Area Forecast Discussion
Issued by NWS Medford, OR

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FXUS66 KMFR 180546

Area Forecast Discussion
National Weather Service Medford OR
1045 PM PDT Fri Aug 17 2018

Updated AVIATION Section

.DISCUSSION...18/00Z NAM in.

Mostly clear skies prevail across the Medford CWA this evening,
except for areas of smoke emanating from numerous wildfires. Low
level easterly flow is now developing.

A long wave ridge over the west coast will weaken a bit and drift
slowly east through Saturday night. A weak trough will dig into
the Pacific Northwest Sunday as it moves out. While this trough is
weak, it will develop a closed low that will remain over the
Pacific Northwest through at least Monday.

Inland highs through Monday will be 5-10 degrees above normal
over the Umpqua Basin and the east side. The remainder of the west
side will see highs 10-15 degrees above normal. The highs will be
several degrees lower in areas covered by thick smoke. The hottest
day will be Saturday with a slight cooling trend into Monday with
the trough digging overhead. It will remain quite dry through
Sunday afternoon, and the digging trough may bring breezy
conditions to the ridges and east side.

There is also the possibility of thunderstorms, which depends on
just where the upper trough sets up. That, in turn, will determine
where monsoonal moisture will get advected. There is a lot of
uncertainty in that aspect of the forecast at this time, but the
models hint at convective feedback beginning as early as Sunday
evening. The current forecast has a slight chance of thunderstorms
over the northern Cascades and points east beginning Sunday
evening and continuing through Sunday night, then returning Monday
afternoon and evening. Any changes to that forecast will be minor.

Extended discussion from the Friday afternoon AFD...Tuesday 21
August through Friday 24 August 2018...The low should exit to
the northeast around midweek.

A weak cold front will follow shortly behind the departing
trough, arriving Thursday or Friday of next week. given the
difficulty the models are having with the early week low, the
timing and strength of this front could easily change, so we will
keep an eye on it and make adjustments to the forecast as
necessary. -BPN


.AVIATION...For the 18/06Z TAFs...For inland areas, especially
across northern California and in the immediate vicinity of any
wildfires, expect visibility restrictions to continue overnight
through Saturday evening due to smoke. Those areas will have MVFR
with occasional IFR. Meanwhile, improvement to VFR has occurred at
Medford and Klamath Falls this evening due to winds scouring out the
low-level smoke. Conditions will be mostly VFR away from the fires,
but MVFR should return to those terminals overnight or Saturday
morning as smoke layers settle back into the valleys/basins.
Conditions should return to VFR Saturday evening as WNW winds once
again mix out surface smoke.

Near the coast, LIFR clouds and fog should develop overnight mainly
north of Cape Blanco , albeit later than originally expected. Clouds
won`t reach very far inland Saturday morning so Roseburg should be
clear. Clouds along the north coast Saturday morning should erode
back offshore Saturday afternoon, but probably return Saturday
evening. -Spilde


.MARINE...Updated 800 PM PDT Friday 17 August 2018...A thermal
trough is causing gusty north winds and steep to very
steep hazardous seas, highest south of Cape Blanco this evening.
Through Saturday night, gales and very steep hazardous seas are
expected from Gold Beach southward, mainly beyond 3 nm from shore,
with small craft advisory level conditions expected elsewhere. Gusty
north winds and a mix of steep to very steep seas may linger over
the waters into Sunday then conditions will gradually improve Sunday
night into Monday. -Spilde


.FIRE WEATHER...Updated 130 PM Friday August 17th...The main
concern for the remainder of today and Saturday will be winds and
relative humidities. Expect single digit RHs over the Modoc and
low teens in the Fremont Winema NF, to mid teens in the Klamath NF
this afternoon. Although periods of breezy winds are expected in
this afternoon for Modoc County and Klamath and Lake Counties,
winds are not expected to meet RFW criteria, but area fires will
still have the ability to spread quickly.

A thermal trough will persist over the coast, bringing moderate
offshore winds and moderate to poor RH recoveries tonight,
especially over the mountains of western Siskiyou County in FWZ 280
and the Coastal Range in FWZ 619. While the RHs will be low enough
for poor recoveries, winds will not be strong enough to reach Red
Flag criteria. Isolated areas of Red Flag conditions are possible,
but the area is not large enough to warrant a warning and will
continue to headline.

Saturday will be pretty similar to today, with temperatures running
a few degrees warmer as an upper level ridge moves overhead. Expect
another round low humidities across the east side, with values
generally in the lower teens but winds will trend weaker than what`s
expected this afternoon. The thermal trough will still be in place
Saturday night, but won`t be as strong and recoveries will be
slightly improved.

The thermal trough will move inland Sunday as another upper trough
approaches the area. Breezy conditions are expected Sunday afternoon
and evening ahead of the trough. This trough will cut off on Monday
and linger over the area into at least Tuesday. There are a
considerable amount of differences between the models on the
position of the low, both between the different models and between
the individual model runs. This leads to low confidence in any
detail on where thunderstorms are expected because any change in the
position will result in changes of the location of thunderstorm
chances. For now, it looks like the best potential will be Monday
and/or Tuesday afternoon for the northern portions of the area,
possibly as far south as the Umpqua Divide. Again confidence is low
on this and model instability parameters aren`t all that impressive
at the moment. Stay tuned as the forecast becomes more refined this
weekend. /BR-y




Pacific Coastal Waters...Small Craft Advisory until 5 AM PDT Monday
        for PZZ356-376.
     Gale Warning until 11 PM PDT Saturday for PZZ356-376.
     Small Craft Advisory until 5 AM PDT Sunday for PZZ350-370.


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