Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
900 PM EDT Sun Mar 26 2023

Day 1
Valid 01Z Mon Mar 27 2023 - 12Z Mon Mar 27 2023

...THERE IS A HIGH RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF
CENTRAL ALABAMA AND GEORGIA...

...Southeast...

A mix of discrete to semi-discrete supercell storms continue to
organize linearly from WSW to ESE along a outflow-modified frontal
zone this evening, generally extending from south-central MS into
central AL/GA. A low amplitude shortwave is evident just upstream
near the Ark-La-Tex via GOES-East water vapor imagery, providing
large scale lift and ascent for additional convective organization
going into the overnight hours. The environment will also remain
very supportive of heavy rainfall, given ample deep layer shear of
50-70 kts, ML CAPE of 1000-2000 J/kg, and precipitable water
values of 1.4-1.7 inches (near the max moving average per BMX/FFC
sounding climatology). In addition, a fairly strong (25-35 kts)
southwesterly low-level jet (LLJ) is feeding additional moisture
transport into the region. This LLJ is expected to strengthen
rather significantly tonight (while hardly veering), with a
bulls-eye of 35-50 kts forecast over southern AL. This has
increased the probabilities for excessive rainfall notably across
central AL/GA, where a High Risk has been introduced (encompassing
much of the prior Moderate Risk area). In addition, the Moderate
Risk has been expanded east and west to include the remaining
portions of central AL/GA (while extending a bit westward into
south-central MS as well). HRRR runs since 18z have been rather
consistent in depicting additional localized totals of 3-5 inches
across the High Risk area (with the most recent 23z run depicting
a concerning stripe of 5-8+ inches). These rainfall maxima are
very concerningly close to the same areas that experienced flash
flooding earlier this morning (where 2-4+ inches have already
fallen, and where FFGs are 1-2 inches or less). Widespread
instances of flash flooding are expected in the High Risk area,
and flash flood impacts locally may be very significant to extreme
through the morning hours. Please see ongoing and subsequent MPDs
for more information.

...Southeast Missouri to Southern Indiana...

A compact shortwave crossing the Midwest this evening is
associated with a slow-moving frontal boundary from southern MO to
IN, which has an area of up to 100-250 J/kg of SBCAPE and 200-400
m^2/s^s of 0-3 km SRH to work with. A line of showers and
thunderstorms is slowly developing near the Marginal Risk area and
could move in a linear fashion over similar areas given the
frontal boundary and mean-layer wind profile appear nearly
parallel. Rainfall rates will not be that impressive and likely
remain under 1"/hr, but rainfall totals up to an inch could pose
an isolated flooding/flash flooding risk, especially given this
region experienced over 4" of rain not too long ago. Streamflows
are running high and soil moisture percentiles are very high per
NASA SPoRT. The antecedent conditions were the main driver for the
upgraded Marginal Risk. Additionally, 12z HREF neighborhood
probabilities do show up to 25% chances for 0.5"/hr rainfall rates
between 00z and 06z Mon.

Churchill/Snell


Day 2
Valid 12Z Mon Mar 27 2023 - 12Z Tue Mar 28 2023

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS PARTS OF
THE DEEP SOUTH AND NORTHERN CALIFORNIA...

...Deep South...

By Monday morning, the same frontal boundary impacting much of the
Southeast and Gulf Coast with scattered showers and thunderstorms
will linger throughout the day and lead to additional chances for
isolated flash flooding. Initial convection early in the time
period is forecast to be ongoing across parts of southern AL and
central/southern GA and could overlap with some areas receiving
heavy rain today and tonight. PWATs will remain around 1.5" during
this timeframe and 850 mb flow turning more westerly through
midday will remain around 40 kts. This boundary is forecast to
continue to be a focus for scattered thunderstorms from southern
AL, southern GA, and the Florida Panhandle through the early
evening hours. 12z HREF neighborhood probabilities depict 20-40%
chances for 6-hour QPF to exceed 2" across this part of the
Southeast through 00z Monday night. This signal along with current
rainfall trends aided in the decision to introduce a Marginal Risk
here.

