699
FXUS66 KMFR 182142
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
242 PM PDT Fri Jul 18 2025

.DISCUSSION (Today through Thursday)...

Overview:

Hot, dry, and breezy conditions continue today. There is also a
small chance (10%-20%) for thunderstorms this afternoon across
Modoc County and southern Lake County. This weekend we will
transition to "cooler" temperatures which will be closer to normal
for this time of year. By Monday, areas recently in the triple
digits will see temperatures fall under 90 degrees which will be a
nice relief for some folks. In general, 70s/80s for highs on
Monday across the forecast area for the coolest day in the
forecast. Monday also starts a pattern change with precipitation
chances possibly increasing through the week, but confidence is
low at this time. The upper level disturbance passing that could
give us precipitation will also bring increased chances for breezy
to gusty wind speeds, especially the eastside next week.

Further Details:

At 500mb, there is a trough passing over southern parts of
British Columbia and Washington. Some models are producing weak
PVA across the area today, and this could phase with convective
temperatures through peak heating. This has warranted an area of
10-20 percent PoPs for Modoc County and southern parts of Lake
County. This is a weak forcing event with very little MUCAPE (<300
J/kg), but we just could not rule out an isolated thunderstorm or
two this afternoon/early evening. The last couple afternoons
where we had a similar weak forcing situation where the HRRR and
Rapid Refresh Forecast System (RRFS) did well. We leaned PoPs away
from NBM and towards the HRRR/RRFS for this situation. Not
expecting severe weather, but isolated lightning and gusty erratic
winds are possible today and early this evening. Smoke and haze
is spreading across the region, so this may inhibit thunderstorms
further. The smoke and haze could also bring unhealthy conditions
to sensitive groups today as we continue to deal with wildfires
across the region.

The aforementioned trough over southwest Canada/PacNW will expand
and become a broader area of cyclonic flow over the PacNW through
the weekend. This will usher in cooler air through the weekend
and early next week, and there is good agreement between global
models. Depending on how far south the trough expands, we could
see both wind speeds and precipitation chances increase starting
early next week. By Wednesday, models are in agreement with a cut-
off low developing over the Pacific (~38N/128W) with this low
pushing northeast over our forecast area and through the PacNW
middle to late next week. As we transition to this cut-off low,
there will be small chances for precip/thunderstorms Monday and
Tuesday as well. This time of year (warm summer months), we
consistently hit convective temperatures in the afternoons, so it
doesn`t take a lot of forcing to get a thunderstorm or two to pop
up. However, its extremely difficult to forecast exactly where
this may happen. All that said, we have two periods next week with
thunderstorm chances. First, is the Mon/Tues timeframe as the
shortwave over southwestern Canada starts expanding southward.
Then, once we transition to the possible cut-off low Tuesday
night, we could see those chances increase again Wed/Thurs.
Variables are going to have to phase together, so its a little too
early to say with much confidence the finer details, but those
chances will be there next week. The Wed/Thurs timeframe has the
potential for stronger storms as some models are creating a
negatively tilted trough as it passes over the forecast area with
MUCAPE values progged to be around 400-900 J/kg with 0-6km shear
around 25-40 knots. Stay tuned as we refine the details in the
coming days.

-Guerrero

&&

.AVIATION (18Z TAFs)...

Weaker offshore flow should clean up the lower ceilings along the
coast briefly this afternoon. With some stronger onshore flow
later tonight, we expect LIFR and IFR ceilings to move back into
the coast with a stronger marine push into Saturday. The Board
Shanty Fire in eastern Josephine County could produce some smoke
impacts near KMFR over the next few days. We did drop visibilities
down to 6SM this afternoon, although the KMFR ASOS is still happy
with 10SM as of now. There is also a very low chance of more
thunderstorms in eastern Lake County today.

-Smith

&&

.MARINE...Updated 200 PM PDT Friday, July 18, 2025...Sub-advisory
conditions will persist through early this evening. Thereafter, a
weak thermal trough develops along the coast overnight and
persists through much of the weekend. This will bring the gusty
north winds, but considerably weaker than the last event. Low end
advisory level conditions are expected south of Cape Blanco
beginning this evening and continuing through Sunday morning.
Winds weaken Sunday as an upper level trough settles over the
region and disrupts the thermal trough pattern. Relatively light
winds and seas are expected Monday and through much of next week.

&&

.FIRE WEATHER...Updated 230 PM PDT Friday, July 18, 2025...
Fire weather conditions remain elevated today into Saturday as a
trough passes to our north. RHs settle into the lower teens with
wind gusts around 25 to 30 mph over exposed terrain east of the
Cascades. Humidities trend about 8 to 15% higher along the
Cascades and farther to the west on Cascades by Saturday. However,
look for RHs in the lower teens around Modoc and southern Lake
County

Conditions begin to improve by Sunday as RHs move about 3 to 5
percent higher on Sunday with some breezy conditions still
persisting.

By Monday, the upper level trough deepens over northern OR/WA,
and this will be the coolest day of the forecast period. Minimum
rhs are unusually high east of the Cascades as this "cooler" moist
airmass moves in. By Tuesday, the trough elongates over the
region, resulting in another day of "cooler" temperatures.
Guidance shows energy from this trough forming another low
pressure off the coast of California around mid-week. This pattern
would result in the return of thunderstorms to the forecast for
the remainder of the week. There are differences on the
position/location/strength of this low, so stay tuned for updates
to the forecast as the time gets closer.

-Smith/Firmin

&&

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

PACIFIC COASTAL WATERS...Small Craft Advisory from 8 PM this evening
     to 11 AM PDT Sunday for PZZ356-376.

&&

$$