168
FXUS66 KMFR 030509
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
1009 PM PDT Tue Sep 2 2025

.DISCUSSION...Updated Aviation Section...

&&

.AVIATION...03/06Z TAFs...Along the coast, stratus/fog has
returned to the coast and will return to the coastal valleys again
tonight.

Thunderstorm activity may continue overnight along and east of the
Cascades. Best probability for a thunderstorm is at Klamath Falls,
but one could occur at Medford/Roseburg early in the morning too.
Gusty outflow winds and periods of reduced flight conditions can be
expected with any thunderstorms that occur over the next 24 hours.

Otherwise, VFR will prevail. 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. -
BPN/Hermansen

&&

.PREV DISCUSSION... /Issued 938 PM PDT Tue Sep 2 2025/

DISCUSSION...Thunderstorms have been isolated across the area
through early this evening. Most have occurred or are occurring
in NorCal and east of the Cascades. The only storm report we had
was a gusty outflow from a thunderstorm in Christmas Valley that
knocked out power. A couple of cloud to ground strikes also
occurred within a cell cluster that ran from just NW of Roseburg
to around Florence earlier this evening. Those cells have lifted
off to the NW of our area as of 9 pm. Radar is currently showing a
line of cells developing east of Highway 97 from around Crescent
southeast down to around Bly. The Red Flag Warning for lightning
on dry fuels remains intact for areas from the Cascade Foothills
in eastern Douglas County over the Cascades to the East Side
(Klamath/Lake counties) and also into eastern Siskiyou/Modoc
counties. The Red Flag warning that was out for FWZs 280, 281 and
621 has been allowed to expire (at 9 pm). There is still some
lightning risk in those zones too overnight into Wednesday, but
activity should be isolated. Current CAM guidance shows more/less
an area of showers with embedded lightning developing and moving
northwestward from NW Nevada and NE Cal into SW Oregon overnight
into Wednesday. Isolated thunder is also a possibility west of the
Cascades and even here in the Rogue Valley, but the main risk
remains from the Cascades eastward and into eastern Siskiyou/Modoc
counties. The pattern of showers and thunderstorms generally
remains across the area day to day through the end of the week
with temperatures remaining above normal. However, heading into
the weekend, models are showing a cooling trend back toward
normal for early September. Models continue to show the potential
for the first true taste of fall-like weather beginning next week
as a stronger trough/closed low approaches the coast and moves
onshore. This should also bring a continued chance for beneficial
precipitation, higher humidity and lessening fire weather
concerns. -Spilde

MARINE...Updated 830 PM PDT Tuesday, September 2, 2025...Northerly
winds will persist this week. Winds and seas will hover at advisory
levels south of Cape Blanco through Wednesday night. Conditions are
expected to improve for the latter half of the week as winds ease
and seas lower. Winds turn westerly to southerly into the
weekend. /BR-y

PREV DISCUSSION... /Issued 249 PM PDT Tue Sep 2 2025/

DISCUSSION...

-Hot temperatures and thunderstorms will be the primary concerns
through Thursday.

-Thunderstorm potential continues into the weekend but temperatures
trend cooler from Friday onward.

-Smoke and haze will continue to bring periods of degraded air
quality to the region, worst in the vicinity of ongoing wildfires.

-Early next week, upper level trough likely moves over the region,
potentially bringing below normal temperatures and the chance for
light, but beneficial rainfall.

High pressure will amplify over the Intermountain West this week as
strong low pressure dives southward over the eastern CONUS and low
pressure lingers over the eastern Pacific. Meanwhile, there will be
two weak, negatively tilted, shortwave troughs that move northward
through the region along the western periphery of the ridge. The
first is approaching the area this afternoon and is expected to be
north of the region by Thursday morning. The next swings through
Friday into early Sunday.

With the ridge amplifying, hot temperatures are expected today
through Thursday. With temperatures expected to be 10 to 15 degrees
above normal temperatures during this time, there will be a moderate
risk for heat related illnesses. There could be period this
afternoon when there a major risk of heat related illnesses in the
Rogue Valley, but the overall impact in the region will mainly be
for those who are sensitive to these temperatures, especially those
without adequate cooling/hydration. Afternoon temperatures look to
remain fairly steady today through Thursday with high temperatures
in the upper 90s (80s)/low 100s (90s) for valleys west (east) of the
Cascades. Cloud cover from anticipated thunderstorms (more on this
below) could keep temperatures a few degrees cooler here and there,
but not enough to really notice. Even still, guidance shows the
probability of reaching or exceeding 100 degrees today (for Medford)
around 90% whereas that probability drops to around 50-60% for
Wednesday and Thursday. Temperatures trend cooler Friday into the
weekend, but will still remain above normal until late weekend/early
next week.

