Area Forecast Discussion
Issued by NWS Medford, OR

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FXUS66 KMFR 232257
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
257 PM PST Thu Nov 23 2017

.SHORT TERM...Tonight (11/23) through Sunday night (11/26)...
The main change to the previous forecast was to issue a high wind
watch for Saturday afternoon through Sunday afternoon for the
southern Shasta Valley, and much of the east side that is east of
highway 97. There is still significant model uncertainty regarding
the details of both features that may produce strong, gusty south
winds. Lowering snow levels at the tail end of the second event,
late Sunday night into Monday morning, continues to appear to be
the only potential for impactful snowfall for any portion of our
area during the next week.

First, a wind advisory remains in effect through this evening for
the Warner Mountains and favored areas in Lake County...especially
Highway 31 between Paisley and Summer Lake. Summer Lake recorded
southwest winds at 25 mph with gusts to 44 mph around noon and
winds will remain nearly as strong for the duration of the event.
These winds are associated with a cold frontal passage. It will
continue to produce rain over much of the area through tonight
while snow levels fall from around 8000 feet now to 5000 to 6000
feet by the time precipitation tapers off late tonight.
Precipitation will remain as rain on the mountain passes with the
bulk of snow falling at elevations above 6000 feet.

A stabilizing air mass and lingering low level moisture will
result in areas of low clouds and patchy fog in west side valleys
on Friday morning. Cloud cover will diminish in late morning with
mainly partly cloudy skies into the afternoon and a brief period
of sunny skies possible for much of the area. High clouds will
gradually increase Friday night.

The main impacts in the short term will occur during the weekend.
Model agreement is very good into Saturday then quickly diverges
on the strength, track, and timing of a strong cold front and
another front following closely behind. The 12Z models exhibit a
general trend toward a slower progression and arrival time of the
initial front by about 6 to 12 hours. As a result, the probability
of precipitation and more notably the quantity of precipitation
forecast have been adjusted lower for Saturday through Saturday
evening. That being said, light rain is still expected at the
coast on Saturday with at least a chance from western Siskiyou
County across all of southwest Oregon into northern Klamath
County.

Models still suggest significant potential for a high wind event
over portions of the forecast area this weekend...Saturday
afternoon through Sunday afternoon. The GFS operational model
features the deepest low pressure center compared to other models,
but it is not very different than the 12Z ECWMF solution of
yesterday. Therefore, reading between the lines, models suggest
the atmosphere could potentially generate a rapidly deepening low
off the coast, and send high winds into the Shasta Valley and East
Side (the favored locations for high winds in a strong southerly
wind pattern). The High Wind Watch is in effect for Saturday
afternoon through Sunday afternoon for this potential. We`ll
likely have to adjust timing and maybe locations as the model
spread shrinks over the next 24 hours. Another low center will
drive into the coast Sunday. This low helps to maintain the strong
winds at least over the East Side, and it may bring enhanced
winds to the coastline. Details on this low are even more
uncertain per model differences. Travelers east of the Cascades
and those planning to drive over Interstate 5 this weekend should
stay tuned to the potential for high winds in the area.

The timing, strength, and track of the parent upper level low will
affect the snowfall forecast for Sunday night into Monday morning.
For now, a model blend suggests snow levels remaining above the
passes at least into and potentially through Sunday evening...then
falling to its lowest point at around 3500 feet by the time the
last of the snow showers end.


.LONG TERM...Mon (11/27) through Thu (11/30)...The
21/12Z EC and GFS start off the extended in good agreement.  They
both show a long wave trough just moving onshore over the Pacific
Northwest. The associated cold front will not be as wet and windy as
previous fronts, but it will be significant as snow levels will
likely be below most of the passes by this time, and that will
affect Thanksgiving related travel.

A long wave ridge will move onshore in the wake of the trough, and
the ridge axis will break to the east of the area sometime Tuesday.
Post-frontal showers will diminish Monday into Monday night, and
Tuesday and Wednesday look to be dry, or at least mostly dry.

The medium range model solutions were out-of-phase with the
Wednesday forecast, but the 23/12Z runs have come into agreement.
Both models show a long wave trough shearing as it approaches the
coast, leaving a cutoff low off the northern California coast
Wednesday. Overall this is a light wind scenario, and the cutoff
forms well southwest of the Medford CWA, so will keep a mostly dry
forecast over the area into Thursday.

&&

.AVIATION...23/18Z TAF CYCLE...Conditions will generally worsen from
west to east this evening into the overnight hours as a front brings
lower ceilings and mountain obscuration to most of the forecast
area. A mix of MVFR and IFR conditions are expected in the evening,
lowering to IFR overnight. Tomorrow, conditions should improve to
VFR late morning or early afternoon as a drier air mass moves in.
It`s likely that the coast conditions will improve first and valleys
will improve last. Confidence in the forecast is lowest at the coast
where a clearing line offshore will reach the coast in the evening,
but our current thinking is that low clouds at KOTH will be stubborn
and remain through early morning before clearing. SK

&&

.MARINE...Updated 200 PM PST Thursday 23 Nov 2017...Steep seas
will lower below advisory level conditions late this afternoon.
South to southwest winds will also ease during this time frame.
Relatively calm conditions are expected through Friday night. Then,
another frontal system will bring strengthening south winds
Saturday, with gales possible and very steep seas late Saturday into
Sunday. Timing is still quite uncertain for the gales, but there is
moderate to high confidence that a round of gales will occur this
weekend as multiple strong low pressure systems pass nearby the
coastal waters. Seas will remain high and steep into early next
week. SK

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Wind Advisory until 10 PM PST this evening for ORZ030-031.
     High Wind Watch from Saturday afternoon through Sunday afternoon
     for ORZ030-031.

CA...Wind Advisory until 10 PM PST this evening for CAZ085.
     High Wind Watch from Saturday afternoon through Sunday afternoon
     for CAZ085.
     High Wind Watch from Saturday afternoon through Sunday afternoon
     for CAZ081.

Pacific Coastal Waters...Gale Watch from Saturday afternoon through Sunday
     morning for PZZ350-356-370-376.
     Small Craft Advisory for hazardous seas until 4 PM PST this
     afternoon for PZZ350-356-370-376.

$$

DW/JRS/SK


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