Area Forecast Discussion
Issued by NWS Medford, OR

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FXUS66 KMFR 201525

Area Forecast Discussion
National Weather Service Medford OR
825 AM PDT Sun Aug 20 2017

.UPDATE...The overall forecast for today seems to be mostly on
track as current observations are matching the forecast. That
being said, extremely smoky skies were observed enroute to the
office today, and the ongoing wildfires will likely to continue to
produce more smoke which will be blown into the west side valleys
as forecast. This affected the official high temperature readings
yesterday, and will likely do the same again today. Thus have
lowered the temperature forecast for the Rogue Valley, Illinois
Valley, and Roseburg for today.

Otherwise, the main concerns for this forecast cycle are going to
be the wildfire conditions around the area--including thunderstorm
chances this week, and sky and smoke cover for tomorrow morning`s
eclipse. Again, since the forecast looks on track, I do not
anticipate needing to make extensive changes, and have left the
previous forecast below which details the forecast pattern nicely.


.MARINE...Updated 815 AM PDT Sunday 20 August 2017...High
pressure offshore and a thermal trough along the coast will
generate moderate to strong north winds and very steep seas
through this evening. Conditions will at least be hazardous to
small craft over all of the forecast area, and warning level
winds/seas will occur beyond 5 NM from shore from Gold Beach
south. Winds and seas will be highest this afternoon and evening,
easing during the overnight and early morning hours. The thermal
trough will weaken beginning Monday, and winds will diminish but
seas are expected to remain steep through the middle of the week
due to short period swells. Have extended the warnings and
advisories in the morning update to account for this.


.PREV DISCUSSION... /Issued 501 AM PDT Sun Aug 20 2017/

DISCUSSION...The main concern this morning is on-going strong,
gusty northeast winds and low humidity affecting the Chetco Bar
wildfire. A red flag warning /PDXRFWMFR/ remains in effect until
noon. The Quail Prairie observation site in the vicinity of the
fire has had gusts of 20 to 25 MPH overnight. Meanwhile, Red
Mound (a ridge top site nearer to Brookings) has had gusts of 30
to 40 MPH. The good news is that these winds are likely near a
peak, but northeast gusts will remain strong this morning then
continue at 15 to 20 MPH this afternoon through tonight with a
modest improvement in humidity recovery of about 10% higher than
this morning. More substantial improvement will come with a change
to light onshore winds during the day on Monday.

The other main items of interest are persistent smoky skies over
much of our area, eclipse viewing conditions on Monday morning,
and the potential for new fire starts (mainly southward and
eastward from the southern Oregon Siskiyou and Cascade Mountains)
due to lightning on Tuesday and Wednesday.

The path of totality will be north of our area, but there will be
many viewing the partial eclipse (with certified eye wear). There
is still high confidence that cloud cover will be limited to
dissipating stratus...retreating to the beaches north of Cape
Blanco, and a few high level clouds over southern Modoc County.
But, smoke will make for filtered viewing over the entire area.

Highs will be several degrees higher today as a weak trough exits
the Pacific Northwest and offshore high pressure nudges into the
region. The other main upper level feature to affect our weather
is a closed low offshore from southern California. It will send
gradually increasing amounts of mid-level moisture northward with
a few afternoon cumulus clouds likely to pop up over the higher
terrain of the far southern portions of our area on Monday

Instability will increase on Tuesday while increased moisture
rotates northwestward into northern California and south central
Oregon. A lower but still significant probability of thunderstorms
will extend to the southern Oregon Cascades and Siskiyou

A significant probability of thunderstorms will also exist on
Wednesday. But, the pattern will change from Tuesday as a broad
low approaches British Columbia. This will shift the steering flow
to southwesterly but the main risk will once again be from the
Siskiyous and Cascades southward and eastward.

A cold frontal passage associated with the trough will bring a
slight chance of light drizzle or showers to the Douglas and Coos
county coast late Wednesday night into Thursday morning. The front
will bring gusty afternoon winds to the east side, but also the
coolest high temperature readings of the week.

There is increased model agreement toward ridging developing over
the west coast late in the week into the weekend with a warming
and drying trend.

AVIATION...20/12Z TAF CYCLE...Gusty north to northeast winds will
continue in the coastal mountains through Sunday with a persistent
thermal trough. Patches of IFR/LIFR are possible north of Cape
Blanco, but the models suggest mostly clear skies. Expect similar
wind speeds with gusts of 35 kt at OTH again this afternoon. North
winds are also expected to surface at RBG with gusts up to 25 knots
in the afternoon.

Areas of MVFR visibility can be expected in wildfire smoke inland
with local IFR in the immediate vicinity of the fires.

FIRE WEATHER...Updated 305 AM Sunday, 20 August 2017...
High pressure offshore and a strong thermal trough over California
will produce gusty northwest through northeast winds once again
across much of the area this afternoon into tonight. Conditions will
be most critical in the far southwestern Oregon Coastal Mountains as
gusty northeast winds continue through Sunday morning. Winds will
weaken some today into early Monday but still be moderate. Humidity
recoveries will still be moderate to poor on the upper slopes and

Lower elevation west side locations will continue to experience mid
level Haines 5 conditions through Monday where the inversion breaks,
while higher elevations experience mostly Haines 4 to 5 where the
inversion breaks.

The flow aloft weakens and shifts to southerly Monday as moisture
and, then, instability arrive. Thunderstorms will be possible
Tuesday and Wednesday from approximately the Cascades and Siskiyous
south and eastward. Tuesday evening through Wednesday morning some
guidance is indicating the possibility of thunderstorms cresting the
Siskiyous and west of the Cascades, so we`ll continue to keep an eye
on this. Wednesday afternoon and evening the threat begins to kick
out to the east and is just about out of the area Thursday afternoon
and evening. -Sandler


OR...Red Flag Warning until noon PDT today for ORZ618-619.


Pacific Coastal Waters...Small Craft Advisory until 5 PM PDT Monday for
     Gale Warning until 11 PM PDT this evening for PZZ356.
     Hazardous Seas Warning from 11 PM this evening to 5 PM PDT
     Monday for PZZ356-376.
     Gale Warning until 2 AM PDT Monday for PZZ376.


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