649
FXUS66 KMFR 220514
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
1014 PM PDT Thu Aug 21 2025

.Updated AVIATION and MARINE Discussions...

&&

.AVIATION...22/06Z TAFs...VFR conditions prevail across the region
and will do so through the TAF period. The exception will be along
the coast north of Cape Blanco where LIFR conditions in marine
stratus will return tonight into early Friday morning. Conditions
along the coast will improve to VFR by late Friday morning. /BR-y

&&

.MARINE...Updated 900 PM PDT Thursday, August 21, 2025...A thermal
trough will maintain gusty north winds and steep wind-driven seas
across all area waters through tonight. Winds ease on Friday, with
advisory level winds retreating to the outer waters and south of
Cape Blanco. Have made adjustments to the small craft advisory show
this improvement for areas north of Cape Blanco by Friday morning.
Winds ease even more so over the weekend, hovering near advisory
criteria in the outer waters, which may maintain steep seas through
the weekend and potentially into early next week. /BR-y

&&

.PREV DISCUSSION... /Issued 443 PM PDT Thu Aug 21 2025/

DISCUSSION (Today through Wednesday)...

Overview:

Very little change from previous forecast with items of interest
below:

* Heat: A heat wave is expected to start tomorrow and will likely go
  through early next week. Heat Advisories and Extreme Heat Warnings
  are in effect.

* Rain/Thunderstorms: Increased chances starting Saturday and
  continuing into next week. Widespread severe weather not expected,
  but a couple storms could become strong with lightning and strong
  outflow winds as the main hazards.

* Fire Weather: Not seeing any widespread overlap of both critical
  winds and RH for the needed hours of Red Flag criteria albeit
  tomorrow is close for the Rogue Valley, but certainly lightning
  over dry fuels could warrant fire weather hazards by this
  weekend. Please see the Fire Weather discussion below for more
  information.

Further Details:

Not much has changed in regards to the upper levels. We will have a
broad area of high pressure consuming much of the southern CONUS.
Hurricane Erin will push north and disrupt the high pressure to the
west and northwest which will result in height rises over our
forecast area tomorrow through early next week as Hurricane Erin
gets absorbed into the main flow. This will result in a very warm
airmass with triple digit temperatures expected through the end of
the week and potentially into early next week. Heat Advisories and
Extreme Heat Warnings start tomorrow. The peak of the heat wave will
be Fri-Sun, and records will be within reach for several of our
official climate sites.

The placement of the high pressure will allow for monsoonal moisture
to get pulled northward. At the same time, we will have weak
perturbations riding the northern periphery of this high pressure.
The combination of upper level energy and moisture could result in
precipitation across mainly eastern parts of the forecast area
Saturday through Monday. Saturday being the least areal extent at
this time, but this day also has the highest uncertainty. This means
we could see coverage expand west and north, but currently the
orientation of the upper level ridge may not be favorable quite yet
for both moisture and dynamics aloft. Any shift in the high pressure
could result in both the energy aloft and moisture getting diverted
in either a positive or negative manner as far as precipitation
goes. Will continue to monitor this threat. At this time, widespread
severe weather does not appear likely, but if a thunderstorm forms
it could become strong with lightning and strong outflow gusts (45-
55mph) as the main threats. These storms will likely be tied to
daytime heating, so these would likely be pop-up storms through peak
heating (2pm-8pm) as we reach convective temperatures. The
combination of convective temperatures and small impulses aloft
should be enough to trigger thunderstorms this weekend into early
next week. Lastly, we will need to monitor the unlikely (at this
time) scenario for nocturnal thunderstorms. Saturday night could be
the timeframe we see this nocturnal threat, but this is a very low
confidence scenario, and really Saturday in general has the least
confidence overall at this time.

-Guerrero

FIRE WEATHER...Updated 200 PM PDT Thursday, August 21, 2025...

Stable southwest flow will continue this afternoon with clear
skies over the entire area.

Two main concerns lie ahead in the forecast. First will be hot
afternoon temperatures for the interior starting Friday and
lasting through at least the early part of next week, and
excessive heat warnings and heat advisories remain in effect.
Please see NPWMFR for more details. Second will be the potential
for thunderstorms as early as Saturday afternoon, with storm
potential lasting into the first half of next week.

Moderate overnight recoveries are expected tonight near and at
the ridges in Fire zones 618, 619, and 280. However winds are not
expected to sufficient enough for critical conditions to be met
with 925 and 850 mb winds between 5-10 kts. The mid slopes and
ridges for the interior will also have moderate overnight
recoveries, but winds will be light.

A portion of the four corners high will nudge northwestward into
our area Friday through Saturday with afternoon temperatures
heating up into triple digit territory for the interior westside
valleys and low to mid 90s east of the Cascades. Only the
immediate coast will be immune to the heat with afternoon
temperatures near seasonal norms along with moderate to strong
winds, especially from Cape Blanco north. At the same time
overnight lows will be warm, especially near and at the ridges,
thus there will be little to no overnight relief from the heat.
While relative humidities will be low, Friday afternoon and
evening, winds are not expected to be sufficient enough for
critical conditions to be met for the interior westside valleys
and east of the Cascades. For now, we`ll just headline for near
critical conditions for portions of Fire Zone 622.

Saturday, monsoonal moisture will begin to move up from the
south, and move into the area towards max heating Saturday
afternoon. The trigger is weak, but the combination of
temperatures reaching and possibly exceeding their convective
thresholds, weak trigger and increasing mid level moisture does
raise the concern for at least isolated thunderstorms in portions
of Fire Zones 285, 624 and 625 in the afternoon and evening. If
nothing else, would not be surprised if isolated storms pops up
over the mountains in the above mentioned Fire Zones.
Additionally, winds aloft will be light, therefore any storms
that pop up will be slow movers.

There`s some evidence supporting nocturnal thunderstorms
overnight Saturday as mid level moisture increases with a
continued weak trigger. Some of the models show convective
feedback by way of precipitation fields overnight Saturday.

Sunday, conditions are more favorable for thunderstorm
development as mid level moisture and instability continues to
increase, along with a weak to moderate trigger. Thunderstorms
Sunday will impact the same areas as Saturday. However, the
Cascades and portions of Fire zones 280 and 284 could be in play
for storms. For now areas west of the Cascades should be in the
clear, but it`s not our of the question a couple of storms could
come off the mountains and slip into the southern portions of Fire
Zones 620 and 622 late Sunday afternoon and evening. However with
light steering winds, most likely they will remain along the
terrain. It`s also worth noting the convective feedback
(precipitation fields) is a bit more robust Sunday afternoon and
evening. Nocturnal storms could still be a part of the equation
Sunday night, but confidence on this is low.

Afternoon and early evening thunderstorms remain a concern for
the first half of next week as week. Monday, we`ll be under a
south flow with monsoonal moisture still being pushed into the
area along with marginal to moderate instability. After Monday,
we`ll be sandwiched between the weakening four corners high and
general upper troughiness over the area.

-Petrucelli

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Extreme Heat Warning from 1 PM Friday to 8 PM PDT Monday for
     ORZ024-026.

     Heat Advisory from 1 PM Friday to 8 PM PDT Saturday for ORZ023-
     025-027>031.

CA...Extreme Heat Warning from 1 PM Friday to 8 PM PDT Monday for
     CAZ080-081.

     Heat Advisory from 1 PM Friday to 8 PM PDT Saturday for
     CAZ082>085.

PACIFIC COASTAL WATERS...Small Craft Advisory until 5 AM PDT Saturday
     for PZZ356-370-376.

     Small Craft Advisory until 5 AM PDT Friday for PZZ350.

&&

$$