403
FXUS66 KMFR 311730
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
1030 AM PDT Sun Aug 31 2025

.UPDATE...Updated Aviation Discussion.


&&

.AVIATION...31/18Z TAFs...Along the coast and just offshore, Marine
stratus is dissipating along the north coast, including North Bend
with VFR conditions likely through at least 0z. Meanwhile, marine
stratus remain along the south coast with IFR ceilings and limited
visibilities. The marine stratus in this location could push briefly
offshore with VFR ceilings for the coast later this afternoon, but
confidence on the timing is low.

Inland, conditions will remain VFR through the TAF period. The only
exception may be areas of MVFR visibilities due to wildfire smoke,
which may affect Klamath County (mainly north of Klamath Falls) and
western Siskiyou County at times. -Petrucelli


&&

.PREV DISCUSSION... /Issued 249 AM PDT Sun Aug 31 2025/

DISCUSSION...An upper low has moved northward and is located off
the Washington coast near 47N 129W. Meanwhile over inland areas,
an upper level ridge is present, centered just east of the area.
This is bringing continued warm and dry weather to the area today.
Temperatures will be similar to yesterday. Breezy afternoon and
early evening winds are expected again today. This pattern
continues on Monday as the upper low gradually shifts further
north and the ridge remains in place. Temperatures are forecast to
be similar or slightly warmer on Monday.

Smoke from area wildfires will continue to impact the region,
especially across western Siskiyou county and into Klamath and
northern Lake counties. With southwest to southerly flow aloft,
the bulk of the smoke impacts from the Emigrant fire should remain
north of the forecast area. The HRRR Smoke model continues to
indicate a pattern of higher smoke concentrations moving into
Klamath County and northern Lake County in the evening and
overnight as northerly winds bring smoke from the Emigrant fire.
This is followed by improvement in the afternoons as southwest
winds increase east of the Cascades 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 will continue to bring impacts
across western Siskiyou County, with higher smoke concentrations
settling in the Scott/Shasta/Klamath River Valleys in the evening
and overnight, then improving in the late morning and early
afternoon. The HRRR models also indicates that smoke will filter
into central Siskiyou County, including the Shasta Valley. In the
Rogue Valley, less but still significant impacts are expected. The
HRRR model shows smoke from the Dillon Fire drifting into the
Illinois/Applegate Valleys, then being carried eastward into the
Rogue Valley in the afternoons, before shifting east in the early
evening. Expect periods of hazy conditions and degraded air
quality for the foreseeable future.

The pattern changes heading into Tuesday and Wednesday with a
strengthening upper level ridge, resulting in hotter
temperatures, and an upper level disturbance moving into the area
from the south. Models continue to indicate sufficient mid level
moisture and weak instability on Tuesday to allow for a slight
chance (15-20%) of thunderstorms across northern California and
from the Cascades east. West of the Cascades in Josephine and
Jackson counties, the potential for thunderstorms is lower
(5-10%). There is a low chance (5-15%) for some isolated overnight
thunderstorms or showers Tuesday night, mainly east of the
Cascades. The slight chance for thunderstorms persists on
Wednesday for these same areas as the disturbance tracks northward
over the area. The exact timing and location for thunderstorms
will be dependent of the track and strength of this disturbance.
Ensembles continue to show variability on this feature, so
confidence is low the details. We will continue to monitor and
update as needed. For additional details on the upcoming heat and
thunderstorms, please see the previous discussion below.


MARINE...Updated 200 AM PDT Sunday, August 31, 2025...A weak
thermal trough develops along the coast today, bringing northerly
winds. North winds increase Monday into Tuesday, resulting in low
end advisory level steep seas south of Cape Blanco. These
conditions may persist through mid-week, with winds easing below
advisory levels for the latter half of the week.

FIRE WEATHER...Updated 200 AM PDT Saturday, August 30, 2025...The
area will remain under southwest flow today and Monday with an
upper low off the Washington coast and a upper level ridge
centered inland over the region. This will bring a warm and dry
pattern to the area. Temperatures will trend slighter hotter going
into Monday. Breezy to gusty winds and dry humidities are
expected across inland valleys through Monday. Overall, winds and
humidities are not expected to reach critical conditions.
However, some locally, brief near critical conditions are possible
in the late afternoons, mainly in the Shasta Valley and east of
the Cascades. So, we have continued to headlined the gusty winds
and dry humidities.

The upper level ridge will continue to strengthen on Tuesday and
Wednesday with hotter temperatures expected. Additionally, there
is a slight chance for thunderstorms (15-20%) across northern
California and form the Cascades east as an upper level
disturbances moves up from the south and into the 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. Conditions could be favorable for
nocturnal storms Tuesday night. However, there is more evidence
that does not support storms Tuesday night. We`ll continue to
monitor this.

Wednesday, the operational models differ with the progression of the
upper low. The ECMWF has the low positioned in northern California
as it becomes cut off from the main flow. The GFS also hints at the
low becoming cut off from the main flow, but progresses the upper
low through our area by Wednesday afternoon which would put the best
chance for storms north of our forecast area. In contrast the ECMWF
solution would at least maintain the chance for thunderstorms
Wednesday afternoon and evening. Gut feeling is the slower
progression of the low makes more sense given both models show the
upper low becoming cut off.

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.

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.

PREV DISCUSSION... /Issued 1027 PM PDT Sat Aug 30 2025/

DISCUSSION...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 a
weak low that takes on a negative tilt as it moves northward
through the region Tuesday through Thursday, 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. There is some question as to whether storms develop west
of the Cascades on Tuesday and Wednesday. Guidance the potential
for storms as far west as the Illinois Valley, but not so much
north of the Rogue-Umpqua Divide. This is still 4-5 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 this weekend, 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 will result
in another warm up across the region for the Tuesday-Thursday
timeframe. Upper 90s into the triple digits for the West Side (low
90s/upper 80s for East Side) is expected for this warm up. 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. Additionally, smoke from area wildfires could also play
into how warm temperatures get. HeatRisk values are indicating a
moderate to locally major risk for heat related illnesses Tuesday
through Thursday, primarily for the valleys west of the Cascades, so
we`ll be evaluating the need for any heat related headlines over the
coming shifts. The ridge starts to break down and shift eastward on
Friday, so temperatures are expected to trend cooler but still
remain above normal for early September.

&&

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

CA...None.

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

&&

$$

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