Siskiyou County
overnight into Saturday morning. VFR will prevail east of the
Cascades, but patchy fog may develop in some of the deeper valleys,
like the Klamath Basin, toward morning. VFR will return to most
areas Saturday afternoon, but low ceilings may last through mid-
afternoon in some of the deeper west side valleys. -Spilde

&&

.MARINE...Updated 845 PM PST Friday 17 November 2017...High pressure
will remain over the coastal waters with mostly light winds and low
seas through Saturday. South winds will increase again ahead of the
next front Sunday with gales and very steep seas likely Sunday
afternoon through Sunday night. Confidence is high that seas will
build to around 15 feet Sunday night from Gold Beach north, with
seas possibly building to 17 feet north of Coos Bay. Unsettled
weather will then continue through much of next week. -Sven/Spilde

&&

.PREV DISCUSSION... /Issued 239 PM PST Fri Nov 17 2017/

SHORT TERM...Tonight through Monday...Current visible satellite
shows plenty of cumulus across the forecast area. Only a few very
brief, light returns on the radar and only light amounts being
reported in the area. Satellite shows these clouds thinning out
and showers should continue to diminish in coverage as the
afternoon progresses. There`ll be a break in the weather tonight,
Saturday and into Sunday morning as a long wave ridge moves
through the forecast area.

Tonight, as skies continue to clear and with a cold air mass in
place, expect temperatures to drop after sunset. Temperatures
east of the Cascades look to be in the mid to upper teens, with
mid to upper 20s expected in the valleys west of the Cascades. The
exception will be the Umpqua Basin where fog is expected to
develop early enough to keep temperatures relatively warmer. Even
still, it will be cold enough to warrant frost and freeze concerns
for these areas as they haven`t experienced enough of these cold
temperatures to end growing season. Have issued a frost advisory
for the Umpqua and Coquille Basins and a freeze warning for the
southern portion of Douglas county. See NPWMFR for more details on
this. Fog is expected elsewhere in most of the west side valleys
and with temperatures being as low as expected, freezing fog is
likely. If you still have sensitive outdoor vegetation, be sure to
take protective actions to prevent any potential damage.

On Saturday, the challenge will be how long morning fog lingers.
This will make for a challenging high temperature forecast. With
a ridge in place, it`s possible that fog lingers for much of the
day, if not all day with only a few hours of sun making it
through. If the fog never breaks, expect temperatures to be much
cooler than what is in the current forecast. If fog does break,
expect a quick return after sunset for most valleys. Aside from
fog and low clouds, conditions will remain dry through Saturday.
Temperatures will be relatively warmer Saturday night, but frost
conditions could be possible again for the same locations as
tonight. Another round of fog is expected for valleys Saturday
night.

Sunday will be another transition day back to a more active
pattern. The day will start off much like Saturday morning with
cool temperatures and valley fog. However, it`s less likely for
fog to linger through the day as another strong front approaches
the area Sunday. High level clouds will begin to stream in ahead
of this front and winds will also increase Sunday afternoon.
Strong, gusty winds are expected in the Shasta Valley, along
Coastal headlands and east of the Cascades late Sunday afternoon
into Monday morning. Moderate to heavy rain is expected to start
Sunday afternoon and continue into Monday morning. Snow levels
will rise to around 8,000 feet with the peak of the rainfall, so
we aren`t expecting any major winter impacts at the current time.
Snow levels will stay around 8,000 feet Monday as the front begins
to lift back north. /BR-y

LONG TERM...Tuesday through Friday...Models are showing the area
located between a trough offshore and an upper ridge inland to
east with a stream of southwest moisture directed at the coast next
week. A moist frontal system is forecast to move around the trough
and northward along the Oregon coast on Tuesday. This is expected to
bring a warm front into the area on Tuesday which will allow rain to
develop across the area. Snow levels are projected to be elevated,
rising to around 9000 ft elevation or higher on Tuesday. Confidence
is low to moderate on the exact rainfall totals due to the run to
run variability in the models. Current models have trended lower on
rainfall for Tuesday. Still expect a potential for moderate to
locally heavy rain along the coast and inland over the coastal
mountains and western Siskiyou county with mainly light to moderate
rain expected elsewhere.

A cold front is forecast to move into the coast and stall near the
coast on Wednesday and into Thursday.  This front will likely bring
rain to the coast and to areas just inland, such as the coastal
mountains, western Siskiyou county and portions of the Umpqua.
Elsewhere expect a chance to slight chance for rain, with rainfall
depending on the exact position of the front and any disturbances
moving over the area ahead of this front. In general expect higher
chances from the Cascades west with weaker chances east of the
Cascades. Snow levels are likely to remain elevated, above 8000 feet
elevation Wednesday through Thursday. Temperatures are expected to
remain generally mild and above normal Tuesday through Thursday with
this warmer air masses in place.

Models show increased variability in the forecast weather pattern
Thursday night into Friday, with the GFS indicating a potential for
remnants of the front to linger near the coast while weak ridging
builds into the area. The ECMWF shows a stronger ridge and a brief
period of dry weather across the area Thursday night into Friday
morning. By late Friday there is broad agreement that another system
will track up from the southwest and into the area bringing
additional wet weather. -CC

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Frost Advisory from 2 AM to 8 AM PST Saturday for ORZ021>023.
     Freeze Warning from 2 AM to 8 AM PST Saturday for ORZ023.

CA...None.

Pacific Coastal Waters...Gale Watch from Sunday morning through late
     Sunday night for PZZ350-356-370-376.

$$


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