Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1116 AM EDT Sun Mar 26 2023

Day 1
Valid 16Z Sun Mar 26 2023 - 12Z Mon Mar 27 2023

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

...Southeast...

Ongoing convection this morning has organized into a linear
orientation along a draped frontal boundary and has already
produced flash flooding across parts of GA ahead of a primary low
amplitude shortwave. This initial area of thunderstorms containing
1-2"/hr rainfall rates, with locally higher amounts, should
continue to push eastward across SC midday and lead to the
potential for scattered flash flooding. This prompted the
expansion of the Slight Risk eastward into central SC. Farther
west, lingering scattered convection is likely throughout the day
as persistent southerly low-level flow from the Gulf of Mexico and
recharging instability fuel continued thunderstorm activity until
a more organized round of heavy rain enters this evening. The
overlap between the ongoing/morning heavy rain and the expected
evening/overnight storms prompted the upgrade to a Moderate Risk
for Excessive Rainfall from parts of central AL to central GA. A
broad longwave trough over the Rockies will contain a potent
shortwave to eject into the Mid-Mississippi Valley this evening an
create an environment ripe for additional upper divergence and a
sufficient low-level jet. PWATs of 1.5"-1.75" combined with around
2,500 J/kg MUCAPE and 40 kts of 850 mb inflow tonight is plenty
for organized thunderstorm development, with an area to focus
along a lingering frontal boundary. Some supercells are possible
within a more broad area of thunderstorm activity, which will
produce areas of particularly intense rainfall rates. Additional
rainfall amounts along a narrow corridor within the MDT risk could
approach 4" and occur over a similar area that has already
received 2-4" this morning. 12z HREF neighborhood probabilities
depict elevated chances for additional rainfall over 3" from
southeast MS to central GA tonight. HREF also highlights much of
the MDT risk area as having 40-60% chances for over 5" of QPF for
the 24-hour period ending 12z Mon. If any of this overlap does
occur overnight, significant flash flooding impacts are possible.
There is the potential for convection to initiate farther to the
south and closer to the Gulf Coast this afternoon along with the
diurnal heating, which could drag a surface boundary and overnight
convection further south. This would lead to less overlap and a
lower threat of numerous instances of flash flooding. However,
current thoughts are that the approaching shortwave, low-level
jet, and current visible satellite trends supports solutions
showing a heavy axis of additional rain over the MDT risk area.

...Southeast Missouri to Southern Indiana...

A compact shortwave crossing the Midwest this afternoon will be
associated with a slow-moving frontal boundary from southern MO to
IN, which is likely to have an area of up to 500 J/kg of SBCAPE
and 400 m^2/s^s of 0-3 km SRH to work with. A line of showers and
thunderstorms is forecast to develop near the upgraded Marginal
Risk area after 21z 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.

Snell


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

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

By Monday morning, most of the convective activity associated with
the front in the Southeast will have drifted south of the main
areas impacted by the strong storms today, i.e. closer to the Gulf
Coast. With little in the way of overlap from Day 1 and good
agreement overall that convective coverage and intensity will be
greatly reduced as compared with Sunday's storms, a somewhat low
confidence forecast of no flash flooding remains for this forecast
update. However, should there be any northward trend with the
forecast convection on Monday, then a Marginal Risk will be needed.

Wegman

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 ADJACENT MOUNTAINS...

...Central California Coast...

In coordination with the MTR/Monterey, CA and LOX/Los Angeles, CA
forecast offices, the inherited Slight Risk area was expanded both
north and south with this morning's forecast update. 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 3/4 inch, which is about 2 sigma above the
climatological normal. This will bring with it a brief increase in
snow levels as high as 7,000 ft 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 anywhere close to 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 expanded due to increasing forecasted rainfall through
the Bay area and especially the mountains north of there, and
expanded 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.

...Potential for Future Risk Changes...

Despite the expansion of the Slight Risk area, at this point it
appears further upgrades to the Slight Risk area are unlikely, as
precipitation totals have remained fairly steady, and the lack of
tropical moisture will preclude rainfall rates more typical of
true atmospheric rivers.

...Gulf Coast through Coastal Georgia...

The Marginal Risk area along the Gulf Coast is largely unchanged
from the Day 4 update yesterday. There has been a small southward
shift in most of the guidance, so the immediate Gulf Coast and
points up to 100 miles inland remain in the Marginal Risk area for
this update. The front will be in roughly in the same place
Tuesday as it was on Day 2/Monday. Thus, expect Monday's rainfall
to have lowered FFGs a bit in the Marginal Risk area by the time
we get to Day 3/Tuesday. Forecasted rainfall is a little higher in
this area than on Day 2/Monday, particularly around southern
Mississippi. Thus, think continued convection in the same areas on
Tuesday as on Monday warrants the continued Marginal Risk for the
same stalled out front.

...Potential for Future Risk Changes...

As with the current Day 1 update, if future model trends move
higher for forecasted rainfall, it's very possible a Slight Risk
may be needed for portions of the Marginal Risk area on Tuesday.
Since a large component to any upgrades will be the rainfall
distribution in this area on Day 2/Monday, confidence and current
forecasted rainfall are low as to where any upgrades would be.

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