FOUS30 KWBC 120741
QPFERD

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

Day 1
Valid 12Z 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...

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 (beginning at 12Z) 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 later this morning. 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 00Z HRRR, which depicts 5-9 inch rain amounts 
across southern Alabama through 12Z Thursday. Uncertainty in the 
specific placement of boundaries/heavy rainfall corridors and dry 
antecedent conditions are factors that preclude a higher risk, 
although some higher-end flash flood potential exists in this 
scenario - especially if complexes can materialize over more 
populated areas of LA/MS/AL/GA.

Cook


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
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