Area Forecast Discussion
Issued by NWS Medford, OR

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FXUS66 KMFR 200553
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
1053 PM PDT Sat Aug 19 2017

Updated AVIATION Section

.DISCUSSION...The main forecast impacts for tonight will be
wildfire smoke/air quality and also the offshore northeast winds
expected across the Coast Range in SW Oregon/portions of western
Siskiyou County.

Once again, low-level up valley flow kicked in here in the Rogue
Valley late this afternoon/evening. This was able to increase
visibility somewhat. But, in contrast to last night, the winds
just weren`t enough to scour out the smoke completely. This is
due to the fact that the fires burning to the north/northeast of
the area generated a lot of new smoke today and the winds aloft
were more out of the north/northeast, rather than north/northwest
like they were yesterday. An air quality alert remains in effect
through Sunday and this will be re-evaluated Sunday morning for
possible extention.

Offshore flow will peak tonight, then should ease Sunday night
into Monday as the thermal trough pushes inland. A Red Flag
warning is out for tonight into Sunday morning for fire zones
618/619. Please see fire weather discussion below for details.

Monday morning eclipse viewing will come down to thickness of
wildfire smoke and also the marine layer along the coast. Expect
some low stratus initially along the immediate coast as the flow
begins to turn onshore. Inland areas will be clear, but the
wildfire smoke is the wild card. Expect smoke to be a potential
issue along and east of a line from Brookings to Roseburg with the
thickest smoke from Western Siskiyou County up the Cascades.
Smoke should also be thinner the farther southeast you go across
the CWA, especially over toward Alturas/Lakeview.

Confidence continues to grow that moisture and instability will
increase across the area Tuesday into Wednesday. This will bring
a renewed threat of significant lightning, especially from the
Siskiyous/Cascades eastward. Stay tuned for details. -Spilde

&&

.AVIATION...20/06Z TAF CYCLE...Gusty north winds will continue in
the coastal mountains overnight through Sunday with a persistent
thermal trough. North winds ease briefly at OTH late tonight into
Sunday morning. While patches of IFR/LIFR are possible north of
Cape Blanco, latest models are trending away from long-lived stratus
at KOTH, so have removed it. Expect similar wind speeds with
gusts of 35 kt at OTH again Sunday afternoon. North 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. -Spilde

&&

.MARINE...Updated 200 PM PDT Saturday 19 August 2017...There
will be little change in the overall pattern through Sunday night.
High pressure offshore and a thermal trough along the coast will
generate moderate to strong north winds and very steep seas through
this weekend. 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 during the afternoons and evenings each day through
Sunday, easing a bit during the overnight and early morning hours.
The thermal trough will weaken early next week, and winds will
diminish but seas are expected to remain steep through the middle of
next week due to short period swells. /FB

&&

.FIRE WEATHER...Updated 345 PM Saturday, 19 August 2017...
High pressure offshore and a strengthening thermal trough over
California will produce gusty northwest through northeast winds
across much of the area this afternoon into tonight. Northwest winds
will peak over the west side inland from the Coast Range this
afternoon and evening, while northwest through northeast winds will
peak east of the Cascades this evening through Sunday morning. Along
the Coast Range westward winds will peak this evening at some
locations, and the overnight into the morning across the Coast
Range. Conditions will be most critical in the far southwestern
Oregon Coastal Mountains as gusty northeast winds continue through
Sunday morning. Winds will weaken some Sunday into early Monday, but
RH recoveries will still be moderate to poor on the upper slopes and
ridges.

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 some 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. BTL

&&

.PREV DISCUSSION... /Issued 400 PM PDT Sat Aug 19 2017/

SHORT TERM...The upper level ridge will remain centered offshore
through Sunday with a surface thermal trough along the coast. The
ridge will shift inland over the region on Monday and the thermal
trough will move inland Monday afternoon. Although there will be
some weak upper level disturbances in the flow, expect little
impact and the area will see continued very warm and dry weather
through Monday.

The thermal trough pattern is bringing gusty north to northeast
flow west of the Cascades with strongest winds over the coastal
mountains. These combined with low humidities are resulting in
critical fire weather conditions and active burning on area fires.
A red flag warning is in effect for much of Curry county. See the
RFWMFR for details.

Smoke from area fires will continue across southwest Oregon and
Northern California through early next week. Significant areas of
smoke are expected over portions of Curry, Douglas, Josephine,
Jackson, and Klamath counties. This is creating air quality and
air quality alerts are in effect these areas in southwest Oregon.
Areas of smoke are also present across portions of Northern
California especially in Siskiyou County. Smoke is expected to be
least along the coast from Gold Beach north and over Coos and
northwest Douglas county.

Temperatures over the next two days will be very warm with highs
in the mid to upper 90s for western valleys and in the upper 80s
to low 90s. Areas along the coast and inland valleys west of the
Cascades will experience breezy to gusty northerly winds in the
late afternoon and evening today and Sunday.

On Monday, the dry and warm weather will continue across the area
with a ridge moving inland. Gusty easterly winds over the ridges
will decrease by late morning. Winds aloft over southern portions
of the area from around the Siskiyous south, will gradually
change to a southeast direction late Monday morning. As the
thermal trough moves inland though, expect winds to shift to a
north to northwest direction Monday afternoon. Smoke will continue
to be a concern for areas in southwest Oregon and northwest
California but the areas which see the worst conditions may
fluctuate slightly.

Cloud cover is expected to be clear for mostly clear for most
inland areas on Monday, the day of the eclipse. Smoke will remain
the main concern for inland locations. Also models are showing
some low clouds and fog along the Oregon coast north of Cape
Blanco and into the Coquille valley. However gradual clearing is
expected in the mid to late morning on Monday. Will continue to
monitor this closely with future model runs.

LONG TERM...Tuesday through Saturday...Expect a chance for
thunderstorms and showers to develop Tuesday through Wednesday as
as southerly moisture feeds into the area from an upper low off
the south-central California coast. The best chances for storms
will be across northern California and from the Cascades eastward
during the afternoon and evening Tuesday and Wedensday. There is
also a slight chance for thunderstorms and showers overnight
Tuesday night into Wednesday morning.

On Thursday, the risk for thunderstorms decreases and models show
an upper trough a weak front moving inland on Thursday. This
front is expected to bring some isolated to scattered light
showers to Coos and Douglas counties along with a marine push of
low clouds. There is a slight risk for thunderstorms in eastern
portions of the area as well with this frontal passage, mainly
Lake county. Otherwise, expect cooler temperatures to develop on
Thursday over the area with highs in the 80s for most inland
valleys.

Friday and Saturday, models show good agreement that an upper
ridge will rebuild over the region and a surface thermal trough
will develop along the coast. This will bring another period of
dry and hot weather to the area with periods of gusty to breezy
northeast winds possible in the coastal mountains.

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Red Flag Warning until noon PDT Sunday for ORZ618-619.

CA...None.

Pacific Coastal Waters...Small Craft Advisory until 11 PM PDT Sunday
     for PZZ350-356-370-376.
     Gale Warning until 11 PM PDT Sunday for PZZ356-376.

$$

MAS/CC/BTL/FB


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