929
FXUS66 KMFR 270517
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
1017 PM PDT Sat Jul 26 2025


.UPDATE...Updated Aviation Discussion.


&&

.AVIATION...27/06Z TAFs...Along the coast, areas of IFR will develop
tonight into Sunday morning, mainly north of Cape Blanco and from
Gold Beach southward. These low clouds will clear to VFR late Sunday
morning.

Inland, overall, skies are VFR and will remain that way through the
TAF period. However, isolated to scattered thunderstorms Sunday
afternoon and evening, mainly from the Cascades east and across
NorCal. These could temporarily lower ceilings and/or visibility,
but the main risks with thunderstorms will be gusty outflow wind
gusts, cloud to ground lightning and even small hail.

&&

.PREV DISCUSSION... /Issued 200 PM PDT Sat Jul 26 2025/

DISCUSSION...The main weather impact over the next several days
will be thunderstorms. With strong upper ridging over the southern
United States (east of the Four Corners) and an anomalous closed
low centered over the Gulf of Alaska, SW Oregon and NorCal are
in an area of SW flow aloft. We are mainly under the influence of
an upstream trough, and this is keeping temperatures at levels
that are fairly typical of late July levels -- highs in the upper
80s to mid 90s for the valleys west of the Cascades and generally
80-90F over the East Side. It will remain that way from now
through the end of next week.

A weakness within the upper trough is traveling into the Great
Basin this afternoon and this will result in another round of
isolated to scattered storms, mostly for SE portions of the
forecast area. The isolated activity focused around the Mt Shasta
region with more scattered (or slightly higher coverage of storms)
storms from the Sierra up into south-central Oregon. Storms that
form this afternoon will be much like those from Thu/Fri, with
wetting rainfall, strong, gusty outflow wind gusts and even some
hail.

Models have maintained S-SW flow over the area for Sunday, so
thunder chances have increased compared to yesterday`s thinking.
With a bit more moisture expected in the mid-levels and the
presence of a subtle shortwave disturbance moving through at peak
heating, the resultant instability should bring another round of
isolated to scattered thunderstorms from NorCal up across the East
Side during the afternoon/evening. Storm motions should also
increase a bit (15-20 kt) compared to recent days. We`ve issued a
Red Flag Warning for this, please see Fire Weather section below.
Newest CAMs (not all, but a few) are even showing an isolated cell
tracking across SE Jackson County during the evening. The
probability of this occurring is around 15%, so we`ve added that
to the forecast, but convective inhibition is greater the farther
NW you are west of the Cascades. It looks like best opportunity
for this is from Phoenix/Talent southeastward.

On Monday, the flow back subtly to the south. In the absence of a
more distinct short wave, we think activity will be more isolated
and shift to the north from around the Cascades north of Crater
Lake across northern Klamath/Lake counties, then arcing down to
around the Warner Mtns. Isolated thunderstorms could also pop up
again over the Siskiyou Mtns of western Siskiyou County. However,
with the flow being from the south, if strong enough, any isolated
cells that form over western Siskiyou County could travel
northward into Jackson County before dissipating. Right now,
confidence is low in this scenario, so we`ll continue to monitor
and adjust the forecast.

As we`ve been talking about for a while now, Tue-Thu next week
appear to be another active period of showers/thunderstorms across
our CWA. A disturbance will round the base of the long wave trough
and move onshore into the California Coast (south of SF Bay) on
Tuesday. This system will head into the Great Basin Tuesday night,
but the orientation of the trough will take on a negative tilt.
This will allow for upper level flow from the south and perhaps
even SE at times. This is when thunderstorms have their best
chance of making in onto the west side of the Cascades. We have
continued to increase PoPs during this period with even the
potential for some nocturnal activity Wednesday night. It won`t
be until Friday or Saturday that we get into a more typical WSW
flow aloft and thunder chances decrease again. -Spilde

MARINE...Updated 200 PM PDT Saturday, July 26, 2025...Sub-advisory
north winds and low seas of 5 ft or less, will persist through the
weekend into early next week with nightly stratus and fog that
will last at least into the morning hours each day. Winds and
seas may trend slightly higher around Tuesday of next week,
especially south of Cape Blanco.

FIRE WEATHER...Updated 100 PM PDT Saturday, July 26, 2025...
Temperatures, humidities, and breezy afternoon winds will all remain
near seasonable levels this week, so the main fire weather concern
will continue to be daily thunderstorm chances. We`ll be in a
relative lull in terms of thunderstorm coverage today through
Monday, with a slight uptick in activity on Sunday. Then from
Tuesday onward, we expect a heightened level of thunderstorm activity
through Thursday/Friday, and this could include areas west of the
Cascades.

For today, another round of isolated thunderstorms is anticipated
for northern California, mainly east of the Shasta Valley into the
Modoc as well as southern and eastern Lake Counties. There is the
potential for some scattered activity across far eastern Modoc/Lake
Counties along the Warners, but given the recent rainfall and in
coordination with partner agencies, a headline in the Fire Weather
Planning Forecast (FWFMFR) is sufficient for today`s activity.
Models have trended more active for Sunday`s activity, indicating
scattered coverage from the Scott Valley in Siskiyou County
eastward, extending across the Shasta Valley to the Medicine Lake
area northward into Klamath, and Lake counties. We`ve decided to go
with a Red Flag Warning for these areas for Sunday
afternoon/evening, and details can be found at RFWMFR. Isolated
storms are expected across the Modoc, and maybe even as far west as
the Cascades and Rogue Valley (10-20% chance), but confidence wasn`t
high enough in scattered coverage to include these areas in the Red
Flag at this time. Guidance holds steady with isolated thunderstorms
anticipated on Monday, focused over northern portions of Klamath and
Lake counties into central and eastern Oregon. Current guidance
shows a scenario where storms develop on the northern and eastern
peripheries of FWZs 624/625, then continue moving north and
eastward. Some isolated development is also possible across western
Siskiyou County, but again, coverage does not warrant any
watches/warnings at this time.

For the upcoming week, attention remains focused on another trough
that swings into central California and turns the flow southerly as
it tracks northward into eastern OR/western ID. This pattern will
maintain daily thunderstorm chances through at least Thursday, if
not into Friday. Confidence is highest for areas east of the
Cascades and across northern California to see at least scattered
coverage during this pattern, but the more challenging aspect is how
far west will storms make it, and will there be any overnight
(nocturnal) thunderstorm potential this week. For now, the forecast
supports increased thunderstorm activity for East Side/norCal
beginning on Tuesday. Areal coverage of storms looks greatest on
Wednesday with this day seeing the best potential for storms west of
the Cascades. Current forecast maintains similar conditions on
Thursday, and is even hinting at some nocturnal potential for the
West Side Wednesday night into Thursday morning. By Friday, activity
should trend less, though confidence in details is low at this
point. Differences in timing and location of these shortwaves will
be challenging to resolve until just a few days in advance, so
expect changes to the forecast over the coming days. /BR-y

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Red Flag Warning from 2 PM to 11 PM PDT Sunday for ORZ624-625.

CA...Red Flag Warning from 2 PM to 11 PM PDT Sunday for CAZ280-281-
     284.

PACIFIC COASTAL WATERS...None.

&&

$$