FOUS30 KWBC 130812
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
312 AM EST Thu Feb 13 2025

Day 1
Valid 12Z Thu Feb 13 2025 - 12Z Fri Feb 14 2025

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF SOUTHERN CALIFORNIA...

...California...
Overall, not much change to the previous forecast, with the 
unfolding atmospheric river expected to peak near the start of the 
period.  A long warm front currently extending along the coast is 
expected to press inland this morning, with IVTs climbing to around 
700-800 kg/m/s near the upstream cold front as it approaches the 
coast later this morning. The strongest moisture flux is expected 
to center near SF Bay at the start of the period, before steadily 
shifting south along the coast today. Models continue to show IVTs 
decreasing as the axis moves south, dropping to around 400-500 
kg/m/s as the AR reaches Southern California later this afternoon 
into the evening. Guidance has remained fairly consistent, 
indicating precipitation amounts of 4-6 inches along the central to
southern coastal ranges and the Sierra Nevada, falling mostly as 
rain below 6000 ft. 

A Moderate Risk (level 3/4) remains in place across portions 
Southern California, from Santa Barbara County southeastward into 
the San Gabriel and San Bernadino mountains, where damaging and 
potentially life-threatening burn scar flash flooding and debris 
flows remain a primary concern. Even outside of the burn scar areas, 
intense rain rates over complex terrain and urban areas may produce 
scattered to numerous instances of flash flooding. Elsewhere, a 
broad Slight Risk (Level 2/4) exists for much of the remaining 
coastal ranges as well as parts of the Central Valley and the lower
elevations of the Sierra Nevada. 

...Florida Panhandle and Southern Georgia... 

Lingering heavy rainfall will continue this morning across 
portions of the Florida Panhandle and southern Georgia ahead of an 
approaching cold front. HREF neighborhood probabilities indicate 
that additional rainfall amounts of 1-2 inches are likely within 
the highlighted area, with some potential for localized rain rates 
high enough to cause some urban/localized flooding concerns.

Pereira


Day 2 
Valid 12Z Fri Feb 14 2025 - 12Z Sat Feb 15 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE MID-SOUTH...

...Mid-South... 

Although most of the convective activity, and thus heavy rainfall 
threat, in the region should be concentrated in the Day 3 period 
(after 12Z Saturday), model guidance does indicate that convection 
is likely to begin blossoming late Friday Night. Conditions will 
become increasingly favorable after 06Z Saturday with a strong 55-65 
knot low-level jet expanding east from the Plains, and significant 
northward moisture transport leading to rapid moistening of the 
boundary layer. MUCAPE values of around 500-1000 j/kg should support 
organized convection with the potential for rainfall rates in excess 
of 1 inch per hour at times. Although there is a lack of a 
significant front or boundary to help organize a band of convection, 
the strength of the LLJ may support backbuilding within a broad zone 
of developing convective clusters. This may be enough for some brief 
training and a ramp-up in flash flood potential prior to 12Z 
Saturday. The 00Z runs of the NAM Nest and FV3 show more widespread 
convection than other models; if this trend persists as additional 
hi-res guidance becomes available, an expansion of the Marginal Risk 
and/or the addition of a Slight Risk area may become warranted with time.

...Southern California... 

The Marginal Risk was removed for southern California given a more
limited QPF signal on the latest guidance, and hi-res models in 
excellent agreement on a lack of organized convection. The bulk of 
the threat should be in the Day 1 period.

Lamers


Day 3
Valid 12Z Sat Feb 15 2025 - 12Z Sun Feb 16 2025

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE TENNESSEE AND OHIO VALLEYS AND THE CENTRAL APPALACHIANS...

...Ohio and Tennessee Valleys...

Confidence remains high in significant rainfall-related impacts on 
Saturday and Saturday Night with model guidance in excellent 
agreement depicting a swath of 3-6 inches of rainfall in the 24-hour 
period across the region. Although models vary slightly with the 
placement, they almost unanimously depict a corridor of heavy rain 
about 750mi long centered somewhere in TN/KY. This same region has 
generally seen above average precipitation in recent weeks, with the 
14-day precip in much of KY, N TN, WV, and W VA at least double the 
normal values. The confluence of these factors should make the 
region especially vulnerable to flooding from the predicted 
rainfall. For additional context on the current hydrologic picture 
across the region, refer to the National Hydrologic Discussion from 
the National Water Center at water.noaa.gov.

Convection may be ongoing at the beginning of the forecast period 
and should become increasingly focused along a developing warm front 
to the east of a surface low that will be emerging out of the Plains 
on Saturday. The front will be positioned on the nose of very strong 
SW, or even WSW, inflow, which should favor backbuilding and 
training and a corridor of very heavy rainfall. The environment will 
be atypical of mid-February with PWATs reaching around 1.5 inches 
and MUCAPE of 500-1000 j/kg. For example, the Nashville sounding 
archive exists back to 1948 and they've only ever reached 1000 j/kg 
MUCAPE once in February, and 1.5 inch PWATs a few times. The 
environment will support rainfall rates in excess of 1 inch per 
hour, and given the potential for training and backbuilding, these 
rates may be sustained for several hours in some locations. These 
would be the areas of greatest concern for flash flooding -- not 
only in terms of likelihood but severity -- particularly if the 
higher rain rates are sustained over urban areas or vulnerable 
terrain for extended periods of time.

The inherited Moderate Risk was mostly maintained as-is with a 
slight expansion to the southwest in the vicinity of Memphis, TN; 
and a more significant expansion eastward into S WV and W VA. The WV-
VA border region just received some significant snowfall, and model 
guidance indicates a fairly high probability of warm, moist air (dew 
points in excess of 45 degrees) accompanying the rainfall into this 
area, which may lead to a combination of runoff from rain and snow melt.

It's worth noting a couple additional things. First, the amount of 
rain forecast from E KY into S WV and W VA in particular is quite 
unusual for the heart of the winter season. Calendar day precip 
records across that area for January and February are generally in 
the 2-3 inch range, and NBM probabilities indicate it is likely (60+ 
percent chance) we will see these types of 24-hour rainfall totals 
in those areas. Second, this relatively rare winter rain event would 
be delivered to an area that has been quite wet recently, and in 
some areas has snow on the ground. And third, the high-end scenarios 
in much of the Moderate Risk area would be significant by historical
standards. The NBM 90th percentile from N TN into S/E KY and S WV is 
on par with the top several wettest days (for any season!) in the 
station records in the area. This corridor is where the greatest 
concern for severe impacts exists, and it's possible an upgrade to 
High Risk may be needed for portions of the area in subsequent updates.

Lamers
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