FOUS30 KWBC 160823
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
423 AM EDT Fri May 16 2025

Day 1
Valid 12Z Fri May 16 2025 - 12Z Sat May 17 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
OHIO AND TENNESSEE VALLEYS ALONG WITH WESTERN MAINE...

...Arklatex into the OH/TN Valleys...
A very active day convectively is expected across large areas of
the OH/TN Valley region as a deep layer closed low and associated
trough ejects east across the Upper Midwest which will drive a 
cold front gradually southeastward through this afternoon and 
evening. A very unstable airmass is expected to pool across much of
the Mid- South and OH Valley region in general with MLCAPE values 
likely to reach as high 2500 to 4000+ J/kg while coinciding with a 
significant amount of deep layer shear. Much of the shear will be 
associated with stronger 500/700 mb wind fields rounding the 
southeast flank of the aforementioned upper-level trough with 
associated jet energy. An EML is already showing up in regional 00Z
RAOB soundings and is at least suggested in GOES-E low-level WV 
imagery early this morning. And this coupled with strong warm air
advection/boundary layer heating ensuing by midday and along with 
deeper layer jet-aided ascent, very strong/severe- mode convection 
is likely to develop which will include a threat for supercells 
across areas of the OH/TN Valley region.

Cell-merger activity and localized training of these organized 
clusters of convection is expected which aside from the severe mode
of the convection will support concerns at least scattered areas 
of flash flooding. In fact, the 00Z GFS shows a corridor of strong
850/700 mb moisture flux anomalies taking aim on KY and TN in the
18Z to 06Z time frame, and the 00Z HREF guidance along with recent
HRRR/RRFS runs suggest areas of especially central/southern KY and
northern TN seeing as much as 2 to 4+ inches of rain as these 
clusters of convection eventually become aligned in a more linear 
fashion with QLCS evolution expected in time. Moist/wet antecedent 
conditions will favor locally enhanced runoff concerns with these 
rainfall totals. The Slight Risk area has been adjusted a bit 
farther south and southwest compared to continuity to account for 
the overall organized convective footprint and with locally high 
rainfall rate (1 to 2+ inch/hour rates) expected. The Marginal Risk
area has been expanding as far southwest as the Arklatex vicinity.

...Mid-Atlantic/Northeast...
A more uncertain evolution of convection is expected today and
tonight for much of the Mid-Atlantic and Northeast. Initially,
strong warm air advection and shortwave energy lifting across the 
lower Great Lakes/northern Mid-Atlantic region early this morning 
may favor some scattered clusters of fairly organized shower and 
thunderstorm activity, but the latest hires model guidance shows a 
fair amount of disagreement with overall rainfall amounts and 
placement early this morning as this energy lifts off to the 
northeast. However, in the 18Z to 00Z time frame, the arrival of 
additional shortwave energy/divergent flow aloft and greater 
instability is expected to favor scattered areas of redeveloping 
convection that will be possible across portions of the northern 
Mid-Atlantic and also northward into central and northern New 
England in close proximity to a warm front that will be riding 
northeastward. This will include a threat for locally slower-moving
and more concentrated convective cells over some of the higher 
terrain, including western Maine. The 00Z HREF guidance suggests 
some pockets of 2 to 3 inch rainfall totals with some locally 
elevated 3-hour FFG and 5-year ARI exceedance probabilities being 
depicted here. Some isolated to scattered areas of flash flooding 
cannot be ruled out given the moist/unstable environment favoring 
high rainfall rates and some of these locally heavier storm totals.
As a result, the Marginal Risk of excessive rainfall has been 
expanding into the Northeast with a Slight Risk introduced over the
higher terrain of western Maine.

Later tonight, the central Mid-Atlantic also may become a target 
for additional convection as the upstream QLCS activity over the 
OH/TN Valley region arrives, and with already sensitive/wet 
antecedent conditions here, additional localized flash flooding 
concerns will be possible. The Marginal Risk area has been
expanded to include much of the central and northern Mid-Atlantic.

Orrison


Day 2
Valid 12Z Sat May 17 2025 - 12Z Sun May 18 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM CENTRAL TEXAS 
THROUGH CENTRAL ALABAMA AS WELL AS FOR PORTIONS OF THE NORTHEAST...

