861
FXUS66 KMFR 220542
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
1042 PM PDT Mon Jul 21 2025

...Updated the Aviation section...

.AVIATION (06Z TAFs)...LIFR stratus is expected to persist near
Brookings through around 17Z Tuesday morning. Meantime, a mix of
LIFR/IFR stratus offshore is expected to surge into the coast
overnight from Cape Blanco northward. This includes North Bend, the
Coquille Valley, and the lower Umpqua Valley. Toward sunrise, a few
hours of patchy MVFR is possible in the central Umpqua Valley,
including Roseburg. Otherwise, the main factor for inland sites
Tuesday afternoon through Tuesday evening will be slight chances of
showers and thunderstorms to the south and east of the Rogue Valley.
This probability looks to be highest for Lake County, extending into
northern and eastern Klamath County. -DW

&&

.MARINE...Updated 800 PM PDT Monday, July 21, 2025...North winds
will approach advisory levels on Tuesday afternoon and evening from
Cape Blanco to Gold Beach and within 20 nm from shore. Otherwise,
with high pressure centered far offshore and a thermal trough far
inland, sub-advisory north winds and low seas at or below 5 ft will
likely continue through the week, and possibly into early next week.
-DW

&&

.PREV DISCUSSION... /Issued 830 PM PDT Mon Jul 21 2025/

DISCUSSION...Radar is showing one cell near the Lake County
border that is heading east, but otherwise things are quiet.

Hopefully today was enjoyable for you, as it was 1 to 15 degrees
cooler than normal in the 70s and 80s! In the coming days high
pressure will build in the area and this will bring temperatures
back to near normal in the 80s and 90s.

There was a minor change for tomorrow`s thunderstorms chances as
they extend into far eastern Lake County at 15-20%, but the area
between Silver Lake down to Mount Shasta still stands. The
forecast is still on track for the next few days to have
thunderstorm chances and Thursday looks to be the larger impact
day. Please see the previous discussion for more details.
-Hermansen

PREV DISCUSSION... /Issued 452 PM PDT Mon Jul 21 2025/

DISCUSSION (Today through Sunday)...

Key Messages:

    * Below normal highs (~10 degrees for some areas) today
    * Thunderstorm chances essentially every day this week
    * Thursday has the highest potential for widespread activity
        - Strong to severe storms possible
        - Hazards: Abundant lightning, strong/severe outflow gusts
            - Cannot rule out hail around 0.50" - 1.00"

Further Details:

The overall pattern--more or less--can be summed up as a trough
over the Pacific Northwest through the upcoming week. For
tomorrow, models are consistent with keeping the trough in an area
favorable for thunderstorms. This is right before the cut-off low
breaks away on the base of this trough. The trough axis location
will be very crucial tomorrow so its worth monitoring the location
for activity east of the axis. Moisture is lacking early through
the day, but theta-e values at 700mb do ramp up later in the day
to the east of the trough axis. This setup would help allow
thunderstorms to spread over northern California, as well as areas
east of the Cascades through the afternoon and early evening
hours. Lightning would be the main threat in addition to strong
erratic outflow gusts around 40-55mph.

By Tuesday night, the parent trough over the PacNW will
transition east, and models continue to produce a cut-off low from
the base of the parent low. This cut-off low will likely form
over the Pacific just west of San Fransisco. This will meander
around this location through roughly Friday night or as late as
Saturday morning before getting absorb into the mean flow. As this
cut-off low forms, it will spread PVA across the area and this
pattern would be conducive for thunderstorms as we reach peak
heating each afternoon Wed-Fri; however, at this time Wednesday
has very little coverage area over eastern Modoc County.

Thursday could be an active day given the potential for higher
end variables to phase together. We could have scattered to
perhaps widespread activity across the area. However, it should be
noted that split flow patterns and cut-off lows can be difficult
to forecast , especially this far out because slight changes can
have large impacts. Many ensemble members continue to show the
potential for MUCAPE to reach values around 1000 J/kg with ample
bulk shear (0- 6km) in some cases around 25-40 knots. Moisture
advection will be ample with very high 700mb theta-e values
advecting into the region on Thursday. This setup could bring
strong to severe storms across the region with abundant lightning.
This being several days out and a cut-off low situation, we could
see changes to the orientation of the trough which would affect
moisture pull and timing/coverage of convection. There is low
confidence scenario where some storms spread over into both the
Rogue and Illinois Valleys with the Rogue being more likely.

Friday will have the potential for thunderstorms as well, but a
lot will depend on how the cut-off low tracks and gets absorbed
into the mean flow. That said, cannot rule out further activity on
Friday with lightning being the main threat at this time.
Isolated thunderstorms possible on the eastside Saturday, but this
is another low confidence scenario given uncertainty in the upper
level pattern.

-Guerrero

FIRE WEATHER...Updated 100 PM PDT Monday, July 21, 2025...With a
relatively "cool" and moist air mass over the region today, fire
weather concerns are relatively low today. Measurable
precipitation has been recorded along the coast, coastal mountains
and across northern portions of Fire Weather Zone 625. Today will
be the coolest day of the forecast period and generally improved
humidities across the area. Conditions trend warmer and drier on
Tuesday, with near normal temperatures returning for the remainder
of the week. Generally good recoveries are anticipated through
the week.

Late Tuesday onward, the attention turns to thunderstorm
potential as low pressure develops offshore of California. At this
time, Thursday looks to be the most active day with the greatest
coverage potential, including west of the Cascades. Please see the
discussion above for current thoughts on the upcoming
thunderstorm pattern.

&&

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

CA...None.

PACIFIC COASTAL WATERS...None.

&&

$$