931
FXUS66 KMFR 020546
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
1046 PM PDT Mon Sep 1 2025

.UPDATE...Updated AVIATION Section.

&&

.AVIATION...02/06Z TAFs...MVFR/IFR conditions are developing along
the coast late this evening with the first clouds showing up around
Reedsport. Expect stratus/fog to fill in at the coast and in the
coastal valleys overnight, then clear back to the near-shore waters
late Wednesday morning/early afternoon. These will return again in
the evening. Moisture returning from the south and east Wednesday
afternoon/evening will lead to isolated to scattered thunderstorms.
Best probability for a thunderstorm is at Klamath Falls, but one
could occur at Medford/Roseburg too.

Smoke from area wildfires isn`t expected to result in lower flight
conditions for most locations, except in the immediate vicinity or
just downwind from wildfires, but brief periods of lower visibility
and/or haze are possible. -Spilde

&&

.PREV DISCUSSION... /Issued at 239 PM PDT Mon Sep 1 2025/

DISCUSSION...

-Increasing confidence in thunderstorms for Tuesday, possibly into
Wednesday.
-High temperatures have trended slightly cooler for the Tuesday-
Thursday timeframe, limiting the risk for heat related illnesses to
Moderate.
-Hazy/smoky conditions will continue to bring periods of degraded
air quality through the forecast period.

Through tonight...Low pressure will linger offshore near the OR/WA
border, putting the forecast area under dry southwesterly flow. Not
much change is expected in the overall weather today, and conditions
will be quite similar to those of recent days. Afternoon breezes
will be a bit enhanced east of the Cascades and in the Shasta/Scott
Valleys today, but again, quite similar to what occurred yesterday
afternoon with gusts of 20 to 25 mph.

The pattern shifts Tuesday with an amplifying ridge over the Great
Basin as low pressure settles over the eastern CONUS. As this
happens, low pressure off the Washington coast retrogrades back into
the Gulf of Alaska and a shortwave, negatively tilted, trough passes
through the region Tuesday through Thursday. This will be the time
period worth watching, for two reasons: thunderstorm potential and
heat.

First, the thunderstorms...confidence is increasing for
thunderstorms to impact the region on Tuesday as models remain
consistent in the timing of the arrival of this shortwave trough.
With it`s arrival, moisture and instability will be sufficient for
storm development and the negatively tilted trough coming through
the region at peak heating will be more than enough of a trigger.
Current guidance shows showers/thunderstorm activity first
developing over the East Side and northern California during the
early part of Tuesday, with thunderstorm activity developing over
Jackson/Josephine Counties late afternoon/early evening. Storm
motion is forecast to be around 15-20 kts, which is sufficient for
storms to move off the terrain. Additionally, storm motion will
generally be from the south to southeast, which is favorable for
storms to move off the Siskiyous into the Rogue/Illinois/Applegate
Valleys Tuesday evening. There`s some uncertainty as to how far
north storms develop, but the Douglas County Foothills stand a fair
chance (15-25%) of seeing some storm activity as well.

Confidence decreases for storm activity for Wednesday and Thursday
due to differences in how fast models move this shortwave trough
north of the forecast area. The EC is slower and would maintain
storm activity into Wednesday while the GFS is faster and takes the
threat north of the area by Wednesday. We`ve left some thunderstorm
chances in the forecast for portions of the Cascades/East
Side/northern California for Wednesday and Thursday to account for
the slower solutions.

The other item we`ve been watching for during this time period was
the potential for hot temperatures and elevated risks for heat
related illnesses. Guidance maintains above normal temperatures
during this time, but still not quite high enough for any heat
related headlines. Tuesday still looks to be the hottest day of the
forecast period with high temperatures in the upper 90s (80s)/low
100s (90s) for valleys west (east) of the Cascades. High
temperatures trend lower by a few degrees Wednesday and Thursday
compared to Tuesday, with highs in the upper 90s (80s) for West
(East) Side Valleys. Take Medford for example...the probability of
reaching or exceeding 100 degrees on Tuesday is about 80% whereas
that probability drops to around 30-50% for Wednesday and Thursday.
With high temperatures 10 to 15 degrees above normal in West Side
Valleys, there will still be a moderate risk for heat related
illnesses. So those who are sensitive to these temperatures,
especially those without adequate cooling/hydration, will need to
take steps to prepare for this uptick in temperatures.

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 likely see additional shortwave troughs
pass through the region through the weekend. At this point in the
forecast, it`s hard to pinpoint details like timing and locations,
but we could see additional thunderstorm chances under this pattern.

