Area Forecast Discussion
Issued by NWS Medford, OR

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FXUS66 KMFR 162147

Area Forecast Discussion
National Weather Service Medford OR
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 it 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 were 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


.AVIATION...16/18Z TAF CYCLE...VFR will prevail across the area
today with the exception being where wildfire smoke degrades
visibility. This is most likely along and west of the Cascades and
also in western Siskiyou County. Also at the coast and offshore,
gusty north winds are expected this afternoon. IFR could return to
the coast tonight, especially north of Cape Blanco. -BR-y/Spilde


.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.


.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
     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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