661
FXUS66 KMFR 312324
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
424 PM PDT Sun Aug 31 2025

...Updated AVIATION discussion for 00Z TAFs...

.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.

In the near term (through Monday night)...Low pressure will linger
offshore near the OR/WA border, putting the forecast area under dry
southwesterly flow. Not much change is expected through Monday, and
conditions will be quite similar to those of recent days. Afternoon
temperatures today will end up a few degrees cooler than yesterday,
then trend a few degrees warmer on Monday. Afternoon breezes will be
a bit enhanced east of the Cascades and in the Shasta/Scott Valleys
today and Monday, 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 to account for the slower
solution. The forecast for Thursday remains dry at this time, but
future modifications may be needed depending on model trends.

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. Recent guidance has trended lower with the
temperature forecast for this time frame, likely catching up with
the models showing this shortwave trough over the region. As such,
it now looks like Tuesday will 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 75% whereas that probability drops to
around 30-50% for Wednesday and Thursday. Considering this trend in
the forecast, we don`t plan on needing any heat related headlines.
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. With
a general southwest to southerly flow over the region through much
of the forecast period, the bulk of the smoke impacts from the
Emigrant fire should remain north of the forecast area. 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 has issued a 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 after 5pm this evening, but a similar pattern is
likely again tomorrow. 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...01/00Z TAFs...VFR levels continue across northern
California and southern Oregon this evening. Marine stratus looks to
rebuild later this evening or early tonight, bringing IFR to LIFR
conditions to coastal areas. This layer could clear out late Monday
morning or early in the afternoon. For inland areas, VFR levels will
continue through the TAF period. Smoke from area fires could
periodically affect visibilities at the Medford and Klamath Falls
terminals. -TAD

&&

.MARINE...Updated 100 PM PDT Sunday, August 31, 2025...Northerly
winds will persist this week, strengthening to advisory levels south
of Cape Blanco tonight into Monday. This will result in low end
advisory level steep seas south of Cape Blanco that will likely
persist through mid-week. Conditions improve for the latter half of
the week as winds ease and seas lower. /BR-y

&&

.FIRE WEATHER...Updated 200 PM PDT Sunday, August 31, 2025...Overall
fire concerns are low through Monday.

The latest satellite image shows marine stratus gradually peeling
back towards the coast north of Cape Blanco, but south of there it
remains persistent and it`s possible if were to clear out, it would
be for only a brief period of time. Elsewhere, it`s clear with a
thin layer of high level clouds that will filter out the sun some,
but the perception is it will be clear.

Smoke for the Emigrant Fire is being carried north to northeast of
our area. Some of the guidance shows some of that smoke shifting
south into northern Fire zone 624 early this evening into tomorrow
morning, then clearing out again as instability increases and it
mixes out. Also smoke from the Blue and Dillon Fires remain in
portions of Fire Zone 280.

It will remain dry through Monday with the typical mid to late
afternoon and early evening breezes in the Rogue Valley, Umpqua
Basin, Shasta Valley and east of the Cascades. Even though relative
humidity will be on the low end in these areas, the wind component
is not sufficient enough for critical conditions to be met. The one
exception could be in the Shasta Valley this afternoon and early
this evening and again Monday afternoon and evening where there
could be brief periods in which critical conditions could be met.
This will continued be highlighted in the fire weather forecast.

Thunderstorm concerns remain low Monday, with continued dry weather.
However, afternoon temperatures will warm up as upper ridging builds
towards the area as the upper low continues to move north west of
Vancouver Island.

Tuesday is starting to become more problematic in terms of
thunderstorms in the afternoon and evening. An upper trough/weak
upper low will move up from the south and will set up over northern
California at the time of max heading and instability. The track and
location of the upper low has all the makings for thunderstorms to
develop early Tuesday afternoon, with storms increasing in coverage
and frequency as the afternoon progresses. Current data suggest Fire
zones 280, 281, 284, 285, 623, 624, and 625 are at highest risk for
thunderstorms. Additionally, models soundings show a very dry sub
layer (inverted "V" sounding) which means storms that do form will
likely produce little to no rainfall. This along with dry and
unstable conditions, receptive fuels and ability for lightning to
start new fires, a Fire Weather Watch has been issued for Tuesday
afternoon and evening. Please see RFWMFR for more details.

It`s worth noting, storms that form over the Siskiyous could come
off the terrain and drift into the southern end of Fire zones 622
and 620, including portions of the Rogue and Illinois Valley late
Tuesday afternoon into early Tuesday evening (4pm-8pm). It`s also
not out of the question storms could form in above mentioned areas.

Conditions are favorable for nocturnal storms Tuesday night mainly
in portions of Fire zone 617, 624 and 625. However much will depend
on the timing and evolution of the upper trough.

Wednesday, the operational models differ with the progression of the
upper low. The ECMWF is slower and has the upper trough getting hung
up over our forecast area which would give us another day of
afternoon and early evening thunderstorms. This has been the case
the past couple of days, although last nights run was showing signs
of leaning towards the faster progression.

The GFS and NAM shows a faster progression with the upper low moving
north of our area by Wednesday afternoon which would put the best
chance for storms north of our forecast area. Given the difference
in the tining, we`ll stay the course and keep a mention of thunder
for Wednesday, but forecast confidence 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. If
nothing else, another thing we may have to consider, but there`s
still plenty of time to address this.

Isolated storms could still be a part of the equation Thursday
afternoon and evening, but right now there`s nothing in the forecast
and much will depend on the track of the upper low.

The operational models show an upper trough approaching from the
west Friday and possibly moving overhead Saturday with thunderstorms
being a concern. Looking at the clusters suggest the upper
low/trough will be positioned farther west which makes more sense
from a global standpoint because the clusters all slow deep upper
troughing with lower anomalies in the northeast part of the U.S.
which typically results in stronger riding for the western U.S.
-Petrucelli

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Fire Weather Watch from Tuesday afternoon through Tuesday
     evening for ORZ617-621-623>625.

CA...Fire Weather Watch from Tuesday afternoon through Tuesday
     evening for CAZ280-281-284-285.

PACIFIC COASTAL WATERS...Small Craft Advisory from 11 AM Monday to 5
     PM PDT Tuesday for PZZ356-376.

&&

$$