Area Forecast Discussion
Issued by NWS Medford, OR

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FXUS66 KMFR 190551
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
1052 PM PDT Fri Aug 18 2017

Updated AVIATION Section

.DISCUSSION...Low-level northwest winds were able to scour out
some of the smoke from the Rogue Valley earlier this evening, but
expect the low-level flow to turn back to the north and northeast
later this evening and overnight, so smoke from the fires burning
in the Umpqua National Forest and the High Cascades will likely
return. There`s just a lot of smoke out there south and east of
the fires, so just about everywhere in the CWA south and east of
the Umpqua Divide will have to deal with it. To escape it, it is
recommended to go west to the coast (north of Brookings) and/or
north/northwest to Roseburg/Eugene/Portland. The forecast is on
track, so will not update this evening. -Spilde

&&

.AVIATION...19/06Z TAF CYCLE...IFR ceilings will prevail along
the coast north of Cape Blanco overnight into Saturday morning. Some
MVFR ceilings may also try to form in portions of the Umpqua Basin
toward sunrise, however, confidence is only moderate that will
occur. The remainder of the area will be VFR, but widespread smoke
from wildfires will result in visibility degradation to MVFR at
times. -Spilde

&&

.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

&&

.PREV DISCUSSION... /Issued 410 PM PDT Fri Aug 18 2017/

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, Josephine, 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.

&&

.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.

$$

MAS/CC/BTL/FB



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