497
FOUS30 KWBC 121534
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1034 AM EST Wed Feb 12 2025

Day 1
Valid 16Z Wed Feb 12 2025 - 12Z Thu Feb 13 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS MUCH OF THE
DEEP SOUTH AND COASTAL AREAS OF CALIFORNIA...

...Southeast...

Large Marginal and Slight Risk areas extend from
Louisiana to the central Appalachians and portions of Virginia. The
Marginal and Slight Risk areas cover multiple regimes for
excessive rainfall potential:

1) Long-duration, moderate to heavy rainfall along an axis
extending from east Texas east-northeastward into northern Georgia
and the southern Appalachians. Much of this axis has experienced
appreciable antecedent rainfall amounts (in the 2-3 inch range on
February 11) and models are consistent in depicting at least one
more round of heavy rain across that same axis as a thunderstorm
complex organizes and moves east- northeastward from in Texas today.
Another 1-3 inches of is expected (heaviest from western in to
central Mississippi). Given the wet soils from prior rainfall,
several instances of flash flooding are expected from central
Louisiana through the southern Appalachians and northern Georgia.

2) Somewhat lighter rainfall should occur across portions of
Virginia and West Virginia through the day. Not only are soils
water-logged and streamflows above average across these areas, but
recent snow/ice could melt under persistent warm advection and
temperatures slowly rising above freezing through the forecast
period. Light to moderate rainfall could enhance excessive
runoff/ponding especially in sensitive areas and low spots.

3) CAMs are in very loose agreement in depiction of east-west
oriented outflow boundary left behind by convective complex(es)
across portions of southern MS/AL through early afternoon. Strong
south-southwesterly 850mb flow and abundant moisture should result
in reignition of deep convection focused along boundaries for much
of the afternoon and early evening. The orientation of these
boundaries should focus convective development and may allow for
training and intense rainfall rates at times. This is supported in
particular by the 12Z HRRR, which depicts 4-8 inch rain amounts
across central Alabama and north-central Georgia through 12Z
Thursday. There remains some uncertainty in the exact placement of
the heaviest axis, but initial 12Z guidance is clustering on this
area for the potential of higher-end flash flooding (locally
significant), particularly for the urbanized areas like Birmingham
and Atlanta metros.


...California...
The initial wave of the AR is set to arrive after 00Z this evening,
with the best IVT values reaching the coast around 06Z. As a result
of this slightly faster timing and consensus of the models for rain
rates to approach or exceed 0.5"/hr after 09Z, a Slight Risk was
introduced. Through 12Z Thursday, rainfall totals of 1-1.5" will be
possible along the favored coastal terrain areas with some isolated
amounts nearing 2". Given this, isolated to scattered instances of
flooding will be possible.

Cook/Taylor

Day 2
Valid 12Z Thu Feb 13 2025 - 12Z Fri Feb 14 2025

..THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL PARTS OF THE
SOUTHERN CALIFORNIA COAST...

...California...
The peak of the atmospheric river is expected during this period
with the initial focus on coastal central California at the start
of the period, then the best IVT shifts southward through the
period, reaching southern California by the end of the period
Thursday night. IVT values from the GFS peak at around 600-700
kg/m/s initially along the central coast then settle around 400-500
kg/m/s as the AR reaches SoCal. The greatest QPF amounts will be
confined to the coastal terrain areas as well as for the Sierra.
Total amounts for the 24-hour period look to be between 1-3" with
the favored terrain areas and peaks topping out between 4- 6"+.

The primary concern is the potential for damaging and potentially
life-threatening burn scar flash flooding and debris flows,
particularly for southern California where the Moderate Risk (Level
3/4) is in place. Even outside the burn scar regions, the intense
rain rates and rainfall totals over the highly urbanized area could
bring scattered to numerous instances of flash flooding. Elsewhere,
a broad Slight Risk (Level 2/4) exists for much of the coastal
ranges as well as the Central Valley and lower elevations of the
Sierra Nevada.

A Moderate Risk of excessive rainfall continues from yesterday`s Day
3 ERO across portions of the Southern California coast, and for the
new Day 2 ERO was expanded slightly on the northwest side to
encompass the Lake Burn Scar (2024), while also eastward to include
the southwest San Bernardino Mountains, including the Line Burn Scar
(also 2024). Again, the primary concern is for damaging flooding and
debris flow from the numerous fresh burn scars in the region given
the 24-hour QPF in the 1.5 to 3.5 inch range (with localized higher
amounts). In addition, the area is highly urbanized and an
environment in which much of the rainfall will turn immediately to
runoff.

Farther inland, given the QPF trends and snow levels, the Slight
Risk was extended into eastern portions of the San Joaquin Valley
and southern Sierra/adjacent foothills. This expansion includes the
Borel burn scar (2024).

...Florida Panhandle and Southern Georgia...
Lingering heavy rainfall will continue Thursday across portions of
the Florida Panhandle and southern Georgia, ahead of the approaching
cold front. Rainfall amounts look to be generally in the 1-3"
range, with some potential for localized rain rates sufficiently
high enough to cause some urban/localized flooding concerns.

Hurley/Taylor


Day 3
Valid 12Z Fri Feb 14 2025 - 12Z Sat Feb 15 2025

...Southern California...
IVT plume weakens while pushing southward across SoCal to begin
the Day 3 Period. While the bulk of the event (Slight-Moderate
excessive rainfall risk) is expected on Day 2 or through Thursday
night, will maintain a Marginal Risk into the early part of Day 3
(Friday morning), especially if the southward AR progression slows
down a bit.

...Mid South...
Amplifying longwave trough across the Southwest-4 Corners Region
Fri-Fri night will lead to backing mid/upper layer flow and
deepening WAA/moistening downstream within a maturing Warm Conveyor
Belt (WCB). Thermodynamic profiles will respond appreciably as a
result, especially late in the period (Fri night) as the low- level
flow backs SSW and increases (to 50+ kts at 850 mb). This leads to
robust low-level moisture transport late Friday night into early
Saturday morning, with 850-700 mb moisture flux anomalies ramping
up to 4-5 standard deviations above normal per the ensembles. There
will also be a corresponding uptick in elevated instability as
well, to the tune of 500-1000 J/Kg towards 12Z Sat. While the main
event will be on Saturday (see Day 4 ERO), will maintain the
Marginal Risk inherited from yesterday`s Day 4 ERO given the
anticipation of the ingredients ramping up fairly quickly Friday
night, which given the probabilistic QPF along with the latest CSU
UFVS-verified ERO first-guess fields, could lead to localized
runoff issues across portions of the Mid South. by the pre- dawn
hours Saturday.

Hurley


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