Debris Flow in California, September 10, 2021

Area Forecast Discussion...CORRECTED
National Weather Service Medford OR
315 AM PDT Fri Sep 10 2021

.DISCUSSION...A significant upper trough is moving inland and
generating plenty of showers and a few thunderstorms over the
forecast area this morning. Precipitable water values of 1.05
inches are being observed, about 50% above normal, over much of
the area. We've observed a tenth of an inch in Western Siskiyou
County, no doubt beneficial to firefighting efforts in that area.
Only one cloud to ground lightning flash has been observed so far,
and that's been in Western Siskiyou County.

Models show precipitation continuing through this morning, with some
lightning expected, mainly from the Western Siskiyou County
northward to areas around the fires in Eastern Douglas County...and
points eastward. The most significant energy arrives this morning
between 4am to 7am and will likely deliver more in the way of
heavier rain and some lightning. Gusts to 40 mph have been
observed with some heavier showers this morning over the Siskiyou
Mountains, so we expect that potential to continue this morning. We've
examined whether current rainfall rates will warrant any flash
flooding concern for wildfire burn scars or otherwise, and we
don't expect any flooding concerns at this time. Rainfall rates
could reach about 0.25 inches per hour, and we expect totals to
get up to around 0.5 inches from the Cascades eastward. Lesser
amounts are expected west of the Cascades. There will be sharp
gradient from significant rainfall along the Cascades to little to
no rainfall from around Roseburg southward to the Illinois
Valley.

Later today, some daytime heating and instability will align with
the energy/dynamics moving southwest to northeast, and this
intersection looks most likely over Northern Klamath and Northern
Lake Counties. Instability peaks early this afternoon as the cool
pool of air aloft moves overhead, and there is a marginal risk for
severe thunderstorms due to this favorable environment for storms.
The energy moves north and east out of the forecast area this
evening, moisture decreases, and the chance for showers and storms
decreases.

Relatively quiet weather is expected from tonight through
much of next week. Smoke conditions should improve today for most
areas. Smoke could increase this weekend with warming
temperatures, but some of this rainfall could maintain improvement
in air quality for a bit longer into the weekend. Temperatures
will generally trend warmer this weekend into the middle of next
week, with one notable bit of energy moving through Sunday
afternoon, just enough to bring breezier conditions and maintain
westerly winds aloft. Ensembles continue to point towards
increasing chances for rain for much of the forecast area by
Saturday, Sep 18. Specifically, about 85% of European, Canadian,
and American ensemble members suggest some rainfall next weekend.
Aware of how these Sep fronts can often fizzle, with models
generally overforecasting rainfall late in the summer, we should
temper expectations with the models at this point. Still, rainfall
is in the forecast, and we expect the next chance to be next
weekend. Lightning isn't expected to accompany this rainfall, and
it could be more significant in amounts and coverage but more
focused from the Cascades westward. Keene

&&

.AVIATION...10/06Z TAFs...IFR/LIFR conditions will spread across the
coast and coastal valleys overnight, persisting into Friday morning.
Over inland areas...thick smoke from the fires will result in areas
of MVFR/IFR visibilities downwind of the fires into Friday morning,
including Medford and Klamath Falls. The lowest visibilities will
mostly be from the Cascades east and Siskiyous south. Scattered
showers will become more widespread across northern California and
southern Oregon through Friday morning along with isolated
thunderstorms. Sandler

&&

.MARINE...Updated 200 AM PDT Friday 10 September 2021..Winds
have decreased overnight and light north winds and seas are
expected today through Friday night. Then the thermal trough will
redevelop Saturday and strengthen Sunday into early next week.
This will bring strong north winds and steep seas to the waters
south of Cape Blanco on Saturday. Winds and seas will increase
further Sunday and Monday and a west-northwest swell builds into
the waters and combines with building wind seas. By late Sunday
afternoon, steep to very steep seas may develop for areas south of
Cape Blanco with steep seas and gusty winds developing north of
Cape Blanco. Gusty winds and elevated seas may persist through the
middle of next week.
-CC

&&

.FIRE WEATHER...Updated 300 AM PDT Friday, 10 September
2021...The main concern through this morning will be moderate to
strong outflow wind gusts near thunderstorms and the potential
for new lightning-caused wildfire starts. A tenth to a quarter
inch of rain has accompanied showers this morning, and a handful
of cloud to ground lightning flashes have been observed in
Siskiyou and Modoc Counties. In short, more rain than lightning is
being observed so far with this system moving through. Still,
can't rule out lightning outside of heavier showers, and we
maintain a Red Flag Warning for most areas from the Cascades
eastward and Siskiyous southward.

Thunderstorm potential west of the Cascades will decrease later
in the morning, but more significant energy will arrive from the
south, bringing increased lightning (with significant wind gusts
and outflows possible) potential for areas east of the Cascades.
Most of Fire Weather Zone 625 and eastern through northern 624
will be fair game in thunderstorm potential through this
afternoon. For this reason, a Red Flag Warning will continue into
the afternoon for those areas, while the Red Flag Warning for
areas south and west of this area ends later this morning. Please
see the Red Flag Warning text for details.

Elsewhere, excellent RH recoveries are expected this morning
except the high terrain of Siskiyou County and in Modoc and
Eastern Lake County late tonight into Friday morning.

Thunderstorms will no longer be an issue this evening as the best
dynamics will be northeast of our area. We'll see a return to dry
and stable conditions Saturday through at least the middle of
next week with thunderstorms chances practically zero.
-Petrucelli/Keene

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Red Flag Warning until 5 PM PDT this afternoon for
     ORZ617-623>625.

CA...Red Flag Warning until 5 PM PDT this afternoon for
     CAZ282-284-285.

Pacific Coastal Waters...Small Craft Advisory from 11 AM Saturday to 1 PM PDT Sunday for
     PZZ356-376.