466
FXUS66 KMFR 200947
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
247 AM PDT Wed Aug 20 2025

.DISCUSSION...Clear skies persist across most of the area this
morning, with the exception of some marine layer low clouds and
fog along the immediate coast and just offshore. Some higher
level clouds are skirting across far northwestern Douglas County,
the only indication of the passage of an upper level trough well
to our north. Once this trough passes, the strong "Four-Corners"
ridge will nudge to the west, centering more over the Great
Basin, and becoming the driving force over our forecast area over
the next week as it amplifies up the West Coast.

This ridge, along with the development of a thermal trough along
the coast, will result in another summer heat wave for southern
Oregon and far northern California. The warming trend will begin
today, with highs rising about five degrees over those of
yesterday, and with a Chetco Effect pushing highs along the south
coats near Brookings up by 10 to 15 degrees. Highs across the
interior then rise sharply over the next few days, culminating
with highs near to well into the triple digits for the West Side
valleys, and well into the 90s for the East Side valleys through
the weekend and into early next week. Heat hazard headlines, such
as Heat Advisories or Extreme Heat Warnings will likely be
necessary, starting as early as Thursday but certainly by Friday
as some areas could be around 105-110 for highs. At this time, the
warmest days look to be Fri-Sun with the peak of the heat
potentially Saturday.

The western periphery of the amplifying ridge will also act as a
conduit for monsoonal moisture, which will begin infiltrating
the area this weekend. With the very warm temperatures and a
moistening atmosphere, we will have the first two ingredients for
afternoon thunderstorms in place by Saturday, and remaining there
for several days. The much more uncertain ingredient will be a
triggering mechanism to provide lift. The models are showing some
very weak, broad shortwaves attempting to traverse the periphery
of the ridge, but the timing and strength of these waves will be
the deciding factor, as well as if they can get close enough to
our area to trigger convection. That begin said, the atmosphere
may become unstable enough to produce air-mass convection on it`s
own, with little upper level help. Given the forecast atmospheric
characteristics and the uncertainty of any shortwaves, have kept
a slight chance of thunderstorms in the forecast for each
afternoon Saturday through the end of the forecast term, with the
best chance for showers and thunderstorms arriving roughly the
middle of next week, when the ridge begins to break down and a
larger trough approaches the region form the northwest. If
thunderstorms form, a few could become strong with lightning and
strong outflow gusts as the main threats.

The heatwave is forecast to break along with the ridge sometime
mid week. -BPN

&&

.AVIATION...20/12Z TAFs...Along the coast and just offshore, areas
of IFR to LIFR will persist through much of the morning, before
clearing to VFR by late morning. Lower flight conditions may return
to these areas again late this evening and tonight. Elsewhere, VFR
will prevail throughout the next 24 hours. -BPN

&&

.MARINE...Updated 200 AM PDT Wednesday, August 20, 2025...Relatively
calm conditions this morning will give way to gusty north winds and
steep seas by this evening as the thermal trough rebuilds across the
area. These conditions will then persist through the weekend, with
moderate to occasionally strong north winds and brief periods of
very steep seas south of Cape Blanco. -BPN

&&

.FIRE WEATHER...Updated 200 PM PDT Tuesday, August 19, 2025...
Southwest flow aloft is bringing seasonal temperatures, drier
conditions, and normal diurnal winds to start the week. A shortwave
disturbance looks to move over the area on Wednesday, which will
bring cumulus development over area terrain but other impacts are
not expected.

High pressure in the Four Corners is expected to build a strong
upper level ridge that will affect area conditions on Thursday then
continue through the weekend. This will bring temperatures 5 to 10
degrees above normal across the area on Thursday, possibly into next
week. Most west side valleys have multiple days of 100 degree
temperatures in the forecast. The Umpqua Valley and warmer east side
areas (Modoc and southern Lake counties) are starting to see some
high 90s. Nighttime recoveries are currently forecast to remain
moderate, but periods of easterly flow overnight may bring locally
low recoveries to area peaks and ridges. This possibility looks
higher over western Siskiyou County terrain. While some terrain may
approach critical dryness conditions, right now winds look to stay
light enough to avoid hazardous conditions.

On Saturday, monsoonal moisture is forecast to start increasing over
the area. With ample surface heating expected, signs of convective
thunderstorms are starting to appear on Saturday and Sunday
afternoon and evening. Without significant instability to contribute
to development, coverage is likely to be isolated and generally east
of the Cascades. These possibilities will come into more focus as
the weekend approaches.

The forecast beyond Sunday shows some variability. Temperatures look
to stay above seasonal norms but may cool a few degrees. If
monsoonal moisture continues to move into the area, afternoon and
evening thunderstorms remain a possibility. With current forecast
values, wet thunderstorms seem like a meaningful possibility when
activity does develop. -TAD

&&

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

PACIFIC COASTAL WATERS...Small Craft Advisory from 11 AM this morning
     to midnight PDT Friday night for PZZ356-376.

Small Craft Advisory from 8 PM this evening to midnight PDT
     Friday night for PZZ350-370.

&&

$$