Smoke impacts...Outside of thunderstorm chances, we aren`t expecting
any significant rainfall to hamper fire activity any time soon.
Smoke model guidance maintains the general pattern of higher smoke
concentrations moving into Klamath County in the late evening as
northerly winds bring smoke from the Emigrant fire. Improvement in
the afternoons is expected as the typical diurnal winds increase and
help to clear out the air. The Oregon Department of Environmental
Quality maintains an Air Quality Alert for Klamath County that is in
effect through at least Tuesday morning. Meanwhile, the worst of the
smoke impacts from the Dillon fire in western Siskiyou County should
remain confined to western Siskiyou County, with periods of higher
smoke concentrations settling in the Scott/Shasta Valleys at night,
then clearing out with the increase in afternoon winds as well. In
the Rogue Valley, less but still notable smoke impacts are expected.
Smoke from the Dillon Fire is drifting into the Illinois/Applegate
Valleys and then being carried eastward into the Rogue Valley with
the increase in west to northwesterly winds during the afternoon.
Model guidance does show the higher smoke concentrations moving out
of the Rogue Valley in the evenings. As the flow turns more
southeasterly on Tuesday, there could be some increased smoke/haze
that moves into the Umpqua Basin and possibly even along the coast.
Smoke concentrations don`t look at that high at this time, so
impacts on air quality should be fairly minimal for the Umpqua Basin
and coastal locations. With no significant rainfall expected through
the forecast period, expect periods of hazy conditions and degraded
air quality for the foreseeable future. /BR-y

AVIATION...01/18Z TAFs...MVFR/IFR conditions along the coast and in
the coastal valleys will clear back to the near-shore waters late
this morning into early afternoon, but will return this evening and
tonight as the marine layer once again pushes onshore.

Elsewhere, VFR will prevail with breezy afternoon winds through the
next 24 hours. Smoke is not expected to result in lower flight
conditions except in the immediate vicinity or just downwind from
wildfires, but periods of lower visibility are possible across the
area. -BPN

MARINE...Updated 100 PM PDT Monday, September 1, 2025...Northerly
winds will persist this week. Winds and seas will hover at advisory
levels south of Cape Blanco through Wednesday morning. Conditions
are expected to improve for the latter half of the week as winds
ease and seas lower. /BR-y

FIRE WEATHER...Updated 200 PM PDT Monday, September 1, 2025...The
area will remain under southwest flow today with an upper low off
the Washington coast and a upper level ridge centered along the
Rockies. This will bring a warm and dry pattern to the area.
Temperatures will be similar or slightly hotter than yesterday.
Similar to yesterday, breezy to gusty winds and dry humidities are
expected across inland areas again today, with strongest winds in
the Shasta Valley and east of the Cascades.  Overall, winds and
humidities are not expected to reach critical conditions. However,
some locally, brief near critical conditions are possible in the
late afternoon and early evening, mainly in the Shasta Valley and
east of the Cascades.

The upper level ridge will continue to strengthen on Tuesday and
Wednesday with hotter temperatures expected. Additionally, models
continue to show an upper level disturbance moving up from the south
towards the area on Tuesday. Confidence is increasing in chances (15-
30%) for isolated to scattered thunderstorms to develop from the
Cascades east and Siskiyous south Tuesday afternoon and evening, and
isolated storms could move into Josephine and Jackson counties
Tuesday evening. A Red Flag Warning has been issued for Fire Weather
Zones 280, 281, 284, 285, 617, 621, 623, 624, and 625 due to the
potential for abundant lightning on dry fuels. There is a potential
for storms to move into zones 620 and 622 Tuesday evening, with
generally south/southeast to north/northwest steering flow pushing
isolated storms off terrain and into the valleys. Additionally,
models soundings show a very dry low layer (inverted "V" sounding)
which means storms that do form will likely produce strong, gusty
and erratic outflow winds (35-50 mph) and may produce little
rainfall. As the disturbance slowly shifts northward over the area
Tuesday night, there are chances for lingering overnight showers and
thunderstorms (15-20%), mainly from the Cascades east. However, with
east to southeasterly steering winds, storms may drift into western
portions of the area (10% chance), such as over Douglas County, on
Tuesday night.

Additionally, Tuesday will be the hottest day of the week, and with
dry low level humidities and breezy afternoon winds, we may see some
locations approach but not quite pass critical thresholds. The hot
temperatures, dry RH, breezy afternoon winds, and overall
instability will add to the already heightened fire weather concerns
due to lightning.

Wednesday, there are a wide range of solutions regarding the exact
path of the upper level low. Most ensemble members have the low
either overhead or just north of the area Wedensday, and this would
push the bulk of thunderstorm chances to the north. Some ensembles,
however, are slower with the low, keeping thunderstorm chances in
our area through Wednesday evening.  So, have kept a slight chance
for thunderstorms (10-25%) in the forecast for areas from the
Cascades east on Wednesday, but confidence in the details is low.

If the upper low does end up progressing slower, then we could be
dealing with the potential for nocturnal storms Wednesday night, but
much will depend on the timing and location of the upper low.

Isolated storms (10-20% chance) could still be a part of the
equation Thursday afternoon and evening and again Friday as
additional disturbances move into the area. Late Friday into
Saturday, a low nears the coast and will bring less hot temperatures
but additional chances for showers and thunderstorms over inland
areas on Saturday. -BPN

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Red Flag Warning from 1 PM to 9 PM PDT Tuesday for ORZ617-621-
     623>625.

CA...Red Flag Warning from 1 PM to 9 PM PDT Tuesday for CAZ280-281-
     284-285.

PACIFIC COASTAL WATERS...Small Craft Advisory until 5 PM PDT
     Wednesday for PZZ356-376.

&&

$$