Area Forecast Discussion
Issued by NWS Medford, OR

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FXUS66 KMFR 182311
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
410 PM PDT Fri Aug 18 2017

The Fire Weather section has been updated...

.SHORT TERM...A shortwave trough will drop down the coast tonight
into Saturday then shift south of the area late Saturday.
Meanwhile an upper ridge will remain centered offshore and a
surface thermal trough will persist along the coast. This pattern
will bring continued warm temperatures to the area with dry
conditions.

The main effect of the shortwave is that it will bring a few
degrees of cooling to the area. Also expect an increase in
humidity tonight for areas west of the Southern Oregon Cascades.
This is temporary though since the thermal trough strengthens
Saturday night into Sunday morning bringing another round of dry
conditions along with moderate to strong winds over the coastal
mountains. As a result expect the potential for critical fire
weather concerns Saturday night into Sunday morning. A fire
weather watch is in effect (see the RFWMFR for details).

With the active Chetco fire in southern Curry county and the
northeast winds expect smoke to continue to move into the
Brookings area at times. Also the northeast winds will bring a
Chetco effect with some warm temperatures in the Brookings area.

Numerous fires are present in the area. With the northeast to east
winds occuring each night then north to northwest winds in the
afternoon and evening expect most inland areas to continue to
experience smoke. Air quality alerts are in effect for Douglas,
Curry, Joesphine, Jackson and Klamath counties. This smoke will
continue this weekend and into early next week. The areas most
impacted by smoke may fluctuate though.

The ridge will nudge inland on Sunday then be centered over the
region on Monday. For the day of the eclipse, expect dry
conditions with mostly clear skies except for areas over southwest
Oregon and northern California. Coastal areas north of Cape
Blanco and near Brookings will see the potential for cloudy skies
with low clouds or patchy fog early Monday morning. These clouds
may extend into the Coquille valley and into northwest Douglas
county near the coast. Gradual clearing is expected in the mid to
late morning. Confidence remains low to moderate on the timing and
amount of low clouds though. So will need to monitor this will
future model runs.

.LONG TERM...Tuesday and Wednesday, models show moisture rotating
around an upper low off the south-central California coast. This
moisture combined with instability and upper level disturbances
will bring a slight chance to chance of thunderstorms to inland
areas Tuesday and Wednesday. The best chance for showers or
thunderstorms will be across Northern California and from the
Siskiyous and Cascades eastward.

An upper trough will near the coast Wednesday night and move into
the region on Thursday. This will bring somewhat cooler
temperatures as well as a slight chance for showers along the Coos
and Douglas coast and into the coastal mountains.


&&

.AVIATION...18/18Z TAF CYCLE...
Gusty north winds are expected from about Roseburg and Grants Pass
westward this afternoon, strongest from the Coast Range westward. A
steady stream of MVFR stratus will hover around 10 nautical miles
west of the Coos/Douglas County coasts much of today, but is likely
to migrate coastward this afternoon. IFR ceilings are expected along
the Coos coast early this evening, possibly lowering to LIFR.
Northeast winds tonight into the morning will disrupt the low clouds
after sunrise Saturday morning. There is low to moderate confidence,
however, that MVFR low clouds will reach to KRBG Saturday morning,
where smoke from wildfires is likely to aid cloud formation. For the
remainder of the area, especially Klamath Falls and Alturas
westward, smoke from numerous ongoing wildfires will continue to be
a problem, reducing visibilities most near the sources under a
general northerly flow, and across the valleys the most in the
overnight and morning hours. BTL

&&

.MARINE...Updated 200 PM PDT Friday 18 August 2017...High pressure
centered offshore and a thermal trough along the coast will support
moderate to strong north winds and steep to very steep seas through
the weekend. Conditions will at least be hazardous to small craft,
but 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 and seas will diminish then. /FB

&&

.FIRE WEATHER...Updated 405 PM Friday, 18 August 2017...
A trough of low pressure pushing through to our north will result in
a push of the marine layer onto the coast and west side this evening
through Saturday morning leading to improved relative humidity
recoveries for many west side locations.

Tomorrow afternoon through Sunday morning a surge of northwest
through northeast wind is expected to push across the west side and
Coast Range. Of most concern is wind over and near the Coast Range
in Fire Weather Zone 619. The pressure gradient increases by about
28 percent on the NAM12, 925mb winds on the NAM12 reach 25-35 knots
during the watch period, and they are even higher on the High
Resolution ARW. Given the explosive growth that has been observed on
the Chetco Bar Fire in the past 24 hours, concern is high that the
fire will be substantially driven by wind later Saturday through
Sunday morning. The watch was issued for all of the zone because of
uncertainty related to RHs and because winds under this pattern tend
to be strongest on the ridges and down the river valleys toward the
coast, to include the Chetco.

While dryness and instability on Saturday at most locations will not
be as critical as today, the combination of dry enough conditions
and wind on the west side is likely to result in another active burn
period Saturday afternoon into Saturday night. The high elevation
Haines lowers from 5 to 4, while the mid-level Haines will remain a
5 through the day Saturday.

Winds are expected to weaken some on Sunday. The flow aloft shifts
to southerly Monday and moisture and instability are expected to
arrive Tuesday. Thunderstorms will be possible Tuesday and Wednesday
from approximately the Cascades and Siskiyous south and eastward. BTL

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Fire Weather Watch from Saturday afternoon through Sunday
     morning for ORZ619.

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.

$$

CC/BTL



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