449
FXUS66 KMFR 040349
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
849 PM PDT Sun Aug 3 2025

.DISCUSSION...More showers and thunderstorms moved through Klamath
and Lake county earlier today, and Klamath Falls had one right
over the observation site which picked up around 0.5" of rainfall
from the passing cells! Any remaining cells will continue moving
to the northeast and weaken.

Tonight will be slightly cooler with more spots falling into the
50s west of the Cascades and 40s to low 50s east. Monday
afternoon`s temperatures will be comparable to today, 5-15 degrees
below normal. There will be a 20-30% chance for showers in
Lake County tomorrow morning as the trough passes over the region.
Please review the previous discussion for more details.


&&

.AVIATION...04/00Z TAFs...Conditions will be VFR this evening, with
isolated thunderstorms from eastern Siskiyou County into Klamath and
northern Lake counties through around 04Z.

IFR conditions in low clouds and fog are expected at the coast and
into coastal valleys from Cape Blanco northward and Brookings
southward from around 09Z tonight into 17Z Monday morning.
Otherwise, VFR will persist into Monday evening. Weak instability
will bring a slight chance of thunderstorms to far eastern Lake
County on Monday afternoon. -DW

&&

.MARINE...Updated 800 PM PDT Sunday, August 3, 2025...We are
expecting sub-advisory conditions with seas in the 5 ft or less
range through midweek. A slight increase in seas is possible on
Wednesday as a light northwest swell (4-6 ft @ 10-12 seconds) moves
into the waters. A stronger thermal trough could develop Thursday,
with a possible peak of strength on Friday, as things heat up
inland. This would bring the potential for stronger north winds and
steep to very steep seas over the coastal waters through the weekend.
-DW


&&

.PREV DISCUSSION... /Issued 109 PM PDT Sun Aug 3 2025/

DISCUSSION (Today through Saturday)...

Overview:

Today and tomorrow will have thunderstorms in the forecast
through the afternoon and early evening hours. Tomorrow will be
for a very small isolated area over eastern Lake County. Main
threats with these are going to be lightning and potentially
strong outflows. A Red Flag Warning is in effect today for
lightning over dry fuels. Please see the "Fire Weather" section
below for further details. Thereafter, the forecast becomes less
impactful with only light rain/drizzle (coast and Umpqua Basin)
through mid- week with relatively cooler temperatures for this
time of year. Temperatures return to near normal by Friday with
triple digits returning through the weekend. This will likely lead
to the strengthening of a thermal trough and a potential Chetco
Effect occuring Friday/Saturday.

Further Details:

An upper level trough is currently passing through the PacNW, and
this will help support thunderstorm activity this afternoon for
areas mainly on the eastside where the better dynamics will be
located. Lapse rates in the low to mid levels will be steep today
as noted by inverted-v soundings, and effective shear around 20-30
knots is possible. The result will be isolated strong to perhaps
severe cells through early this evening, and these will likely be
very pulsy in nature as well. A couple storms could produce
strong outflows with gusts mainly around 40-55 but an isolated 60
mph gust is not out of the question. Small hail will be the
general theme as well, but the strongest storm could produce hail
near an inch. These threats would be primarily between 2pm and
9pm.

The aforementioned trough will be exiting to the east by
tomorrow, but the trough axis may lag slightly tomorrow
morning/afternoon which could result in an isolated storm or two
over Lake County. We are talking about a very small area.
Convective allowing models are hinting at some isolated activity
tomorrow so felt inclined to include this in the forecast given
the threat of lightning.

The flow then becomes more zonal with a ridge of high pressure
over the Desert Southwest/Southern Plains. Models are indicating
weak perturbations within the zonal flow Tues/Wed that could help
produce weak shower activity mainly along the coast the the Umpqua
Basin. Overall, not expecting much--if any--QPF either of these
days, but the chance is nonetheless there for western areas to see
light precipitation. Chances are much higher towards the northern
coast of Oregon.

500mb heights will slowly rise Friday through the weekend which
will result in a much hotter airmass. This could result in triple
digits for our typical hot spots like westside valleys. This may
result in at least a small chance for heat related hazards, but we
have several more days to analyze before making that decision.


FIRE WEATHER...

Scattered strong to possibly severe thunderstorms with frequent
cloud to ground lightning will be the main fire weather concern
today into this evening as an upper trough swings into the PacNW.
A Red Flag warning is in effect for FWZs 624 and 625 for abundant
lightning on dry fuels. Current radar is showing cells developing
right now from the Cascades eastward across northern Klamath/Lake
counties. This is also the main area of concern for potential
severe weather this afternoon (wind gusts >=58 mph and/or hail
>=1.00" in diameter), but isolated strong storms will also extend
to the south and west into portions of NorCal (NW Modoc County
into central and eastern portions of Siskiyou County). Even in the
absence of thunderstorms, occasionally moderate to gusty breezes
will develop this afternoon/evening. Add in gusty, erratic
outflows around thunderstorms and this could result in increased
fire behavior for existing fires.

Storm motions today will be faster than in previous days. So,
while atmospheric moisture content is still adequate to support
rainfall, wetting rainfall chances are a bit lower due to the
faster-moving storms. Activity will move from the SW toward the
NE. Soundings are still showing inverted V, a good indicator for
potential strong outflow due to a drier sub-cloud layer and
increased downdraft CAPE. These are most pronounced over the far
east and northeast portion of the area.

Activity could last into the evening, but should become more
isolated around or after sunset. Models move the trough axis
through the area tonight into Monday, so showers and isolated
thunder could linger in far eastern sections, but should drop out
of the forecast the remainder of the week as a drier, more stable
pattern takes hold.

Another upper trough offshore will graze portions of the area at
midweek and this should keep temperatures near to below normal
area wide. There might even be light showers or some drizzle along
the coast during the Tue night to Wednesday night time period.

After that, a strengthening thermal trough will develop in NorCal
and the SW Oregon coast, bringing some increased E-NE winds
starting Thursday night and continuing into next weekend. This
likely will bring moderate to locally poor overnight recoveries
near and at the ridges in southwest Oregon and Fire zone 280.
These offshore winds could extend to the SW Oregon coast Friday
with warm temperatures making it all the way to the beaches near
Brookings. The interior will also heat up, but more so this
weekend. Temperatures will approach and possibly exceed 100F in
the Rogue Valley Sat/Sun.

-Spilde

&&

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

CA...None.

PACIFIC COASTAL WATERS...None.

&&

$$