219
FXUS66 KMFR 161017
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
317 AM PDT Mon Sep 16 2024

.DISCUSSION...This morning, a closed low is spinning over the
Sacramento Valley of northern California. The broad circulation
will continue to move SE, pivoting to the east and moving into
Nevada by this evening, and then eventually into northern
Utah/western Wyoming overnight into Tuesday.

This low caused a cool down across our area yesterday along with
a smattering of showers/thunderstorms, which were focused in
NorCal and portions of southern Klamath/Lake counties. We tallied
around 200+ cloud to ground lightning flashes in southern/eastern
portions of the CWA yesterday. It`ll remain active across the East
Side today as easterly flow on the northern side of the low
continues to bring wrap-around moisture focused in Lake and Modoc
counties. Over there, where forcing and instability are maximized,
we expect numerous showers and isolated to scattered
thunderstorms. Rain amounts near and to the south/east of Highways
139/299 in CA and US Highway 395, will be 0.25-0.75 of an inch,
with some locations possible nearing an inch from the Warner Mtns
eastward. Farther west, amounts of 0.10-0.25 of an inch can be
expected back to around the Cascades, though in these areas,
amounts will be more hit or miss. Shower chances diminish
significantly near the Cascades and especially over the west side.
Even so, we can`t completely rule out a shower or two this
afternoon/evening over the Rogue Valley. East side areas will be
the coolest with respect to normal, about 15-25 degrees below
normal for highs (55-65F), while inland west side locations will
average 5-15 below normal (65-75F).

As the low ejects through the Great Basin, showers and
thunderstorms this evening east of the Cascades will gradually
shift to the east overnight into Tuesday. Most areas will get a
break from precipitation Tuesday morning, but the next deep upper
trough digging south from the Gulf of Alaska will spawn another
closed low offshore of the PacNW. This will bring a renewed risk
of rain along the coast beginning late Tuesday morning, with
increasing PoPs west of the Cascades during Tuesday afternoon.
East Side areas should remain dry most of Tuesday. The low will
pinwheel southward just offshore Tuesday night, then settle to
near or just off SF Bay area during Wednesday. This system will be
a more typical wet system for the area with the bulk of
precipitation expected along and west of the Cascades, but also
into NorCal. Overall 0.25-0.75 of an inch is likely (highest
amounts along the coast), perhaps a little more 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 (including here in the
Rogue Valley), though with a showery regime, some areas could have
more or less. Thunder chances with this system look low
(generally 15% or less), but suppose there could be isolated
cells, especially out over the marine waters where the core of the
upper low moves. We`ll see showers gradually diminish across the
area from NW to SE Wednesday and especially Wednesday night as the
low moves onshore into south-central CA.

After that, we`ll see a return of higher heights late this week,
with the core of upper ridging offshore and the main jet and storm
track aimed to our north in toward British Columbia through the
weekend. Shower chances should remain south and east on Thursday
as the 2nd closed low ejects into the Great Basin. It`s also still
possible another system coming over the top this weekend could
throw a monkey wrench in the warming/drying trend, but this is
looking less and less likely. Overall, we`ll see more in the way
of sunshine with temperatures getting back closer to normal levels
heading toward the autumnal equinox. -Spilde

&&

.AVIATION...16/06Z TAFS...Conditions are mainly VFR this evening,
but with scattered showers. Areas of showers will persist east of
the Coast Range through Monday evening, with a focus of shower
activity east of the Cascades. Mountain obscuration and MVFR
ceilings are possible in the vicinity of showers, but conditions
will remain mainly VFR through Monday evening.

Also, low stratus with MVFR ceilings are expected along the coast
and into the lower Umpqua Basin around 10Z-18Z overnight into Monday
morning. After brief improvement, a thick cover of MVFR ceilings is
expected to reach the coast early Monday evening. -DW

&&

.MARINE...Updated 200 AM Monday, September 16, 2024...Wind
speeds will strengthen today as an upper low continues to gradually
move over the west coast. This will result in steep to very steep
seas through tonight, with the strongest winds and highest seas
south of Cape Blanco where a Hazardous Seas Warning is in effect
through tonight.

An incoming front Tuesday will briefly turn winds to a southerly
component, while allowing seas to briefly diminish. During this
time, rain showers and isolated thunderstorms will also be possible
over the waters. The chance for rain is high (85+%), but
thunderstorm chances are only around 15-20%. That said, any
thunderstorm that develops will have the potential for strong
erratic gusty winds. Moderate to occasionally strong north winds and
steep seas will develop Wednesday, likely bringing the return of
conditions hazardous to small craft through the end of the week.

Guerrero

&&

.FIRE WEATHER...The lightning count across NorCal and south-
central Oregon east of the Cascades yesterday was 200+ CG flashes.
Many of these storms, however, came with plenty of rainfall.
Surface observations showed most locations that got the
storms/lightning yesterday had 0.25-0.50 of an inch of rainfall
with a few spots in western Siskiyou County, CA, coming in with
around an inch (Fort Jones) and even Summer Lake RAWS in Oregon
near 0.75". A similar story is expected today with showers and
storms across eastern areas, particularly Modoc and Lake counties.
Where there is lightning, there will be rainfall, and it will be
cool with high humidity, limiting fire ignition/spread risk. The
cool, moist pattern will continue through mid week with yet
another deep upper trough expected to bring more wetting rainfall
Tuesday night into Wednesday. Then, we`ll see things get back to
normal late this week and into the weekend with a warming and
drying trend. -Spilde

&&

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

PACIFIC COASTAL WATERS...Small Craft Advisory until 5 AM PDT Tuesday
     for PZZ350-356-370-376.

Hazardous Seas Warning until 5 AM PDT Tuesday for PZZ356-376.

&&

$$

MAS/JWG