620
FXUS66 KMFR 241555
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
855 AM PDT Mon Jun 24 2024

.DISCUSSION...Stratus sits in the southern Coquille and Umpqua
valleys, as well as a small sliver in the Brookings area. Smoke
is also continuing to settle south over Applegate Lake from the
Upper Applegate fire. Overall, these conditions will make for some
calm conditions today, but that will change over the next few
days.

Our forecast concerns revolve around:

- Tuesday, enhancement of diurnal winds as the thermal trough
  moves inland, which could lead to critical fire weather
  conditions east of the Cascades and in Siskiyou County.

- Tuesday night, there is a faint signal for nocturnal lightning
  as moisture moves in from the south. This will be our main
  forecast element to assess as storms would be high-based and
  dry, and followed by dry, windy conditions.

- Wednesday, an approaching front & trough will increase winds
  east of the Cascades, and could lead to further critical fire
  weather conditions.

&&

.AVIATION...AVIATION (24/12Z TAFs)...VFR conditions continue across northern
California and southern Oregon tonight. A marine push has brought a
stratus layer near the Coos and Douglas counties line. Northerly
winds could push these clouds into the southern Umpqua Valley, but
is not expected over Roseburg.

Other areas will remain VFR through the TAF period with elevated
afternoon winds, especially at the coast. There is a 40-50%
probability for MVFR ceilings at the coast tomorrow morning, so have
added that to TAFs. -TAD/Hermansen

&&

.MARINE...Updated 845 AM Monday, June 24, 2024...An update was made
to the forecast to increase the wind wave (and thus total sea
height) as observations were showing wave heights one to two feet
high than forecast both north and south of Cape Blanco. Otherwise,
the forecast beyond today looks on track.

The thermal trough continues today, and steep seas will persist for
much of the waters south of Cape Blanco into this evening, with very
steep and hazardous seas south of Pistol River. The approach of
another front then disrupts the pattern with improving conditions
tonight into early Thursday. The front is expected to provide a deep
layer of low clouds and a slight chance of showers Wednesday night.

A return of the thermal trough pattern is expected Thursday into at
least Friday night, with steep seas possible south of Cape Blanco.
-Miles/Hermansen

&&

.PREV DISCUSSION... /Issued 445 AM PDT Mon Jun 24 2024/

DISCUSSION...Satellite imagery this morning is showing some low
clouds in the Coquille Valley, and patchy fog along the coast, but
other than that, skies are clear. Dry weather will persist today
with lots of sunshine. High temperatures in the upper 80s east of
the Cascades to the low to mid 90s across NorCal will be about
5-10F above normal. Over the west side, highs will be around 90F
or so in the Rogue/Illinois valleys, but in the low to mid 80s
near Roseburg. Coastal locations will have highs in the upper
60s/low 70s. Expect breezes to pick up again for most areas mid-
late this afternoon and evening before subsiding around sunset.

Mid-level SW flow will increase tonight and Tuesday ahead of an
upper trough offshore, but with a dry air mass in place, we aren`t
expecting any precipitation. It will turn breezy again Tuesday
afternoon/evening, even gusty at times over portions of the east
side. Over there, SW winds peak at 15-25 mph with gusts to 35 mph.
Please see the fire weather section below for more details on
that. Tuesday will be the hottest day, especially for the west
side valleys where temps will peak in the low to mid 90s for
Medford/Ashland/Grants Pass and up around 90F in Roseburg.

Models are showing some mid and high level moisture arriving from
the south that will graze areas south and east of the Cascades
later Tuesday pm/night and into Wed am. So, some cirrus or
altocumulus are possible then, but model precipitation
probabilities remain 10% or less. It isn`t out of the question
there could be some returns on radar over there, but given the dry
air mass below, it`s likely to be virga and not reach the ground.
It should be noted that instability is lacking since this
moisture is arriving primarily Tuesday night, so thunder
probabilities are also very low (10% or less).

The upper trough offshore will move closer on Wednesday and mid-
level flow will continue to increase with another dry cold front
passing through. This will bring the potential for strong, gusty
WSW winds east of the Cascades during the afternoon/evening
(shifting to NW) with gusty NW breezes just about everywhere
else. Local in-house guidance is showing peak wind gusts of 35-45
mph in NE California and over the east side deserts with gusts of
20-30 mph for most other locations. Temperatures will be lower by
at least 10F Wednesday west of the Cascades and by 5F or so over
the east side. Moisture remains limited with this system, but
another strong marine push could induce some light rain/drizzle or
mist along the coast and mountains of Douglas County.

The cool upper trough will swing onshore into the PacNW Wednesday
night into Thursday. The best precip chances are to our north,
but there remains a low probability (<20%) of showers across
northern Douglas County. Thursday is probably the coolest day for
most inland areas with highs only in the upper 70s/low 80s for the
west side valleys and in the 70s to near 80F east of the Cascades
and over NE California.

Warm, dry weather is expected Friday and Saturday with the next
trough/mostly dry frontal system expected to arrive on Sunday.
-Spilde

FIRE WEATHER...Conditions yesterday east of the Cascades and, in
particular, Modoc County, were certainly within the realm of
"critical" in terms of fire weather with gusty winds and very low
humidity during the afternoon/early evening. Some of the more
prone RAWS stations in Modoc County (Timber Mtn, Devil`s Garden)
and also east of the Cascades (Summit, Coffee Pot) reported wind
gusts of 30-35 mph with afternoon RH values down in the 10-15%
range. Today, it will be very dry again with breezes picking up in
the afternoon. Despite similar low afternoon MinRHs, we don`t
think it will be quite as breezy as yesterday. So, while fire
risk is elevated, we`ll have no headlines.

However, as an upper trough offshore approaches on Tuesday, mid-
level flow increases modestly. That, combined with an already very
dry air mass will lead to critical fire weather conditions again
in the afternoon/early evening. We got word from our local fuels
experts that fire danger is increasing to high for areas from the
east slopes of the Cascades eastward. As such, we have issued a
Fire Weather Watch for strong, gusty SW winds of 15-25 mph with
gusts to 35 mph for portions of the Fremont/Winema NF and Modoc NF
in FWZs 624/285. Criteria are a bit higher in FWZ 625 (Deserts),
so no headline there. It will also be quite breezy with low
humidity in portions of the Shasta Valley, but we don`t expect it
to meet the 6-hour duration criteria there.

By Wednesday, the upper trough offshore will move closer and mid-
level flow will increase more substantially. This will be the
windiest day of the next several. We expect another period of
stronger, gusty WSW winds (15-30 mph, gusts 40-45 mph) in
combination with low humidity (around 15%) during the
afternoon/evening, especially east of the Cascades. For most of
the rest of the area, guidance is showing a drop in temperature
and an increase in humidity to help mitigate this threat, but
areas farther east are more susceptible. We`ll see if further
headlines are necessary. Right now, thunder probabilities remain
very low (10% or less), grazing the SE portion of the area.
-Spilde

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Fire Weather Watch from Tuesday afternoon through Tuesday
     evening for ORZ624.

CA...Fire Weather Watch from Tuesday afternoon through Tuesday
     evening for CAZ285.

PACIFIC COASTAL WATERS...Small Craft Advisory until 11 PM PDT this
     evening for PZZ356-376.

     Hazardous Seas Warning until 11 PM PDT this evening for PZZ356-
     376.

&&

$$