494
FXUS66 KMFR 050543
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
1043 PM PDT Thu Sep 4 2025

...New Aviation Section...


.AVIATION (06Z TAFs)...At the coast and just offshore, IFR to
LIFR conditions are expected through the TAF period. It is taking
the longest to drop visibility at the Coos County coast, but have
added IFR visibility towards the start of the TAF period. Tonight
between 11Z-17Z fog is likely, so this has been added to the coast
TAF.

Elsewhere, VFR conditions will continue through the TAF period,
except in the vicinity of the Emigrant Fire, and fires in central
and western Siskiyou County. Tomorrow`s rain and thunderstorm
chances remain along the Cascades north of Crater Lake and in
northern Klamath County and across Lake County. Gusty and erratic
winds will be a concern in and in the vicinity of any
thunderstorms, along with lightning and potentially, small hail.
For this reason, the inland TAFs do not have any mention of
rain/thunder at this time.

-BPN/Hermansen

&&

.PREV DISCUSSION... /Issued 210 PM PDT Thu Sep 4 2025/

DISCUSSION...It has been a relatively quiet day so far in the
forecast area. Some cumulus buildups are occuring this afternoon,
but nothing has appeared on radar. Yet. Meanwhile, the marine
layer will remain well entrenched with stratus and fog right along
the immediate coastline and across the coastal waters. Expect the
marine fog and clouds to push back into the coastal valleys in
this evening and remain in place well into Thursday morning. This
cycle is likely to repeat for several more days.

Hot temperatures will continue today for inland areas. With the
lack of thick cloud cover today, temperatures should be able to
reach their full potential with plenty of sun, and upper 90s to
triple digit highs are not out of the question for many of the
inland West Side valleys this afternoon.

These warm temperatures will also allow for increased instability
this afternoon, when another, albeit weaker, upper disturbance
rotates into the area around the larger upper low to the west.
This could trigger isolated storms again this afternoon and
evening, with the high resolution models showing cells forming
over the Klamath and Siskiyou mountains and across portions of the
Cascades and East Side this afternoon and evening.

Friday into the weekend, temperatures will trend cooler but still
remain above normal for early September (normal high for Medford
being 88 degrees). Upper level troughing will be persistent over
the eastern Pacific and we`ll see additional shortwave troughs
pass through the region through the weekend. The next is expected
to approach the region Friday, bringing the return of scattered
thunderstorm chances Friday into Saturday. Guidance is in pretty
good agreement showing the best chance for thunderstorms should be
in the northern Cascades, and portions of Klamath and Lake
counties. Friday night ,the upper trough will become negatively
tilted, and while models are only barely hinting at nocturnal
thunderstorms, this is the pattern that has decent potential for
nighttime convection. There are some indications thunderstorms
could set up as far west as the coast and over the marine waters
with this shortwave, especially overnight and into Saturday
morning as the shortwave axis passes directly overhead.

Beyond Saturday, the parent trough that has drifted around over
the eastern Pacific firing shortwaves at us this week will finally
move onshore and stick around through most of next week.
This pattern change will result in below normal temperatures and
several days (roughly Sunday night through Wednesday) of light,
but beneficial, precipitation chances that would make the area
feel more like October than early September. It is still yet too
early in the season,  and precipitation may be too spotty, to
call this a season ending event for area wildfires. That being
said, we are more confident that healthy measurable rainfall
will occur across the area over the first half of next week,
which would definitely be helpful with local firefighting
efforts, and would put a large dent in fire weather concerns. -BPN

AVIATION...04/18Z TAFs...At the coast and just offshore, IFR to
LIFR conditions are expected through the TAF period. Ceilings could
come up to MVFR for a brief period between 22--1z, but should
quickly lower again late this afternoon through tonight.

Elsewhere, VFR conditions will continue through the TAF period,
except in the vicinity of the Emigrant Fire, and fires in central
and western Siskiyou COunty. The main concern for this afternoon and
early this evening will be isolated thunderstorms. Isolated storms
could be in the vicinity or over the Klamath Falls and Medford
Airfield between 21-3z.  Gusty and erratic winds will be a concern
in and in the vicinity of any thunderstorms, along with lightning
and potentially, small hail. -BPN

MARINE...Updated 200 PM PDT Thursday, September 4, 2025...Moderate
north winds and steep seas will continue for most areas through this
afternoon as the thermal trough gradually weakens. Marine fog and
low clouds will persist for the next several days. An upper level
low will pass over the area late Friday into Saturday, with showers
and drizzle likely, and a stray thunderstorm or two possible.
Conditions are expected to improve tonight through the weekend.
Expect showers with no significant swell and light winds becoming
south ahead of an area of low pressure moving onshore early next
week. -BPN

FIRE WEATHER...Updated 200 PM PDT Thursday, September 4, 2025...
Slight chances for afternoon and evening thunderstorms remain in the
forecast. The highest chances (15-20%) stretch across Klamath and
Lake counties, while lesser chances (10-15%) are over Jackson,
Josephine, and Douglas counties. With normal winds aloft and decent
atmospheric moisture available, storm motion will likely be slow and
rain showers may be included. Isolated storms may continue tonight
into early Friday morning.

An approaching trough trough looks to move from south to north
across the area, providing instability for activity on Friday.
Chances for thunderstorms (25-35%) are in the forecast across Lake
and northern Klamath counties as well as the Cascades north of Mt
McLaughlin for the afternoon and evening. Slight thunderstorm
chances (10-15%) are present over Josephine and western Siskiyou
counties, with slight chances also continuing into Saturday morning
over area terrain. With these conditions, a Red Flag Warning is
being issued for abundant lightning for Weather Zones 617, 623, 624,
and 625. Dry air aloft will limit activity behind the trough. This
warning will cover from 1 PM through 9 PM on Friday. Isolated
thunderstorms may continue Friday night into early Saturday morning.

The trough moves over the area through the day Saturday, bringing
elevated winds over terrain east of the Cascades. Thunderstorm
chances remain in the forecast for Saturday evening as well, with
slight chances (20-25%) over the Cascades and northern Klamath
County. Any changes in the timing of the trough`s movement may
change where and when thunderstorms are forecast. Lower chances (15-
20%) remain over the Cascades on Sunday afternoon and evening.

On Monday, a low pressure system looks to settle over the area
through at least midweek. While this feature is in place,
temperatures are forecast to drop below seasonal averages and
chances for rain showers could be present across the area. Periods
of embedded thunderstorms may be possible, but timing and confidence
in these chances are currently slight. -TAD

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Red Flag Warning from 1 PM to 9 PM PDT Friday for ORZ617-
     623>625.

CA...Red Flag Warning from 1 PM to 9 PM PDT Friday for CAZ285.

PACIFIC COASTAL WATERS...Small Craft Advisory until 11 PM PDT this
     evening for PZZ370-376.

&&

$$