934
FXUS66 KMFR 292046
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
146 PM PDT Fri Aug 29 2025

.DISCUSSION...The upper level pattern is kind of "messy" today
with generally high pressure over the western states and a weak
low pressure trying to undercut this ridge. This results in an
area of high pressure cut off over British Columbia. Meanwhile, a
deeper low pressure is drifting eastward over the eastern Pacific
between 130 and 140W. This low pressure will approach the West
Coast today into the weekend, pushing the weaker low/shortwave
northward into the cut off high pressure, weakening each other and
allowing for the deeper low to get closer to the West Coast. This
pattern will maintain southwesterly flow over the region, which
will keep temperatures above normal (mid-upper 90s west/mid-upper
80s east), but limit the degree of monsoon moisture moving into
the region. We expect dry conditions today and into the early
weekend, but there is just enough (marginal) moisture and
instability for some cumulus buildups over the terrain in the
afternoons. It`s not out of the question for one of those buildups
to develop into a stray shower, but the chances for lightning are
pretty low (5-10%) at this time.

Low pressure offshore nudges closer over the weekend, and this will
bring a cooling trend for Saturday and Sunday with high
temperatures trending around 5 to 10 degrees cooler over the
weekend compared to today. Models even try to bring a very weak
front into the Pacific Northwest late Saturday into Sunday, but
this won`t amount to much in our area. Likely a farther inland
push of marine stratus into the Umpqua Basin, and maybe even few
sprinkles/drizzle along the coast. Otherwise, dry conditions are
likely to continue through the weekend, but the flow will turn
more southerly and this could allow enough moisture and
instability to slip into eastern portions of the region late
Sunday into Monday. We don`t expect much to amount from this
increase in moisture/instability since there won`t be an
appreciable forcing mechanism, but the potential for cumulus
buildups and a stray shower will remain present for Sunday and
Monday afternoons.

Tuesday through Thursday will be the time period worth watching for
two reasons: thunderstorm potential and heat. First, the
thunderstorms...Models remain consistent in showing some
energy/weakness in the ridge moving northward through the region
Tuesday through Thursday. In fact, guidance now has a weak low that
takes on a negative tilt as it moves northward, bringing increasing
moisture and instability. We`ve ventured away from the NBM to add a
slight chance of thunderstorms for Tuesday and Wednesday
afternoons/evenings, focused in the typical areas along and east of
the Cascades as well as for portions of western Siskiyou County.
This is still 5-6 days out, so details are likely to change once
this time frame is covered by the models that help us hone in on
timing and location.

Meanwhile, the upper level ridge amplifies over the intermountain
west as energy dives southward into the mid-west. As this ridge
amplifies, the low pressure offshore responsible for the weekend
cooldown, gets pushed northwestward (retrogrades) into the Gulf of
Alaska. We`ll see a warming trend beginning Monday, but models show
the ridge axis being centered over the West Coast States by mid-
week, which is likely result in another warm up across the region
for the Tuesday-Thursday timeframe. Upper 90s for the West Side
(upper 80s for East Side) is nearly certain for this warm up, with a
80-90% chance of triple digits in West Side Valleys for the middle
of next week. While it seems fairly certain that it`ll be hot again
for mid-week next week, with the aforementioned low drifting around
during the same time, cloud cover could end up being a mitigating
factor in the extent of the heat. At this time, the warmth looks to
be shortlived, with the pattern transitioning again toward the end
of next week and a possible cool down for the following weekend.
Regardless, we`ll be evaluating the need for any heat related
headlines over the coming shifts.

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. Latest HRRR
Smoke model does indicate the potential for higher smoke
concentrations in portions of Klamath County as late evening
northerly winds could bring smoke from the Emigrant fire farther
south. Models also show improvement in the afternoons 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, smoke impacts from the Dillon fire in western
Siskiyou County should remain confined to western Siskiyou County.
Models show higher smoke concentrations settling in the
Scott/Shasta/Klamath River Valleys at night, then clearing out with
the increase in afternoon winds as well.

