327
FXUS66 KMFR 302148
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
248 PM PDT Sat Aug 30 2025

.DISCUSSION...A weak frontal boundary remains draped across the
marine waters this afternoon, and this is associated with a cut
off upper level low that will linger just offshore through the
weekend. While the radar is showing some decent returns with this
front over the waters, area webcams show this is most likely to be
virga and the radar beam height is coinciding with the virga
showers. Surface observations to the north haven`t recorded any
precipitation either and cloud bases are pretty high for any
meaningful precipitation. This front is expected to remain just
offshore and continue to weaken through the evening as it moves
northward, so it will largely go unnoticed for inland areas.
Afternoon temperatures today will be quite similar those of
yesterday, and there will be a slight uptick in afternoon breezes
today.

High temperatures will trend a few degrees cooler for Sunday and dry
conditions are expected to continue through the weekend. The flow
will turn more southerly as low pressure offshore drifts northward
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 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.

Smoke impacts...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 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, smoke impacts from the Dillon
fire in western Siskiyou County should remain confined to western
Siskiyou County, with higher smoke concentrations settling in the
Scott/Shasta/Klamath River Valleys at night, then clearing out
with the increase in afternoon winds as well. Latest observations
in the Rogue Valley show an increase in smoke over the last hour
or so. This appears to be smoke from the Dillon Fire drifting into
the Illinois/Applegate Valleys and then being carried eastward
into the Rogue Valley with the increase in west to northwesterly
winds this 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...30/18Z TAFs...Satellite imagery shows high clouds moving
onshore over coastal areas of southern Oregon. Ceilings should
remain VFR along the coast and just offshore. Marine stratus is
likely to form early this evening (2-4z) with IFR ceilings that could
last until the end of the TAF period, although ceilings could
improve to MVFR towards 18z tomorrow.

Inland, VFR conditions will last through the TAF period. The
exception will be in eastern Douglas county where smoke will result
in MVFR visibilities until 21-22z at which point it should mix out
and dissipate. Meanwhile smoke from the Emigrant Fire could drift
south into portions of northern Klamath County this evening and
tonight,  but it`s not expected to result in reduction in visibility
at the Klamath Falls airfield. -Petrucelli

&&

.MARINE...Updated 100 PM PDT Saturday, August 30, 2025...Relatively
light south winds and low seas will persist through Sunday morning.
Winds turn northerly Sunday and increase some as a weak thermal
trough develops. North winds increase further Sunday night into
Monday, reaching advisory level criteria south of Cape Blanco. Seas
also steepen in response, with low end advisory level conditions
expected by late Monday morning. These conditions are likely to
persist through at least mid-week, with winds easing below advisory
levels for the latter half of the week. /BR-y

&&

.FIRE WEATHER...Updated 200 PM PDT Friday, August 29,
2025...Overall fire concerns are low through Monday with not much
variation in the weather.

The latest satellite image shows some cumulus build ups over the
higher terrain in portions of Fire zones 285, 624 and 625. Meanwhile a
weak frontal boundary associated with the the upper low off the
northwest Oregon coast will bring some high and mid level clouds to
the coast, north of Cape Blanco and northwest Douglas county.

Smoke for the Emigrant Fire is being carried 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 start of next week 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 continued be
highlighted in the fire weather forecast.

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 on
Sunday.

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 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 enters the picture and 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. 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. -Petrucelli

&&

.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.

&&

$$