Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
501 AM EDT Thu Mar 22 2018
VALID 12Z Thu Mar 22 2018 - 12Z Fri Mar 23 2018
MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
50 WSW 87Q 30 SW MRY 10 SW WVI 20 E SNS 30 NNE PRB 35 WSW BFL
30 SSW BFL 25 S BFL BFL 15 SE NLC 15 ESE MOD 10 NNE MHR
10 ENE CIC 30 NE CIC 45 ENE CIC 15 NNE BLU 25 W TVL 40 SSW TVL
50 W MMH 35 SW MMH 45 SSW BIH 25 WNW IYK 15 N MHV 15 NNW WJF
10 ESE PMD 30 SW DAG 35 W NXP 15 NW PSP 10 SSW PSP 30 WSW TRM
15 NNE CZZ 10 WNW CZZ SEE NFG SNA 20 NW AVX 40 NW NSI.
SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
10 SW SBP 10 NNE SBP 25 ENE SBP 35 N SBA 20 WNW SDB 10 SE SDB
10 S PMD 35 NE ONT 35 S DAG 25 NNW PSP 15 WNW PSP 10 SW PSP
20 WSW PSP 20 W PSP 15 NE RIV ONT CQT 20 SE NTD 10 W OXR
15 SW SBA 15 S LPC VBG 10 SW SBP.
MODERATE RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
15 WSW SBA 10 SSE LPC LPC 25 ESE SMX 30 NNE SBA 15 SW SDB
15 ENE 3A6 15 SSE PMD 25 NE ONT 35 NE RIV 25 NE RIV 10 NE ONT
CQT 20 ESE NTD OXR 15 WSW SBA.
HIGH RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
10 NNE CMA SBA 20 WNW SBA 15 E LPC 20 N SBA 30 WSW SDB
20 WNW 3A6 10 NNE CMA.
...High Risk of Excessive Rainfall Continues Thursday across Santa
Barbara and Ventura Counties...
...Moderate Risk Extends eastward into Los Angeles and southwest
San Bernardino Counties...
An Atmospheric River or elongated plume of deep subtropical
moisture continues to extend northeast into southern CA early
Thursday morning. Observed GPS PWs aoa 1.25" along the southern CA
coast represent record values for late March, which along with the
850-700 mb layer moisture flux, are easily 5+ standard deviations
above the norm per the SREF and GEFS.
The next round of heavy rainfall will be moving in by 12Z, with
the robust warm conveyor belt (WCB) becoming more efficient in
producing heavier rainfall rates owing to increased divergence
aloft ahead of the next shortwave along with sharpening low-level
frontogenesis within the right entrance region of the departing
upper level jet streak to the north. Changes to the QPF from the
previous forecast cycle were minimal, and once again largely based
on high-res guidance (particularly the HREF means, NBM, ARW, and
ARW2). The HIGH risk area in the new day 1 ERO continues to
capture the Thomas and Whittier burn scars in Ventura and Santa
Barbara Counties, while also coinciding to the multi-model signal
for additional heavy rainfall along the southern (particularly SW)
flank of the Transverse ranges through 00Z. The orographic
component combined with the robust synoptic-scale support (with
coupled subtropical and northern stream forcing) will favor peak
hourly rainfall rates of 0.50-1.00 inches -- especially over the
western Transverse ranges and southern foothills -- which are
supported by the latest HREF probabilities along with the
multi-model consensus per the CAMs. The heaviest rates are
expected between 14-21Z from west to east across this region, with
additional totals after 12Z Thu likely between 3-6" across the
south-southwest facing foothills and mountains.
In addition, the MARGINAL risk for excessive short-term rainfall
was extended northward across eastern portions of the San Joaquin
and Sacramento Valleys, foothills, and western slopes of the
Sierra, where an additional 3-7+ inches of rain can be expected
during the day 1 period per the latest high-res guidance suite.
The models also continue to depict modest instability across this
region (MUCAPES of 500-1000+ j/kg), which will foster heavy
rainfall rates (0.50-1+ inch) that will migrate southward (as with
the PW axis) through 03Z Friday.