322
FXUS66 KMFR 251018
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
318 AM PDT Tue Jun 25 2024

.DISCUSSION...The weather pattern over the next 7 days will be one
that is fairly typical for early summer with upper ridging giving
way to troughs and/or mostly dry fronts as the northern branch of
the upper jet seasonally edges back toward the north. This will
keep the majority of the action well to our north, but the weather
forecast isn`t trivial with a whole host of subtle, but noticeable
day to day changes.

Right now, an upper ridge axis exists near the Cascades. This will
likely bring the hottest day of the stretch to most of the area
today with highs of 90-95F in populated inland areas, perhaps a
degree or two higher/lower depending on location. A minor marine
intrusion is resulting in some stratus near the Coos and southern
Curry coasts, as well as the Coquille Valley this morning, but
these clouds should tend to dissipate through the morning/early
afternoon; it will be cooler there with highs generally 60-70F.
The remainder of the area will start off sunny today with the
usual afternoon/evening NW breezes west of the Cascades and
increasingly gusty WSW breezes over the east side. Satellite
imagery is showing some mid-high level moisture advancing north-
northeastward from California and just offshore this morning. Some
mid-high level clouds will arrive from the south this morning,
but especially this afternoon and tonight, primarily in NorCal and
from about the Cascades eastward.

Models are showing an area of elevated instability (~50-100 J/KG
in the 600-300mb layer) as this moisture axis swings through
tonight into the wee hours of Wednesday morning. This is likely to
generate some elevated showers/virga given a very dry sub-cloud
layer. A few sprinkles could reach the ground. While there isn`t
much forcing for convection, and model ensemble probabilities of
thunder are generally 15% or less, there have been isolated
lightning flashes off the California coast this morning. So, it
certainly wouldn`t be surprising to see a flash or two overnight,
especially from around the Modoc northeast to southern Lake
County. The overall forecaster confidence in lightning is low,
but the official forecast includes this low probability of
occurrence for those areas tonight. We expect this moisture axis
to shift to the north and east away from the area on Wednesday as
another upper trough approaches from the west. The implication of
the nocturnal lightning is that if it occurs and fire starts
result, a period of dry and windy conditions will lead to critical
fire weather conditions east of the Cascades during Wednesday
afternoon/evening. And, this would provide the weather variable
to support any new fires to spread rapidly.

Farther west, however, Wednesday will begin a period of cooling
with high temperatures about 10F lower than today. This will
continue Wednesday night into Thursday morning as the cool, upper
trough pushes inland through the PacNW. During that time, there
is a chance (20-30%) of light rain showers along the
Coos/Douglas/Curry Coasts and into northern Douglas County, but
most everywhere else will remain dry. Temperatures inland on
Thursday will be about 5-8F below normal for highs.

The cool down will be brief, however, as short wave ridging on
Friday brings dry, sunny weather and temperatures back to near or
slightly above normal levels. The next upper trough offshore will
approach the coast Friday night into Saturday and this will lead
to an uptick in afternoon/evening breezes again, especially over
the east side. Shower chances increase (20-40% chance) along the
coast and into Douglas County Saturday night into Sunday as the
upper trough moves onshore yielding yet another cool down for
inland sections. There is also a chance of showers in portions of
northern Klamath and Lake counties (15-25% chance) Sunday
afternoon.

Aside from late night and morning stratus along the coast, dry
weather will prevail early next week. -Spilde

&&

.AVIATION...(25/12Z TAFs)...IFR conditions in marine stratus are
expected to expand in coastal areas overnight into early Tuesday
morning, especially north of Cape Blanco and south of Brookings.
These low clouds are expected to dissipate after sunrise, with VFR
by 18Z. A thicker, more extensive band of IFR stratus is expected
along the coast into the Coquille and lower Umpqua valleys late
this evening into Wednesday morning.

Otherwise, VFR conditions are expected to prevail through the TAF
period for inland locations, with breezy afternoon and evening
westerly winds. Winds will be strongest east of the Cascades with
gusts up to 25 kt.  /DW


&&

.MARINE...Updated 200 AM Tuesday, June 25, 2024...The thermal
trough continues with steep seas for much of the waters south of
Cape Blanco into this morning. The approach of another front then
disrupts the pattern with improving conditions Tuesday into early
Thursday.

The front is expected to provide a deep layer of low clouds and a
chance of showers Wednesday night. A return of the thermal trough
pattern is expected Thursday into Friday night, with steep seas
possible south of Cape Blanco. Another front and deep marine layer
are likely to disrupt the pattern, with improved conditions for the
weekend.
/DW


&&

.FIRE WEATHER...Breezy WSW winds (15-25 mph with gusts to 35 mph)
are expected to develop again this afternoon and evening,
especially for areas east of the Cascades. This, in combination
with another day of minimum relative humidity in the 10-15% range,
will create critical fire weather conditions in portions of the
Fremont/Winema and Modoc National Forests. A Red Flag Warning is
in effect from noon until 8 pm today for that. Meanwhile, mid and
upper level moisture will arrive from the south today. This could
generate some elevated showers/virga tonight, especially from the
Cascades eastward. Models are showing some elevated instability
in concert with this moisture axis, but ensemble model lightning
probabilities are low (15% chance or less). We have seen a few
lightning flashes off the coast of California this morning
associated with this moisture/instability, so it wouldn`t be
surprising to see a flash or two tonight. The best chance for this
to occur will be in eastern Modoc northeast to southern Lake
County. While forecaster confidence in lightning is low, the
implication of the nocturnal lightning is that if it does occur
and fire starts result, we are still expecting a period of dry
and windy conditions Wednesday afternoon/evening. This will lead
to critical fire weather conditions east of the Cascades and any
fires that do start would spread rapidly. As such, we have a Fire
Weather Watch out for the strong, gusty winds and low RH expected
Wednesday afternoon and evening for FWZs 624/625 in Oregon and
FWZ285 in California. Wind gusts could be up to 40 mph. There is
also a chance that winds are strong enough in the Scott Valley
along with low humidity to bring a few hours of critical or near-
critical conditions there. Duration may not be quite long enough,
however. After that, things should calm down a bit fire weather-
wise Wednesday night into Thursday with a cool down, better
humidity recoveries and less wind. It`ll be dry and warmer Friday,
but still not too bad wind-wise. The next upper trough will bring
increasing afternoon/evening breezes Saturday/Sunday.
-Spilde

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Red Flag Warning from noon today to 8 PM PDT this evening for
     ORZ624.

     Fire Weather Watch from Wednesday afternoon through Wednesday
     evening for ORZ624-625.

CA...Red Flag Warning from noon today to 8 PM PDT this evening for
     CAZ285.

     Fire Weather Watch from Wednesday afternoon through Wednesday
     evening for CAZ285.

PACIFIC COASTAL WATERS...Small Craft Advisory until 11 AM PDT this
     morning for PZZ356-376.

&&

$$

MAS/DW