Area Forecast Discussion
Issued by NWS Medford, OR

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FXUS66 KMFR 170533

Area Forecast Discussion
National Weather Service Medford OR
1032 PM PDT Wed Aug 16 2017

.Updated aviation discussion.

.DISCUSSION...Weather concerns continue to center around the
wildfires in our forecast area. The hot, dry, and breezy
conditions (temperatures 5 to 10 degrees above normal) have been
and will continue to be favorable for fire growth and smoke
production. Thus, many plumes of smoke are being observed on
visible satellite imagery and air quality has generally degraded
over much of the forecast area over the past 24 hours. Persistent
northerly flow in the lowest 5000 feet of the atmosphere should
maintain smoke in similar areas the next few days at least. After
coordination with partner agencies, we have issued an Air Quality
Alert at AQAMFR.

Drilling down into a bit more specifics, the northerly pressure
gradient is expected to decrease some Thursday, so afternoon
northerly winds tomorrow should be slightly lighter than this
afternoon. The air mass gradually warms through Friday before an
upper trough Saturday brings slight cooling. There is some
significant uncertainty for the weekend into next week in terms of
how the weather pattern evolves. One model solution brings a
renewed chance of thunderstorms while another keeps most
thunderstorm activity out of our forecast area. Please see the
previous forecast discussion and fire weather discussion below for
details on the expected weather conditions and the potential
impacts on area fires.


.AVIATION...17/06Z TAF CYCLE...VFR will prevail across the area
through Thursday with the two exceptions of IFR stratus returning to
the coast north of Cape Blanco between about 09Z this evening and
16Z Thursday morning, and wildfire smoke degrading visibility. This
smoke is beginning to affect most areas in the forecast area except
for the coast and Lake County. Also, at the coast and offshore,
gusty north winds will be strongest south of Cape Blanco and during
the afternoon and evening hours. -DW/SK


.MARINE...Updated 230 PM PDT Wednesday 16 August 2017...Confidence
has increased that high pressure centered offshore and a thermal
trough along the coast will continue 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 and south of Gold Beach through the
period. Winds and seas will be highest during the afternoons and
evenings each day through Sunday, easing a bit during the overnights
and early mornings. Model guidance shows peak boundary layer winds
of ~50kt on both Saturday and Sunday afternoons, then the pressure
gradient weakens with winds gradually easing early next week.


.PREV DISCUSSION... /Issued 247 PM PDT Wed Aug 16 2017/

DISCUSSION...The upper ridge centered near 40N and 140W will shift
slightly east over the next few days bringing a continuation of hot
and dry conditions over our area through at least Friday. The latest
visible image shows clear skies over most of the area. The exception
to this are patchy marine stratus over the outer waters and a few
patchy cumulus near Crater Lake and the Warners. Also smoke is also
moving from northeast to southwest over the Eclipse Complex in
western Siskiyou County and Miller Complex as fires in these areas
appear to becoming more active. Meanwhile the thermal trough is
just off the coast with offshore flow. The Chetco effect is in full
force with Brookings currently at 87 degrees. It`s possible they
could top out in the lower 90s in the next few hours.

Not much is going to change in the next few days with continued
offshore flow. The Chetco Effect will remain in place, therefore
expect mild temperatures at Brookings tonight followed by another
warm day Thursday. Marine stratus will return mainly north of Cape
Blanco and possibly inland into the Coquille Basin late tonight into
Thursday morning. Elsewhere skies will be clear, but we`ll still
have to deal with patchy to areas of smoke.

A weak shortwave trough will ride over the ridge Saturday and could
bring slight cooling to most inland locations, but only by a few
degrees on average. The models show marginal instability at best,
but mid level moisture is lacking, so we`re not expecting any
thunderstorms. However cumulus build ups are possible over the
mountains in Northern Cal and east of the Cascades.

The models show a weak upper trough developing in California
saturday night into Sunday and could tap into some monsoonal
moisture. The GFS in particular is more aggressive with convective
precipitation in the Sierras and into portions of Modoc County. The
ECMWF has a drier solution, but does show some convection along the
Sierras. For now kept in a slight chance of thunderstorms in
southeast Modoc and Lake County late Sunday afternoon.

The upper trough will close off and remain in central Californaia
Monday and it remains to be seen how much moisture and instability
will move into our area. The GFS shows more instability, moisture
and slightly stronger trigger. The ECMWF is weaker with all of these
elements and has a southwest flow over us which would tend to keep
the best chance for thunderstorms south of our area. Given the model
differences this far out, decided to keep a slight chance of
thunderstorms mainly in northern California and southeast Klamath
and Lake County.

Beyond Monday, the models are in general agreement showing an upper
low moving southeast from the Gulf of Alaska towards Vancouver
Island. The models differ with position of the upper low, but both
would suggest some cooling possible by next week at this time. It`s
a ways out and lot`s could change, so stay tuned. -Petrucelli

FIRE WEATHER...Updated 100 PM Wednesday 16 August 2017...
Northeast offshore winds are occurring in the Coast Range of SW
Oregon and also over the mid slopes and ridges in western Siskiyou
County, where, overall, humidity recoveries have been poor.
Conditions, however, did not meet red flag criteria. Recent RAWS
observations at Red Mound, Quail Prairie, and Slater Butte are
showing winds averaging around 5-12 mph with gusts to 20 mph and
humidity of only 25-35 percent. We are expecting similar conditions
tonight into Thursday morning. Therefore, have cancelled the Fire
Weather Watch for fire zone 619. There may still be brief periods of
Red Flag conditions in isolated areas tonight, but if this does
occur, the time period will be too brief to warrant a red flag
warning. Elevations around and above 2000 feet are at highest risk
of reaching the wind/relative humidity criteria. This, however, does
not include the Chetco Bar Fire because relative humidities are
expected to be a bit higher there.

It should also be noted that all mid slope/ridge locations in fire
weather zones 618,619,620 in Oregon and 280 in California will have
a period of gusty northeast winds tonight into Thursday morning
along with poor to moderate recoveries. This is a critical fire
weather pattern, especially since there are numerous active
wildfires in the region. The rapid drying combined with the gusty
winds could enhance the risk of spread of existing fires and also
perhaps awaken some "sleeper" fires. Winds should ease a bit, but
recoveries will remain poor to moderate each night/early morning
through Sunday. This will be monitored closely and additional
watches or warnings may be needed.

Another item of note is that many areas west of the Cascades will
have about 3-5 hours of gusty north to northwest winds early this
evening, but especially in the north-south facing valleys. Humidity
in these areas will be flirting with critical levels, but is
expected to remain just above 15 percent.

Models show it will remain dry through at least Saturday, with
things becoming a bit more unstable on Friday. Moderate haines will
be expected nearly everywhere, with high haines indices forecast for
portions of fire weather zones 623, 624, and western 625. As smoke
clears out of the fire locations on Friday, this may add to the
nature of the unstable conditions near any existing wildfires.

Models continue to show an upper disturbance closing off near the
central California coast Sunday into early next week. This may be
able to back the flow enough to bring some moisture northward along
the Sierra crest and perhaps into SE portions of our forecast area.
We are continuing to indicate a slight chance of thunderstorms in
Modoc on Sunday afternoon, then into portions of the East Side in
Oregon Monday and Tuesday afternoons. As of now, details are
uncertain, but lightning chances are increasing as we head into next
week. -Schaaf/Spilde




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.      Small Craft Advisory for hazardous seas
until 11 PM PDT Sunday      for PZZ350-356-370.


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