Debris Flow and Heavy Rain in California, October 17 to 18, 2015

Area Forecast Discussion
National Weather Service Sacramento CA
430 AM PDT SAT OCT 17 2015

SYNOPSIS...
An unsettled weekend with cool temperatures and
showers/thunderstorms. This will give way to a breezy beginning
of the week with warming temperatures.

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.DISCUSSION
Quite an eventful overnight shift. A North-South line of
showers/thunderstorms began to develop after 1 am. By 2 am, this
line generally impacted valley areas between Modesto up towards
Red Bluff with the Sacramento region getting hit the hardest.
Multiple reports of lightning with brief, heavy downpours in and
around the Sac Metro region. The Downtown Sacramento rain gauge
measured over 0.30" between 1-2 am. By 3 am, the gauge had
measured 0.38" of rain. This line gradually moved northeastward
and by 3:15 am, it looked like part of the line was headed towards
the Butte Wildfire Scar. Between 3:30-3:45 am, the showers were
diminishing on radar just west of the scar. Hourly rate of rain
with these cells is below debris flow critera for the burn scar,
but we will continue to monitor this morning. As of 4:30 am there
were almost 200 lightning strikes between the Modesto to Oroville
area and including the Western Sierra slopes.

Instability will continue through tonight. Have added slight
chance of thunderstorms to the Western Sierra Slope Foothills and
Mountains between 5 am to 11 am since that's where the line of
showers/thunderstorms is currently located and the HRRR also
indicates activity in that region this morning. In the afternoon,
HRRR shows a isolated shower activity in the valley, but it will
be hit or miss. Daytime highs today will be near to a couple
degrees below normal for this time of year with valley highs in
the mid 70s, mountains in the 50s and 60s.

The low lingers over NorCal into Sunday and shower activity should be
limited to the higher elevations of the Sierra. Daytime highs will
be similar to today...near to a little below normal. By Monday
morning the low will be northeast of our region and a warming
trend begins with dry weather. More notable warming occurs Tuesday
as daytime highs around 10 degrees above normal with valley highs
back around the mid 80s. JBB

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.EXTENDED DISCUSSION (Wednesday THROUGH Saturday)

The extended period starts out dry under a ridge of high pressure
between a low centered over northern Baja and another trough in
the Gulf of Alaska. Wedmesday will likely be the warmest day with
daytime highs expected to come in between 5 and 10 degrees above
normal. A small scale upper trough is forecast to pass through the
Pacific Northwest on Thursday. This should bring a little cooling
but daytime highs are still likely to remain several degrees above
normal. Extended models begin to deviate on Friday in regards to
another upper trough digging out of the Gulf of Alaska. Both models
put this system over the Pacific Northwest coast by Friday
afternoon but GFS somewhat deeper bringing precipitation to the
far northern portions of California while the ECMWF is more
shallow with the system keeping any precip well north of the
state. GFS would indicate the possibility of some shower activity
northern zones next Saturday while the ECMWF would keep the region
dry. Have kept slight threat of precipitation over the northern
zones Saturday in coordination with 00z GFS and surrounding
offices. This represents a trend of pulling precipitation farther
northward and quick look at 06z GFS shows this trend continuing.
If this trend continues...may end up removing precip from forecast
all together next weekend. Either way...cooler temperatures still
in store for end of the week although daytime highs are still
likely to remain at least a little above normal.

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.AVIATION...

Mainly VFR conditions today all TAF sites but widespread showers
and thunderstorms are possible through about 06z Sunday. Surface
winds south to southwest today sustained below 15 knots except
strong and variable near thunderstorms. More variable winds after
06Z.

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.STO Watches/Warnings/Advisories...
None.
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