FOUS30 KWBC 140808
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
308 AM EST Fri Feb 14 2025

Day 1
Valid 12Z Fri Feb 14 2025 - 12Z Sat Feb 15 2025

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

Models continue to show ingredients coming together late in the
period to support a developing heavy rain that will evolve into a
much larger threat on Day 2. Deepening moisture coincident with a
southwesterly 50 kt low level jet, interacting with mid-level
energy, is expected to support showers and storms developing 
across the region Saturday morning. There is some signal in the hi-res
models showing storms beginning to train, raising an increasing 
threat for heavy accumulations near the end of the period. While 
widespread heavy amounts are not expected prior to 12Z, this 
continued signal in some of the guidance for training storms with
locally heavy amounts beginning to develop, warranted leaving the 
Marginal Risk in place across portions of the region. However, the 
areal extent of the risk was reduced some from the previous, 
confining the area mostly to where the 00Z HREF showed the highest
neighborhood probabilities for accumulations of an inch or more. 

Pereira

Day 2
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 heavy rainfall and flash 
flood event across the region on Saturday and Saturday Night. Models 
continue to depict a swath of 3-6 inches of rainfall in the 24-hour 
period, although there continues to be some variability on the exact
placement. Therefore, the maximum risk level was held at a Moderate 
Risk for this outlook, and the biggest change was to shift the 
overall footprint slightly further to the northwest (by 25-50 mi) 
based on trends in the 00 UTC model guidance. The Moderate Risk area 
was also trimmed a bit in the Central Appalachians where model QPF 
has been trending lower and the region will be removed from the area 
of greatest instability (and thus likely to see lower rain rates.)

Based on cluster analysis, a primary source of uncertainty seems to 
be related to the position of the surface low. This makes sense as a 
more northward track of the low would allow for a more significant 
poleward push of the warm front and overall warm sector. Although 
the 00 UTC NAM, NAM Nest, and GFS have trended the swath of rainfall 
further northwest, this is far from unanimous. In fact, AI-based 
versions of the ECMWF and GFS have been fairly consistent in their 
depiction of the overall scenario and are among the furthest south 
with the position of the surface low. Therefore, although the 
overall risk areas were trended slightly to the northwest out of 
deference to the overall ensemble envelope, the changes were 
incremental at this point and still include a reasonable chance of 
some of the southern scenarios unfolding.

An emerging point of consensus among many models, though, appears to 
be a focus of the heaviest rainfall generally between 85W and 90W 
longitude, or likely somewhere in W/C KY or NW TN. This is an area 
that would be favored to receive both the early round of training 
and backbuilding convection along the developing warm front in the 
morning and early afternoon, as well as more vigorous convection as 
the surface low tracks nearby with increasing surface-based 
instability. This would potentially maximize both the peak intensity 
of rain rates as well as the overall duration of heavy rain. It also 
happens to coincide with the most significant above-normal precip 
departures over the past 30 days, with precip in that period along 
an axis from DYR-LEX generally 1.5 to 2.0 times the normal amount. 
Therefore, this is an area that will continue to be monitored for 
any potential risk upgrades to a High Risk if it becomes apparent 
that the duration of heavier rain rates will last for at least 
several hours. More transient bursts of heavy rain would be less 
likely to lead to severe impacts, and it is the uncertainty of both 
the placement of the heavy rain and the extent to which it will be
concentrated in a sustained period that preclude a risk upgrade at 
the present moment.

The environment will be quite unusual for mid-February. The 
combination of MUCAPE of 500-1000 j/kg and PWATs reaching around 1.5 
inches has only rarely been observed in KY/TN at this time of year, 
and would be supportive of 1-2 inch per hour rain rates in the most 
organized and intense convection. A powerful southwesterly LLJ over 
60 knots, oriented at an acute angle to the developing warm front, 
will favor backbuilding into the region of stronger instability. And 
the overall mean flow should favor some periods of training 
convective clusters or bands near the warm front and surface low 
track. All of these ingredients are fairly classic for impactful 
flash flooding cases, and significant impacts are possible in this 
one as well.

Lamers

Day 3
Valid 12Z Sun Feb 16 2025 - 12Z Mon Feb 17 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL IN PORTIONS OF THE
MID-ATLANTIC...

...Mid-Atlantic...

A Marginal Risk was maintained across portions of the Mid-Atlantic 
region on Sunday, primarily due to the potential for briefly heavy 
rain rates with convection along an advancing cold front, and fairly 
low flash flood guidance values. However, the speed of the cold 
front should reduce the duration of any heavy rain, and thus any 
flash flood threat should remain rather isolated.

Lamers
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