399
FXUS66 KMFR 151150
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
450 AM PDT Sun Sep 15 2024

...New Aviation Section...


.AVIATION (12Z TAFs)...

A complicated TAF cycle is underway as an upper level disturbance
continues to pass through the region. We will see low clouds
(MVFR) and showers increase across the region today. There is also
a low (10-15%) potential for a thunderstorm today, but through
this cycle Klamath Falls would be the main terminal for these
thunder chances and overall best chance for rain showers.
Otherwise, isolated to scattered showers today could impact the
other terminals, but confidence was too low to include in this
cycle.

Guerrero

&&

.PREV DISCUSSION... /Issued 251 AM PDT Sun Sep 15 2024/

SYNOPSIS...It`ll feel more like mid-October than mid-September
the next several days as two upper troughs swing through the area.
This will bring a cooler than normal weather pattern with some
showers, isolated to scattered thunderstorms and occasionally
breezy afternoon winds. We`ll maintain this pattern through
Wednesday. However, signs are pointing toward a drier, warmer
pattern moving in late this week (beginning Thursday) and
continuing into next weekend. We will have to watch a system
coming over the top that could graze the area later Saturday into
Sunday.

DISCUSSION...An elongated upper trough, closing off into an
upper low near Vancouver Island this morning, will usher in a
cooler air mass across SW Oregon and northern California today
along with some showers and isolated/scattered thunderstorms. This
will be the first of two upper troughs/lows that traverse the
area today through the middle part of the upcoming week resulting
in a pattern more resembling mid-October than mid-September. Highs
today will be down about 8-12 degrees compared to yesterday for
the west side valleys (low 70s) and 10 to as much as 20 degrees
lower than yesterday over the East Side and in NorCal (60-70F).

The best forcing for showers and thunderstorms will extend from
NorCal northeastward across SE Klamath and into Lake County. This
will be the focus area for wetting rainfall today. Satellite
shows a corridor of moisture in this area right now and KMAX is
picking up some light returns at this early hour. Meanwhile, as
the low moves southward today, there will be an area of subsidence
and mid-level drying on the NW periphery of the low and this will
result in a relative minimum in shower activity, which will be
for areas on the west side of the Cascades. That`s not to say
there isn`t a chance of showers over here (including the Rogue
Valley), but they`ll be much more hit-or-miss than they are over
Siskiyou/Modoc up into southern Klamath/Lake counties. Coverage of
thunderstorms over/down in those areas could reach scattered this
afternoon/evening affecting locations like Mt. Shasta City,
Alturas and Lakeview. We aren`t too concerned about the
thunderstorm activity today since they`ll largely be wet storms,
but they could produce cloud to ground lightning, and some gusty
winds.

Tonight into Monday morning, the closed low will pivot across
northern California and then move into the Great Basin Monday
afternoon. This will turn the mid-level flow to easterly across
our area. Showers and isolated thunderstorms will continue over
the East Side, while shower chances diminish west of the Cascades.
Overall, rain amounts with this first system will be light west
of the Cascades. Some locations will miss out on rain completely
while others could get up to a tenth of an inch. Widespread
wetting rain is likely though east of the Cascades where amounts
of 0.10-0.25 of an inch will be common, with up to 0.50 of an inch
in the mountains where showers tend to focus.

There will be a break between systems late Monday night or first
thing Tuesday morning, but as one upper low departs to the east,
another will dig southward along the PacNW coast. Showers likely
reach coastal areas again Tuesday morning, then spread inland
over to the Cascades by late Tuesday afternoon. Highest rainfall
probabilities will be along the coast (near 90%) Tuesday, but
most areas east of the Cascades and in NE CA should be dry. This
low will move into NorCal Tuesday night and showers will
overspread the remainder of the region, then settle south and
eastward Wednesday into Thursday. We`ll see showers gradually
diminish across the area from NW to SE during this time frame.
Rain amounts this time will likely be highest along the coast over
to the Cascades, but near/north of the Rogue-Umpqua divide.
Overall 0.50-0.75 of an inch is likely, perhaps around an inch in
the coast ranges. A tenth to 0.25 of an inch appear to be the most
likely amounts across the remainder of the area, though with a
showery regime, some areas could have more or less.

Heights are shown to rise Thursday into next weekend with a
drying trend and temperatures rising back closer to normal.
However, guidance does show another system moving over the top to
our north during the weekend. Right now, it looks like mainly a
dry trough/frontal passage Saturday or Saturday night. Rain
chances at this time are less than 10%. While this could change,
as of now, it appears this system is much more likely to cause an
increase in afternoon/evening breezes over the east side than
anything else. -Spilde

AVIATION...15/06Z TAFS...VFR conditions prevail this evening, with
the exception of patchy IFR at the coast. High level clouds
associated with a dry cold front will continue across the east side
into the early morning.

Overnight into Sunday morning, an upper level trough will move into
the region, bringing increasing low and mid level cloud cover west
of the Cascades, and mid level clouds to the east side. VFR cigs are
expected to develop in West Side Valleys after 03z this evening, and
after 12Z on the East Side, with areas of terrain obscuration and
patchy MVFR.

Showers and isolated thunderstorms will develop Sunday afternoon
into Sunday evening from the Cascades eastward and in northern
California.
/DW

MARINE...Updated 200 AM Sunday, September 15, 2024...Northerly
wind speeds will increase today as an upper level trough slides
over the region. This will bring steep wind-driven seas to areas
south of Port Orford this morning. Northerly winds strengthen
today as the day progresses, resulting in steep seas spreading
north of Cape Blanco to all areas. Additionally, very steep and
hazardous seas will develop late Sunday afternoon for areas south
of Gold Beach. These conditions will persist through Monday night,
then winds will ease and seas will briefly lower into Tuesday.

A front will move into the region on Tuesday bringing widespread
precipitation and potentially isolated thunderstorms (10-15%) into
the coastal waters. Behind the front, gusty north winds and steep
seas return late Tuesday into Wednesday, likely bringing the return
of conditions hazardous to small craft.

-Guerrero

FIRE WEATHER...Updated 245 AM Sunday, September 15, 2024...The
main fire weather risk today will be isolated to scattered
thunderstorms this afternoon/evening for portions of the Shasta,
the Klamath/Modoc and Fremont National Forests. While cloud to
ground lightning and outflow wind gusts are the main threats,
thunderstorms will be wet and humidity will be much higher than
recent days. As such, significant fire risk is low. It will remain
that way through at least the middle of this week. We are
expecting a drying, warming trend late this week into next
weekend, but only back closer to what would be considered "normal"
for this time of year. -Spilde

&&

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

PACIFIC COASTAL WATERS...Small Craft Advisory from 5 PM this
     afternoon to 5 AM PDT Tuesday for PZZ350-356-370-376.

Hazardous Seas Warning from 5 PM this afternoon to 5 AM PDT
     Tuesday for PZZ356-376.

&&

$$