The next in a parade of shortwaves to eject out of the western
U.S. trough will aid in a separate area of developing convection
across eastern TX that should push into LA and southern MS after
06z Monday night. The shortwave tracking through the
Mid-Mississippi Valley (similar to today) will allow for
additional upper divergence and an increase in southerly flow out
of the Gulf of Mexico. However, upper-level ridging retrograding
westward underneath this shortwave across the far southern Gulf of
Mexico will allow for convection to develop farther west with this
round. It's possible the upgraded Marginal risk for Day 2 may need
to expanded farther into central TX if CAM guidance picks up on
the potential for earlier/western convection on Monday. By 12z
Tuesday, 12z HREF highlights very high probabilities along the
southern TX-LA border for over 2" of rain in 6 hours, which could
lead to isolated flooding concerns, even for areas with relatively
high FFGs.

...Northern California...

Rapidly deepening upper-level low and surface cyclone off the
Pacific Northwest late Monday night will usher in the next
atmospheric river to California towards the tail end of the Day 2
ERO. IVT values after 06z Tuesday increase to 500-600 kg/m*s north
of the San Francisco Bay Area and will spread southward along the
California coast on Day 3. Areas along the northern California
coast will have the best chances to achieve higher than 0.5"/hr
rainfall rates and the potential for isolated flooding or rapid
runoff through 12z Tuesday, hence the upgraded Marginal Risk area.
Snow levels will rapidly drop to around 3000 feet, limiting any
snowmelt and runoff from the higher terrain.

Snell

Day 3
Valid 12Z Tue Mar 28 2023 - 12Z Wed Mar 29 2023

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE
CENTRAL CALIFORNIA COAST AND SAN FRANCISCO BAY AREA...

...California...

Only minor changes were made to the previous risk areas, with the
main expansion being to stretch the Marginal Risk to include Los
Angeles county. An upper level polar cutoff low begins the day
Tuesday morning centered off the Oregon coast, and tracks
southeastward to be centered over the northern California coast by
Wednesday morning. A 100 kt westerly jet streak wrapping around
the low will direct a plume of Pacific moisture and precipitation
into California as it tracks south. PWATs will approach
0.75-1.00", which is about +1.5-2 sigma above the climatological
normal and IVT peaks at about 500-600 kg/m*s along central
California. This will bring with it a brief increase in snow
levels as high as 7,000 ft along coastal ranges during the day
Tuesday. The attendant surface low will follow a similar track as
the upper level cutoff low through this period, making it a
vertically stacked low, that will result in largely unidirectional
flow throughout the column south of the low. Since the vertically
stacked low has polar origins, it will have plenty of cold air and
a lack of plentiful Pacific moisture with it. Thus, this rainfall
event is not expected to be as intense as the atmospheric rivers
impacting the state in recent weeks. Nevertheless, the residual
impacts from the atmospheric rivers remain across coastal central
California, and the injection of up to 3 inches of rain into the
coastal mountains are still expected to cause or worsen flooding
impacts as that rainwater flows down the mountains and swells
already high rivers. Thus, the Slight Risk was maintained due to
steady forecasted rainfall through the Bay area and especially the
mountains north of there, while also spanning south to include the
westernmost Transverse Ranges of Santa Barbara County. Further
inland, slightly lower snow levels, much higher mountains, and
more recovery time from the last round of atmospheric rivers were
factors that led to the continuance of the Marginal Risk, as much
of the heaviest precipitation is likely to fall as snow. The
highly favorable antecedent conditions for flash flooding remain a
significant factor in the Slight Risk area.

...Central Gulf Coast States...

The Marginal Risk area along the Gulf Coast was confined to the
south and along the central Gulf Coast compared to the recent
issuance in order to match recent guidance and the likelihood that
intense rainfall rates will remain closer to the coastal regions.
By this time, the lingering frontal boundary plaguing the Deep
South will shift farther to the south and limit the available
moisture/instability for areas to the north. Still, convection
ongoing from Monday night moving into eastern LA and southern MS
is likely to tap into 1.5" PWATs and have rainfall rates
potentially exceeding 1"/hr. Urban regions and locations where
rain has fallen over the last few days will be most at risk to
localized flooding concerns.

Snell/Wegman


Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: www.wpc.ncep.noaa.gov/qpf/99epoints.txt