Stay tuned for updates as details become more clear over the coming
days. /BR-y

&&

.AVIATION...28/18Z TAFs...Along the coast and just offshore,
IFR ceilings have improved to MVFR. The marine stratus should remain
in place north of Cape Blanco, but satellite image shows the marine
stratus peeling back towards the coast south of Cape Blanco, and
these areas could briefly experience VFR conditions between 21-02z.
Any improvement will be short lived with IFR and local LIFR ceilings
returning early this evening (2-3z).

Inland, the marine stratus has burned off at Roseburg, and will
slowly burn off at the Coquille Basin and northwest Douglas county
this afternoon. Otherwise, VFR conditions will prevail through the
TAF period with the typical late afternoon and early evening diurnal
breezes, with the exception of some localized lower flight
visibilities in smoke in the vicinity of or downwind from area
wildfires. -Petrucelli

&&

.MARINE...Updated 130 PM PDT Friday, August 29, 2025...Relatively
light south winds and low seas will persist through Saturday. Winds
turn northerly Sunday and increase as a weak thermal trough
develops. Breezy to gusty north winds return Sunday afternoon into
Monday, especially south of Cape Blanco, with steep seas possible by
Tuesday. These conditions are likely to persist through at least mid-
week. /BR-y

&&

.FIRE WEATHER...Updated 200 PM PDT Friday, August 29,
2025...Overall fire concerns are low through Monday. The latest
satellite image shows clear skies over most of the interior, except
for building cumulus in eastern Fire zone 285 and southeast Fire
Zone 625. So far a few isolated showers are showing up on radar east
and southeast of the above mentioned fire zones. It`s not unstable
enough for these showers to produce thunder, but come of the high
res simulated radar reflectivity shows returns, and we`ll have a
slight chance of showers there into early this evening. Given how
dry it is at the lower levels any it`s unlikely any precipitation
from these showers will reach the ground.

Smoke for the Emigrant Fire is being carries north to northeast of
our area. However, some of the guidance shows some of that smoke
shifting south into northern Fire zone 624 late this afternoon and
evening. This could be due to a combination of the typical diurnal
shift in the winds from west to northwest late in the afternoon and
evening and lowering inversion which will allow the smoke to settle
in to the above mentioned areas.

The theme through the weekend will be continued dry weather 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
ind 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 Saturday afternoon and evening
where there could be brief periods in which critical conditions
could be met. This will be highlighted in the fire weather forecast.

An upper low off the Oregon coast will result in a stable southwest
flow Friday and Saturday. Some of the guidance hints at marginal
instability, but the trigger is lacking and mid level moisture is
also marginal at best. Thus the most likely scenario will be
building cumulus in the afternoons, then dissipating in the evening.

Like today, a building pressure gradient will result in breezy winds
over the usual suspects (Rogue, Umpqua Basin, Shasta Valley and east
of the Cascades). With the drying humidities, these winds will push
conditions toward critical values for a few hours each afternoon,
particularly in the southern Shasta Valley and along the ridges.

There`s good agreement the upper low will move little and actually
drift north off the Washington coast Sunday into the start of next
week as an upper ridge axis builds north east of our area.
Temperatures are not expected to vary much day to day and will
remain above normal. We`ll still have to deal with the typical
afternoon and early evening breezes and low relative humidity for
the same areas mentioned above with brief periods of near or
critical conditions. While we may approach headline thresholds, we
do not expect to reach critical conditions at this time.

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

If there`s a window in which thunderstorms could be an issue, it
will be Tuesday and Wednesday afternoon and early evening as weak
upper low drifts north towards our area. Tuesday the position of the
upper low is such that it could result in at least isolated
thunderstorms for portions of northern California, and Cascades
east.

Wednesday, there`s fairly good agreement the upper low will move
north into northern California or southern Oregon. The location of
the upper low Wednesday suggest a greater chance for isolated storms
and also covering a larger portion of the area, and some of the
guidance is already showing precipitation (which is a result of
convective feedback). We have added a slight chance for afternoon
and early evening thunderstorms to the areas mentioned above both
Tuesday and Wednesday.

The other question will be 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.

It`s also worth noting, the upper ridge will remain in place Tuesday
and Wednesday with afternoon temperatures around the triple digit
mark for the interior westside valleys, and upper 80s to low 90s
east of the Cascades. -Petrucelli

&&

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

CA...None.

PACIFIC COASTAL WATERS...None.

&&

$$