Debris Flow and Flooding in California, April 5 to 8, 2018

Area Forecast Discussion
National Weather Service San Joaquin Valley - Hanford CA
250 PM PDT Thu Apr 5 2018

.SYNOPSIS...
A wet system will bring an increase in precipitation chances
beginning Friday through Saturday. More precipitation may also be
expected toward the middle of next week. Slight cooling will take
place on Sunday before temperatures begin trending higher on
Monday.

&&

.DISCUSSION...
A change in the weather is finally taking hold of the West Coast
as precipitation overspreads the Pacific Northwest and Northern
California. Central California is still on the edge of the current
Atmospheric River event as clouds moved in and out for most of
today. While no precipitation has been reported today across the
Central California Interior, the cloud cover has managed to lower
temperatures by as much as 5 to 10 degrees from those observed
Wednesday afternoon. With little fluctuation in the forecast
models for several days now, the district is well on track to
receive significant precipitation in the next 48 to 60 hours.
Therefore, big change will come in the form of precipitation
across California on Friday and Saturday.

With the minimal fluctuation in forecast models, confidence in
what the weather prognosis for the next several continues to
increase. Now that the event is within the next 48 hours, models
continue to converge on a solution that will produce abundant
precipitation that will reach the northern sections of the
district toward this weekend. With even better model consensus on
placing the position of the Upper Low west of British Columbia,
confidence has improved toward developing a good southwesterly
fetch as moisture that is already being drawn in from the Hawaiian
region. The current trajectory of the moisture is toward the
Pacific Northwest where upper air observations are showing the
deeper moisture profile. Therefore, this trajectory will give the
Pacific Northwest and Northern California the first of the
precipitation (which regional Doppler radar is already
registering).

Integrated Water Vapor Transport (IVT) analysis from the
Atmospheric River Forecast Product page has been very helpful in
analyzing the precipitation potential of the first in a series of
upcoming storms to hit California. As models have shown minimal
deviation in their solutions for several days, so too has the
transport of moisture that will hit California by the end of the
week. Therefore, minor adjustments continue to be made to the
overall forecast package as uncertainty continue to decrease with
the strength and timing of the first storm (Atmospheric River
event). IVT analysis is still placing the bulk of the moisture
around the early Saturday morning time-frame which works well with
forecast precipitation values for Central California. In
addition, ensemble analysis of possible precipitation rates
showing the highest accumulation occurring in the period centered
during the mentioned Saturday morning period. While additional
accumulation will occur during the day on Saturday, precipitation
is still expected to diminish leading toward Saturday night.

While this event is still expected to greatly impact the Pacific
Northwest and Northern California, possible flooding, strong winds
and even a slight chance of afternoon thunderstorms could be
possible on Saturday. Even with higher confidence in this event
over the northern half of the district, the southern half
maintains a lowered confidence of occurrence. Kern County will
still receive measurable precipitation but, at lower accumulation
then areas closer to Merced County. On the other hand, Models are
computing as much as 2 inches (plus) of water for areas across the
San Joaquin Valley north of Merced County and up to 1 inch in the
48 from Friday morning to Sunday morning from Merced down to
Fresno (and even touching on Kings and Tulare) Counties. The
potential for this much precipitation as the Sierra Nevada will
double and possibly triple Valley amounts will create a hydrologic
hazard for the foothills and higher elevations of the Sierra
Nevada North of Kern County. Due to the fact that the ground
remains moist from the previous atmospheric river event,
conditions are favorable for flooding of smaller streams and
rivers along with mud and rock slides. While the San Joaquin
Valley may not see the worse flooding, foothill locations and burn
scar areas will be more susceptible to better runoff and increase
danger due to heavier precipitation. However, the San Joaquin
Valley could see a slight chance of possible thunderstorm activity
during the day on Saturday. These thunderstorm will also have the
potential of producing localized heavy precipitation and
exacerbate the flooding concerns (especially over burn scars).

Beyond this event, the pattern remains active for much of the
coming seven days as more precipitation is expected for the mid
week and end of week periods. While uncertainty is much higher for
the second and third storm, measurable precipitation could be seen
for those secondary events. Currently, the second storm is taking
on a more Northern California trajectory which will limit the
possible precipitation over Central California. However, the third
storm shows signs of dropping deeper toward Central and Southern
California.

&&

.AVIATION...
MVFR visibility possible due to showers by 12Z Friday in San Joaquin
Valley at KMCE and KMER and will spread south to KFAT and KVIS by
18Z Friday. Mountain obscurations and precip expected over the
Sierra Nevada beginning 12Z Friday. Locally gusty conditions,
including northwest sustained winds 25 to 30 kts with gusts around
35 to 40 kts, expected near the passes and canyons in eastern Kern
County tonight into Friday morning. Otherwise, VFR conditions can be
expected across the central CA interior 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...
Flood Watch from Friday evening through Sunday morning BELOW
7000 FEET FOR THE CAZ093-096.

&&

public...Molina
avn/fw...BSO
synopsis...Molina

weather.gov/hanford