Area Forecast Discussion
Issued by NWS Medford, OR

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FXUS66 KMFR 190419
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
819 PM PST Sun Nov 18 2018

.DISCUSSION...High pressure is in control of the forecast area this
evening. An upper ridge axis is sitting overhead, and skies are
clear over most of the forecast area. The exception is the coast
where marine layer low clouds and fog are impacting that area. The
marine layer has developed over the past 24 hours per the North
Bend profiler observation and should continue to affect the coast
and coastal waters through Tuesday. As the marine layer lifts
some though, the fog should lift to low clouds over the next day
or so.

Tonight, low clouds and patchy fog should form once again in the
coastal valleys, Umpqua Basin, Illinois Valleys and in the Grants
Pass area. These clouds will clear late Monday morning or early
Monday afternoon. Elsewhere, away from the coast and aforementioned
valleys, expect clear skies on Sunday.

An Air Stagnation Advisory continues through Tuesday for most
valleys, and air quality observations confirm that mixing has
recently become poor again, with Klamath Falls reporting moderate
air quality. We expect mixing to be poor on Monday as the upper
ridge axis remains in place and winds will be generally light for
the forecast area.

Please see the previous forecast discussion below for details on the
big weather pattern change expected midweek.

&&

.AVIATION...For the 19/00Z TAFs...A similar pattern to what`s
occurred over the past few days is on tap. Along the coast and into
the Coquille Valley IFR to LIFR will persist overnight then improve
late Monday morning. In the Umpqua Basin, expect IFR cigs to develop
later this evening, then transitioning to LIFR with fog. Cigs will
scatter out Monday afternoon. Elsewhere, expect widespread VFR,
though some localized fog is possible in the Grants Pass area and
Illinois Valley Monday morning. -MM

&&

.MARINE...Updated 200 PM PST Sunday 18 November 2018...Light
winds and seas along with low level stratus clouds are continuing
across the coastal waters this afternoon. Light southerly winds and
west seas are expected tonight through Tuesday morning. A weak front
will move across the waters late Tuesday into Wednesday, bringing
gusty south winds, rain, and steep seas. South winds are expected to
rise to small craft advisory levels Tuesday evening through
Wednesday morning. Steep seas are expected to develop during this
period and may linger across the waters through Wednesday afternoon.
A stronger front will move onshore Thursday, and this may bring gale
force south winds and very steep seas to the waters. Heavy west
swell may develop Thursday evening and night and continue into
Friday morning, before gradually lowering. -CC

&&

.PREV DISCUSSION... /Issued 235 PM PST Sun Nov 18 2018/

DISCUSSION...Not much change in the weather is expected through
Tuesday with continued dry weather with cold mornings east of the
Cascades, Rogue, Illinois and Applegate valley west of the Cascades.
The latest visible image shows marine stratus along the coast and
portions of the Umpqua Basin. The stratus in the Umpqua could burn
off in the next few hours, but we`re expecting stratus to
redevelop once again this evening and tonight. Also stratus could
develop in the Illinois Valley late tonight.

Our next focus of attention is towards a pattern change starting
Tuesday night. The first thing we`ll notice with this change will be
increasing winds in the Shasta Valley later Tuesday afternoon and
night as a frontal system slowly moves towards the forecast area.
Winds in the Shasta Valley are expected to increase further Tuesday
night into Wednesday and it could approach advisory criteria near
Weed during this time. Meanwhile the models are showing favorable
wind direction aloft (southeast) for stronger winds to surface in
portions of the Rogue Valley, including Ashland and Medford.

There is pretty good agreement among the models with onset of
precipitation. One thing of note is the models have been trending
slower meaning precipitation will be delayed. It`s expected to reach
the coast early Wednesday morning, then move onshore during the day.
The heaviest precipitation amounts will be in the usual locations,
the coast, coastal mountains, Mount Shasta region, including Mount
Shasta City and Cascades. Because of the downsloping winds in the
Rogue Valley, it`s possible it could remain mostly dry into
Wednesday morning with not much more than intermittent light
rain. Once we lose the downsloping effects (late Wednesday
afternoon) then rain will increase in coverage and intensity.

Snow levels Wednesday will be above the major passes, so were not
expecting any concerns. The exception will be up near Crater Lake
and possibly Diamond Lake area where up to an inch is possible by
sunset Wednesday evening.

Showers will continue into Wednesday evening as the upper trough
axis moves overhead, then decrease in coverage overnight Wednesday.
Snow levels will come down to around 5500 feet which is still above
the major passes and only minor accumulations are expected near
Crater Lake. -Petrucelli

Long Term...Thanksgiving through Sunday...The wet pattern will
continue as the second, and strongest of the series of fronts will
pass through the Pacific Northwest and northern California. Analysis
of the newest data still largely supports maintaining the general
consensus of sloppy holiday travel. The models have generally slowed
the front down, which allows for the strong winds ahead of the front
to build in on Thanksgiving. The winds will be strongest at the
Coast and along Highway 101 as well as for Interstate 5 in the
Shasta Valley during the late morning and early afternoon. Then, the
strong winds will spread to the east side, impacting most of the
roadways on Thursday evening and night.

The moderate to briefly heavy rain is expected to arrive Thursday
morning and linger for a soggy Thanksgiving holiday. Although the
precipitation amounts have come down somewhat, we are still
expecting the first big rainfall event this fall for southern Oregon
and northern California. The heaviest rainfall areas will be near
Brookings and Red Mound at the coast, and across the Mt. Shasta area
due to winds blowing up Mt. Shasta allowing for more rain to fall in
the area. Although it is a low chance, we are still concerned about
isolated debris flows in a couple of the recent burn scars (mainly
the Natchez Fire and the Chetco Bar Burn Scar from last year). More
information will come out on the exact nature of these impacts in
the coming days. Meanwhile, rainfall amounts in the Rogue Valley
will be lower due to winds blowing down the Siskiyous. The
precipitation will linger into Friday as well. Although the
intensity will be less, the precipitation will continue to be
widespread. Still, the overall rain amounts are expected to be quite
generous for the entirety of our forecast area.

Snow levels are unlikely to get lower than 4000 feet during the next
week with a snow level forecast to briefly fall to 4000 to 4500 feet
late Friday night into Saturday morning. As a result, a Snow levels
are expected to remain above the major passes. Confidence in snow
levels, however, remains relatively low since we are still so far
out.  The ECMWF is stronger and positioned farther north than the
GFS with the next front on Friday. These model differences are
critical but it is safe to say that snow levels will be lowering
late in the day on Friday into early Saturday, and will likely reach
their lowest point early Saturday morning. Snowfall amounts will be
the highest in the Cascades and at the top of Mt. Shasta, and could
put us back on track for a reasonable November snow pack.

For next weekend, models disagree on the extent to which a ridge
offshore from central and southern California will have an impact on
our weather. The ECMWF shows stronger ridging while the GFS and a
sizable minority of GFS ensemble members continue a series of
disturbances riding over the top of the ridge into our area. At
the least, compared to yesterday`s solutions, there is a decreased
probability of precipitation. A drier solution would be
accompanied by an increased probability of travel difficulties due
to persistent valley fog. Have continued this trend into Sunday
as well. -Schaaf

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Air Stagnation Advisory until 4 AM PST Tuesday for ORZ021.
     Air Stagnation Advisory until 4 AM PST Wednesday for ORZ023.
     Air Stagnation Advisory until 4 AM PST Tuesday for ORZ024>031.

CA...None.

Pacific Coastal Waters...None.

$$


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