Flash Flooding and Debris Flow in California, January 4 to 5, 2017

Area Forecast Discussion
National Weather Service San Joaquin Valley - Hanford CA
243 AM PST Wed Jan 4 2017

.SYNOPSIS...A very moist flow of Pacific air will continue over
the region today and Thursday and then as a ridge of high pressure
develops, shift to the north briefly by late in the week. Another
even stronger fetch of moisture will take aim on the region this
weekend into early next week.

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.DISCUSSION...Atmospheric river event #1 continues this morning
with the centerline of the fetch of moisture along 37N. Embedded
in this flow area several weak short wave troughs and vorticity
centers as seen on water vapor imagery and as each moves through
the region, precipitation will increase in coverage and intensity
from the steady lighter, mountain and terrain enhanced "river"
precipitation. As of 2 am radar is showing an increase in echo
intensity into the Sierra along and north of HWY 198 and rain
gauges are showing a maximum hourly rates as high as .20" per
hour. This rate should easily be infiltrated in the ground thus
for the moment flash flooding shouldn't be an issue. The flash
flood watch remains in effect however.

Snow levels as depicted on the NOAA S band radars at San Luis
Reservoir and Eastman Lake show the freezing levels in free air
have climbed to around 7500-8000 feet and observations in the
Sierra from mesonet stations are indicating rising temperatures as
any trapped colder air is being scoured out.

For the next 36 hours or so, models keep the Atmospheric River
pointed at Central California and the Sierra and foothills will
realize the greatest precipitation. As is typical in these
westerly flow events, the San Joaquin Valley will experience a
pronounced rain shadow as the only lifting mechanism much of the
time is topography and what is lifted over the coastal mountains
is forced downhill into the west side of the SJV and this is where
rainfall amounts will be the least. Meanwhile over Kern County,
both the bulk of the moisture remaining farther north and an
especially long track of air moving over extensive mountains to
the west effectively causes little precipitation too. By later
Thursday as a deeper area of low pressure out over the Pacific
forms and a short lived ridge develops along the West Coast the
flow into the area will shift to the northwest, effectively ending
Atmospheric River event #1.

Friday and Friday night look to be dry before the next potentially
much wetter Atmospheric River #2 takes aim on Central California
Saturday afternoon through Monday. It should be noted that AR
events have a "core" of heaviest precipitation and lesser amounts
to the sides. Like a "fire hose" if a location remains affected by
the core of the flow it could receive some incredible amounts of
rain. At this time, forecast models continue to target areas from
Sequoia Park to Yosemite. If this model forecast holds true,
rainfall amounts exceeding 15 inches area possible below 8500 feet
with yes, several feet of snow over higher elevations. This one
needs careful watching for many impacts that can caused by feet of
rain in the mountains.

As stated in the description above for AR #1, topography will play
a major role in the weekend event and some degree of rain
shadowing is a given in the SJV. All in all, this is a forecast
period that holds some historic potential if the model forecast is
correct!

&&

.AVIATION...
In the San Joaquin Valley, mainly VFR with local MVFR due to rain
and ceilings. In the southern Sierra Nevada and adjacent
foothills,widespread mountain obscuring IFR in upslope clouds and
precipitation. In the Tehachapi Mountains, areas of MVFR and local
mountain obscuring IFR in upslope clouds and rain through 18Z today,
becoming widespread IFR after 18Z today. Otherwise, VFR conditions
are expected during the next 24 hours.

&&

.AIR QUALITY ISSUES...
None.

&&

.CERTAINTY...

The level of certainty for days 1 and 2 is high.
The level of certainty for days 3 through 7 is medium.

Certainty levels include low...medium...and high. Please visit
www.weather.gov/hnx/certainty.php for additional information
an/or to provide feedback.

&&

.HNX Watches/Warnings/Advisories...
Flash Flood Watch through Thursday afternoon CAZ090-092>097.

Winter Storm Warning until 4 PM PST Thursday ABOVE 7000 FEET FOR
THE CAZ096-097.

&&

public...Dudley
avn/fw...DS
synopsis...Dudley

weather.gov/hanford