Area Forecast Discussion
Issued by NWS Medford, OR

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000
FXUS66 KMFR 240417
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
917 PM PDT Wed Aug 23 2017

.DISCUSSION...An update to the forecast is not necessary. Isolated
thunderstorm activity in Lake County early this evening moved off
into eastern Oregon. The southern extent of a weak cold front will
bring a marine push into the coast and the Umpqua Valley tonight
into Thursday morning with areas of drizzle. The front will weaken
as it moves inland, but it will bring an increase of high clouds
and also gusty afternoon and early evening westerly winds that
will be strongest east of the Cascades. Gusts will be in the range
of 10 to 20 mph on the west side of the Cascades and 15 to 25
mph on the east side. There is also a slight chance of late day
thunderstorms in Lake County, with the 00Z NAM and GFS each
showing weak instability from the northern part of the county
northeastward. High pressure will bring a strong warming and
drying trend Friday into next week.

&&

.AVIATION...24/00Z TAF CYCLE...Along the coast...over the
coastal waters...and in the Umpqua Basin...areas of MVFR/IFR
cigs/vsbys will move into the Umpqua Basin tonight...then burn back
to the coast by Thursday afternoon. Elsewhere, areas of MVFR vsbys
in smoke...possibly IFR near fires...will persist through Thursday.
Isolated thunderstorms east of the Cascades will diminish
tonight..then return Thursday afternoon and evening.

&&

.MARINE...Updated 830 PM PDT Wednesday 23 August 2017...A weak
thermal trough near the coast will strengthen Thursday...bringing
increasing north winds and steep seas into the weekend. Winds will
increase to gale force south of Cape Blanco Thursday then persist
into Saturday with small craft advisory winds over the rest of the
area. The trough will weaken Sunday...and winds and seas will
diminish at that time.

&&

.PREV DISCUSSION... /Issued 300 PM PDT Wed Aug 23 2017/

DISCUSSION...Satellite imagery this afternoon is showing our first
lightning strikes of the day in Lake County, in close proximity
to shortwave activity that`s moving northeastward through the
eastern portion of our CWA this afternoon. Thunderstorm chances
will continue through the afternoon and early evening over eastern
Siskiyou, Modoc, Klamath, and Lake Counties. A Red Flag Warning
highlights the expected new fire starts with abundant lightning on
dry fuels in northern Modoc, Eastern Klamath, and Lake Counties.
Drier midlevels today and decreased midlevel moisture should keep
any thunderstorms from forming west of the Cascades and over
Western Siskiyou County.

Tonight, a deep marine layer (2500-3000 ft per the North Bend
profiler) will push inland tonight, reaching the Umpqua Divide with
some clouds likely spilling over into Jackson and Josephine County
valleys. The lift with the approaching frontal boundary, and the
deep marine layer, will help to bring drizzle and few hundredths
of an inch to the immediate coast and coastal valleys, along with
higher terrain from the Cascade foothills in Douglas County
westward.

Tomorrow, a mostly dry frontal boundary will push through the
forecast area. Enough moisture and instability will linger east of
the Cascades tomorrow (in eastern Klamath and Lake) to provide a
slight chance for thunderstorms, so we`ve added that to the
forecast.

A sharp trend towards hotter and drier conditions begins on Friday
and continues through Monday/Tuesday. Triple digit temperatures are
forecast for Medford Saturday through Tuesday. Afternoon readings
will be about 10 to 15 degrees above normal. At this point there
looks like no need for heat-oriented watches or advisories, but
we`ll assess the impact of smoke in the area limiting the ability to
open windows and bring in cool air from outside (for those without
air conditioning). There is no doubt that this will impact fires in
the area. Additionally, a thermal trough pattern at the coast will
set up again from Thursday through the weekend, and this really can
drive fire behavior via persistent northerly to easterly winds, low
humidity, and the potential for unstable conditions as the thermal
trough moves inland early next week. More on this can be found in
the fire weather discussion below.

FIRE WEATHER...Updated 2:00 PM PDT Wednesday, 23 August 2017...Lingering
cloud cover delayed thunderstorm development over the east side,
but visible satellite is showing several cumulus buildups, and a
few CG strikes have been observed in Lake County over the past
hour. Models show high clouds diminishing and shifting eastward
this afternoon. If that doesn`t pan out, thunderstorm activity
could be suppressed. However, with moderate instability in place
and a weak shortwave expected to pass over the East Side over the
next few hours, we should continue to see at least isolated
thunderstorms this afternoon. PW values are between 0.80 and 1
inch, therefore we can expect some localized wetting rainfall.
However, lightning can occur outside storm cores on dry to very
dry fuels and lead to many new fire starts. Also, there is high
potential for gusty and erratic winds near thunderstorms. The red
flag warning remains in effect for this afternoon and evening for
portions of the East Side.

A cold front will pass over the area Thursday, bringing a slight
chance of thunderstorms to zone 624 and 625.

A thermal trough will set up along the coast Thursday night and we
could be looking at moderate recoveries along the mid slopes and
ridges in zone 618 and 619, including the Chetco Bar Fire and
western fire zone 280. Right now recoveries look strong enough to
preclude red flag conditions.

A rapid warm up is expected Friday through at least early next week.
The thermal trough will remain along the coast Friday night into
Saturday, so we could be dealing with gusty northeast to east winds
near and at the ridges with moderate to possibly poor overnight
recoveries in Fire zones 618, 619 and western 280. It remains to be
seen if recoveries will be as low as they were this past weekend,
but it`s something that will need to be monitored.

The models suggest the thermal trough along the coast will weaken
and move inland Sunday night into Monday which could lead to
improved recoveries in fire zone 618 and possibly 619. In any case
weak onshore flow is expected with winds shifting to the north to
northwest. Relative humidities will be higher right along the coast,
but the onshore flow may not be strong enough for higher RHs at the
Chetco Bar Fire. The bigger concern there will be breezy northwest
winds in the afternoon and evening hours.

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...
CA...None.

Pacific Coastal Waters...Small Craft Advisory until 5 AM PDT Thursday
     for PZZ356-376.
     Small Craft Advisory from 8 AM Thursday to 5 AM PDT Saturday for
     PZZ350-356-370-376.
     Gale Warning from 8 AM Thursday to 5 AM PDT Saturday for
     PZZ356-376.

$$

DW/NSK/JRS



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