Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1059 AM EDT Thu Mar 22 2018
VALID 15Z 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 RIV FUL LAX 20 SE NTD 15 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
OXR 15 SE SBA 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.
HIGH RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
15 WNW OXR 20 WNW SBA 15 E LPC 20 N SBA 20 SW SDB 10 NNE BUR
BUR 15 WSW VNY 15 WNW OXR.
...High Risk of Excessive Rainfall Continues Thursday across Santa
Barbara and Ventura Counties...
...High Risk extended into western Los Angeles County...
...Moderate Risk Extends eastward into southwest San Bernardino
...Slight Risk added to the eastern side of the Sacramento/San
Joaquin Valleys and the Sierra-Nevada foothills...
A slight risk area has been added to portions of the
Sacramento/San Joaquin Valleys and the adjacent foothills of the
Sierra-Nevada. The latest radar trends confirm some relatively
organized convective segment developing out ahead the well-defined
southern stream shortwave trough as somewhat greater instability
begins to pool in across central CA. Still seeing rather robust
moisture transport fostering greater rainfall efficiency for
enhanced rates, and with the increase in instability profiles
favored by the latest CAM guidance, there will be some rainfall
rates that could approach 1 inch/hr and lead to some short-term
excessive totals. The CAM guidance including recent runs of the
HRRR favor multiple convective segments with perhaps some brief
cell-training going through the late-morning and afternoon hours
which will enhance the concern for locally excessive totals that
will lead to runoff problems and perhaps some flash flooding.
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 the increased divergence
aloft ahead of the main shortwave trough along with a period of
right-entrance region upper forcing via the jet streak to the
north (the latter also enhancing the low-level frontogenesis).
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, as well as the Rye, La
Tuna, and other burn scars in western Los Angeles County. Over the
entire HIGH risk area, there continues to be a multi-model signal
for additional heavy rainfall along the southern flank of the
Transverse ranges through 00Z Fri. The orographic component
combined with the robust synoptic-scale support (with the
aforementioned coupled subtropical and northern stream forcing)
will favor peak hourly rainfall rates of 0.50-1.00 inches --
especially over south-facing portions of the Transverse ranges and
southern foothills -- as these robust hourly rates are supported
by the latest HREF probabilities along with the consensus of
deterministic high-res guidance. The heaviest rates are expected
between 14-00Z from west to east across this region. Additional 24
hour rainfall totals between 3-6" are likely from 12Z Thu - 12Z
Fri across the south-facing mountains and adjacent foothills.
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.