Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
408 PM EDT Sat Mar 25 2023

Day 1
Valid 16Z Sat Mar 25 2023 - 12Z Sun Mar 26 2023

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


Snell





Day 2
Valid 12Z Sun Mar 26 2023 - 12Z Mon Mar 27 2023

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS PARTS OF
THE SOUTHEAST...

Multiple rounds of heavy rain are possible along and just to the
north of a lingering quasi-stationary front extending from the
Southeast to the central Gulf Coast States, which may prompt
isolated instances of flash flooding. Initial convection across
central AL and into east-central GA should be ongoing by 12z Sun.
These storms by themselves will be capable of 1-1.5"/hr rainfall
rates until modest 850 mb southwesterly flow weakens to below 30
kt by about 15z. The resulting cold pool should aid in suppressing
the aforementioned frontal boundary closer to the Gulf Coast and
setting the stage for the next round Sunday evening. Tropospheric
precipitable water content per the 12z HREF is forecast to peak in
the 1.5-1.8" range by Sunday evening in response to a surge of 850
mb moisture flux and overarching shortwave swinging from the
southern Plains to the Ohio Valley. While not extreme, this
moisture source would equate to around +2 standard deviations
above climatology (per both the ECENS and GEFS) and into the
75th-90th percentiles. There will also be no shortage of
instability, with a corridor of 2000-2500 J/kg MUCAPE Sunday
evening along and just inland from the central Gulf Coast.
Resulting convection is anticipated to initially develop to the
southwest across LA and expand with the increasing 850 mb flow
along the stationary front through southern/central MS and AL.
Sufficient instability and southwest 850 mb flow running into the
frontal boundary will support areas of organized convection
(within a larger area of scattered shower/thunderstorm activity)
and the potential for supercells, which in turn are likely to
contain intense rainfall rates. The slow-moving frontal boundary
will also allow for training cells or multiple cells to overlap
areas that received previous rainfall. Rain Sunday morning across
central AL could also prime soils before the evening convection
rolls through and increase the potential flash flooding threat.
12z HREF neighborhood probabilities depicts the upper-bound of
potential rainfall during this period by highlighting 15-20%
chances for 12-hour QPF to exceed 5" across parts of
central/southern AL. This is mainly due to FV3 and HRRR members,
which are likely overdone spatially but not out of the realm of
possibilities in a localized sense given the overall setup.

A Slight Risk was considered during this update centered around
parts of central AL, but opted for only an upgrade to a Marginal
Risk given recent dry conditions (less than 2% 0-40 cm soil
moisture percentiles per NASA SPoRT-LIS) and some lingering
uncertainty on where exactly the overlapping heavy rain could set
up. WPC QPF is in line with the latest HREF guidance in
highlighting the potential for a swath of 2-4" total rainfall.
Localized amounts over 4" are possible and would likely overcome
even some of the more drier soils in the region. Will continue to
monitor the need for an expanded risk area to the west across
southern MS and the possibility of a targeted Slight Risk if CAM
guidance shows more agreement. Heavy rain to the east into the
Carolinas is expected remain fast-moving and limit any flooding
impacts.

Snell

Day 3
Valid 12Z Mon Mar 27 2023 - 12Z Tue Mar 28 2023

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

...2030z Update...

No changes from the overnight issuance.

Snell

...Northern California...

The inherited Marginal risk area for coastal northern California
north of the Bay Area, and portions of the northern Sacramento
Valley was downgraded to a risk-free map with this morning's
update. A vertically stacked low will drift off the coast of
Oregon on Monday. The upper levels will be supportive of
occasionally heavy rain, especially Monday night, as a 90 kt jet
streak rounds the base of a highly positively-tilted upper level
trough as it slowly moves south down the coast. This trough will
bring with it a renewed push of cold air. Flow orthogonal to the
coastal mountains will support locally heavier upslope
precipitation as the trough, low, and its attendant surface cold
front move south down the coast.

With this synoptic setup in place, there are 2 primary factors
that were considered in the ERO risk downgrade. First, the
aforementioned cold air associated with the trough will hold snow
levels to an average 1,500 to 2,500 ft above sea level range.
Thus, a substantial fraction of the heaviest upslope precipitation
into the mountains will fall as snow. Of course, any snowfall will
not contribute to any flash flooding concerns. Thus, any potential
flooding would be confined to the immediate coast and the
lowlands. Further, the upper level pattern bringing cold Pacific
moisture from the Gulf of Alaska into the Pacific Northwest, as
opposed to tropical moisture, means there will be far less overall
moisture available to fall out as precipitation. PWATs only very
briefly exceed 1/2 inch with the heaviest precipitation Monday
night. Secondly are the antecedent conditions. Of the entirety of
the state of California, the area that has recovered the most from
recent atmospheric river events has been the northern California
coast. Soil moisture has fallen below 70% in this area, showing
that much of the rainfall from recent weeks has had time to drain.
Thus, FFGs in this area have recovered such that the chances this
storm can produce enough rain to exceed those values are very low,
especially considering the low snow levels converting most of the
expected precipitation into higher elevation snow.

In coordination with the MFR/Medford, OR and MTR/Monterey, CA
forecast offices, all agreed the low snow levels will keep the
flash flooding threat minimal.

...Southeast...

Just as on Day 2/Sunday, the same stalled out frontal boundary
draped from northern Louisiana into South Carolina will still be
in place on Monday. The same scenario plays out again on Monday,
where the weak frontal boundary is the only forcing for
convection, despite a juicy airmass featuring PWATs above 1.5
inches and MUCAPE along the Gulf Coast getting up to 1,600 J/kg.
So, in rinse-and-repeat fashion, thunderstorms are likely to
develop again Monday afternoon, perhaps a little further west
along the boundary, favoring the Louisiana-to-Alabama portion of
the front. High FFGs will once again be the dominant factor
preventing flash flooding, as the storms perhaps form around the
front, but have little ability to consolidate. The strongest
storms may cause briefly heavy rain, but those storms will be
isolated enough that the likelihood of resultant flash flooding
will be near zero once again.

There is somewhat less confidence in the lack of flash flooding
threat on Monday, primarily because the rainfall on Sunday will
have possibly lowered FFGs in many of the same areas likely to see
additional thunderstorm activity again on Monday. While the broad
5 state corridor from LA to SC is favored, where within those
states those FFGs will be lowered will only really be known after
Sunday's round of storms has taken its course. Thus, future
forecast updates will need to evaluate how much overlap Monday's
forecasted storms compare with Sunday's storms, of course taking
into account just how much modification the FFGs have undergone
after Sunday. Despite the potential for a reintroduction of the
Marginal Risk at a later time, in coordination with
BMX/Birmingham, AL, MOB/Mobile, AL, GSP/Greenville-Spartanburg,
SC, CHS/Charleston, SC, and ILM/Wilmington, NC forecast offices,
the Marginal Risk was also eliminated with this morning's update.

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