...Texas to Alabama...
Convection will likely develop across portions of central TX
Saturday afternoon as increasing forcing overruns the dryline. 
Enough mid level vort energy, upper jet support and robust 
instability to support likely upscale growth of convection as it 
moves into northeast TX. The extent of the flash flood risk will 
likely come down to convective longevity at any one location as 
cells should be pretty fast moving. This fast movement may limit 
the areal coverage of flash flooding, however certainly an 
opportunity for at least some training as convection grows upscale.
The 00z CAMS and AIFS suggest northeast TX towards the AR/LA 
border has the best chance of seeing convective training 
potentially leading to a flash flood threat. Overall still not 
enough model support to suggest Slight risk coverage, but localized
flash flooding appears probable with this setup, especially over 
more sensitive urban areas. Areal averaged rainfall may only 
average around an inch, but would expect to see localized totals 
around 3", much of which would fall in just a couple hour period.

Convection will likely be ongoing across the eastern portion of the 
Marginal risk (MS/AL) at 12z Saturday, but likely in a decaying 
state. However we could see at least isolated to scattered 
redevelopment along the leftover outflow during the day Saturday, 
and anything that does develop would likely be slow moving and 
capable of resulting in a localized flash flood threat.

...Northeast...
A couple rounds of convection appear likely across the Northeast 
on Saturday...one in the morning and another by afternoon/evening. 
Both rounds are expected to feature quick moving cells with deep 
layer mean flow over 30 kts. This will likely limit the extent of 
any flash flood risk, however localized 1" in an hour amounts are 
still probable given the moisture and instability forecast. These 
higher rates combined with the potential for multiple convective 
rounds supports an isolated flash flood threat and a continued 
Marginal risk area.

Chenard


Day 3
Valid 12Z Sun May 18 2025 - 12Z Mon May 19 2025

...THERE IS SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE 
CENTRAL PLAINS INTO THE LOWER AND MIDDLE MISSISSIPPI VALLEY...

...Plains, MS Valley and Southeast...
A favorable pattern for widespread convection and heavy rainfall 
on Sunday across the middle portion of the country. A longwave 
trough and embedded shortwaves pushing eastward across the Rockies 
and into the Plains will provide ascent over a broad region. The 
frontal pattern will likely feature a developing low moving into 
the High Plains of CO/NE, a warm or stationary front extending east
of this low across portions of KS into MO/AR, and a dryline across
OK/TX. A large pool of instability is expected east of the dryline
and south of the stationary front, with values upwards of 
4000-5000 j/kg. Some convection may be ongoing near the 
warm/stationary front Sunday morning, but more robust development 
is expected by Sunday afternoon into the overnight hours as 
stronger forcing ejects into the Plains. 

The main area of focus for flash flooding appears to be across 
portions of eastern KS into MO and/or AR near the aforementioned
stationary/warm front. Convection should first develop over central 
KS with upscale growth into an MCS likely. Mean flow is off to the 
northeast, however with a strong southwesterly low level jet, 
Corfidi vectors are pointed more towards the southeast. Thus would 
expect convection to turn easterly and then southeasterly as it 
organizes Sunday evening. As this process occurs some 
training/backbuilding on the south/southwest extent is probable 
resulting in a scattered flash flood threat. The exact MCS 
location/track remains uncertain, but think the 00z GFS is likely 
too far to the north and east...with the aforementioned expected MCS
propagation taking it on a farther southern track. Thus tend to 
think something closer to the 00z ECMWF and UKMET is more likely. 
The experimental 00z RRFS (the first CAM to go out into day 3) also 
seems to show a plausible evolution and placement over eastern KS 
into western MO. The setup does have the potential for a swath of 
over 5" of rainfall where training/backbuilding ends up being 
maximized. Thus this is trending towards a higher end Slight risk, 
with at least scattered flash flooding probable.

Another area of interest will be farther northwest into western NE 
and southwest SD, closer to the low track. A multi-model signal 
exists for a second QPF max over this area...which makes sense given 
what should be impressive synoptic driven ascent and instability 
along the stationary/occluded front. Considered expanding the Slight 
risk into this area...but opted against that for now. This area 
remains in severe to extreme drought and was not ready to go with a 
large Slight risk area yet. But we will continue to monitor trends 
and an expansion of the Slight risk may eventually be necessary.

A localized flash flood risk also exists both along the dryline from 
central TX into OK...and over the Southeast where a lingering 
boundary and substantial instability will support heavy rainfall 
with any storms that area able to develop.

...Northeast...
A Marginal risk continues over portions of northern New England on
Sunday as a deep layered low remains overhead. This will be the 
3rd straight day of localized heavy rainfall, and thus some areas 
may be more sensitive by this time. Generally thinking both 
instability and moisture will be lower by Sunday, however cold air 
aloft under the low will likely still support some heavier 
convective cells and an isolated flash flood risk.

Chenard

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