Thunderstorms...the first of two negatively tilted shortwaves is
approaching the region this afternoon as noted by the high level
clouds moving into the area. Overnight and earlier this morning, as
this shortwave moved northward through the Sacramento Valley, it
kicked off thunderstorms overnight and produced a fair amount of
lightning. This shows that the elevated instability is there. Models
have come into better agreement regarding how fast the trough exits
the region, but slowed the arrival a bit. This slow down now means
there is a better chance for some overnight shower/thunderstorm
activity tonight. Before then however, we expect isolated to
scattered thunderstorms in the area this afternoon and evening,
focused along/east of the Cascades and across northern California.
Storm motion is forecast to be around 15-25 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 this evening. The fly in the ointment, however, is the area
of cloud cover currently moving into the region, and to some degree
the amount of smoke around. Cloud cover has been known to limit
surface heating/instability and smoke can choke out thunderstorms as
well. With these two factors present this afternoon, it`s possible
that thunderstorm activity isn`t as widespread as what is currently
forecast.

Overnight tonight, guidance maintains some isolated
shower/thunderstorm potential over the region. The majority of the
high resolution models are bringing a band of showers that stretches
across Modoc/southern Klamath/central Jackson Counties northeastward
into the Umpqua Basin by the early morning hours. The probability of
thunder is pretty low in the CAMs, but given the activity seen in
the Sacramento Valley last night, we`ve maintained a thunder mention
in the forecast for the overnight period. Thunder chances continue
into Wednesday as the trough moves overhead. With the timing of the
trough, it could be one of those days where storms are firing off a
little earlier in the day compared to normal. Expected another day
of isolated to scattered thunderstorms that should wind down after
sunset. Overnight chances Wednesday look to be less with the trough
moving just north of the forecast area Wednesday night. We should
see a relative lull in storm activity on Thursday. The GFS maintains
some energy and instability on Thursday, but looks to be less than
today and Wednesday, so storms should be more isolated.

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 see additional shortwave troughs pass
through the region through the weekend. The next is expected to
approach the region Friday, bringing the return of scattered
thunderstorm chances Friday into Saturday. It`s a bit early to try
to pinpoint details at this time, but current guidance indicates
thunderstorms possible as far west as the coast with this shortwave.
Stay tuned for updates as details become more clear.

Beyond Saturday, the parent trough over the eastern Pacific
responsible for sending these shortwaves through the area moves
closer to the West Coast. While it`s too early to say with
confidence, models generally agree on this trough moving inland
through Oregon, bringing below normal temperatures and chances of
light, but beneficial precipitation. This by no means looks like a
season ending event with the current forecast only having around a
tenth of an inch of rain in the forecast, but certainly a system to
moderate fire weather concerns. There are differences in the
guidance and ensembles on the timing of this beneficial rainfall,
with some saying early next week and others saying more towards the
middle of next week. Again, stay tuned for updates and/or changes to
the forecast as the time gets closer. /BR-y

AVIATION...02/18Z TAFs...Along the coast, VFR prevails today, with
the exception of the near-shore waters and immediate coastline south
of Gold Beach, where marine layer stratus/fog persists. Expect
stratus/fog to return to the coast and the coastal valleys again
this evening and tonight.

Moisture returning from the south and east this 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. Thunderstorm activity may continue overnight
along and east of the Cascades. Gusty outflow winds and periods of
reduced flight conditions can be expected with any thunderstorms that
occur over the next 24 hours.

Otherwise, VFR will prevail. 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. -BPN

MARINE...Updated 100 PM PDT Tuesday, September 2, 2025...Northerly
winds will persist this week. Winds and seas will hover at advisory
levels south of Cape Blanco through Wednesday night. 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 Tuesday, September 2, 2025...A
shortwave travelling along the southwest flow aloft has produced
numerous thunderstorms across California this morning and early this
afternoon. This wave will continue to push north, and will pass over
far northern California and southern Oregon this afternoon. This
impulse should be able to tap into the hot temperatures and
increasing moisture to produce unstable conditions and thunderstorms
over much of the area today. Models continue to show the most likely
area of thunderstorm development to be from the Cascades east and
Siskiyous south this afternoon and evening. A Red Flag Warning
remains in effect for these areas due to the potential for abundant
lightning on dry fuels.

There is a potential for south/southeast to north/northwest steering
flow pushing isolated storms off terrain and into portions of the
Rogue and Umpqua basins this evening.

Then, with the impulse still overhead, thunderstorms could linger
along and east of the Cascades through tonight and into the early
morning hours. Any nocturnal storms are likely to be high based, and
therefore are much more likely to be dry. Instability will redevelop
over portions of the area Wednesday afternoon, with scattered
thunderstorms possible yet again, but this time mainly focused along
the Cascades, the Warners, and other areas of high terrain of the
Eats Side.

Aside from the threat of lightning, 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 to no rainfall.

Additionally, today 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.

Isolated thunderstorms are possible Thursday, again along and east
of the Cascades, as the influence from the larger upper level trough
to our west continues. Friday into Saturday, a low nears the coast
and passes onshore, resulting in cooler, more seasonable
temperatures, and additional chances for showers and thunderstorms
over inland areas Saturday. -BPN

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Red Flag Warning until 8 AM PDT Wednesday for ORZ617-623>625.

CA...Red Flag Warning until 8 AM PDT Wednesday for CAZ284-285.

PACIFIC COASTAL WATERS...Small Craft Advisory until 5 AM PDT Thursday
     for PZZ356-376.

&&

$$