FOUS30 KWBC 180815
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
415 AM EDT Sun May 18 2025

Day 1
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 
today 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 OK/KS into MO/AR, and a dryline stretching 
from central KS into 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.

Complicated convective evolution today into tonight, but the 
greatest risk of flash flooding will likely be with any upscale 
growth of convection that can occur this evening into tonight near 
the warm/stationary front. There is a significant amount of spread 
amongst the 00z HREF members and global guidance regarding 
convective details today, supporting the idea of a lower confidence 
forecast. However, while the confidence on the details may be lower 
than normal for a day 1 forecast, there is decent confidence that 
somewhere will see a more organized flash flood risk develop later 
today into tonight.

The overall expectation is that robust convective development occurs 
along and east of the dryline this afternoon from central TX into OK 
and KS. The activity over TX may try to grow into a few convective 
clusters, and rainfall rates should be intense enough for a 
localized flash flood risk. However it appears like the better 
threat for more widespread organized development is over northern OK 
into central KS, closer to the dryline/warm front intersection. This 
activity will have greater lower level convergence to work with and 
also be closer to the mid level vort energy swinging through the 
High Plains. This activity should grow upscale Sunday evening into 
the overnight, potentially into an MCS, at which point it should 
feel the impact of the strong southwesterly low level jet and begin 
turning easterly and then southeasterly aligned with the upwind 
Corfidi Vectors and along the instability gradient. As this process 
occurs some training/backbuilding on the southwest extent is 
probable resulting in a scattered flash flood threat.

The exact MCS location/track remains a bit uncertain. As already 
mentioned, the HREF members are all over the place, and thus the 00z 
HREF probabilities do not appear all that useful. Best guess at this 
point is something close to the 00z GEM reg might be reasonable. 
Also like the placement of the 00z AIFS and EC mean...with a 
maximum QPF somewhere from northeast OK into southeast KS, 
southwest MO and northwest AR. Do consider these areas to be a 
higher end Slight risk today, as where any training MCS does 
develop we could be looking at a swath of upwards of 3-5" of rain. 
Main thing to watch will be how far north the warm front gets today
over OK/KS and where convection first starts to grow upscale. If 
this all happens earlier/farther south then OK into northern AR 
become more in play for flash flooding...while if it happens later 
and farther north then we are looking more at eastern KS into 
central MO. Tend to think both the 00z and 06z HRRR are too far 
north, but also can not completely rule that scenario out either.

Another area of interest will be farther northwest into central and 
western NE, 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 drought and possible that a majority of the training 
convection ends up over the high FFG sand hills. These factors 
should keep the flash flooding localized in nature, and thus will 
maintain the Marginal.

The extension of the Marginal risk into Southeast accounts for 
organized convection that should be ongoing this morning, and then 
potential isolated redevelopment later today near the lingering 
boundary where substantial instability will support heavy rainfall 
with any storms that are able to develop.

...Northeast... 

A Marginal risk continues over portions of northern New England on 
Sunday as a deep layered low remains overhead. Instability and PWs 
are lower today, however cold air aloft and some surface heating 
should allow for showers and a few heavier convective cells to 
develop this afternoon. Both the 00z HREF and 18z REFS indicates a 
30-40% chance of exceeding 1" of rain in an hour, and cells will be 
slow moving.

Chenard


Day 2
Valid 12Z Mon May 19 2025 - 12Z Tue May 20 2025

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

Another day of widespread heavy rainfall expected across the middle 
portion of the country on Monday as the longwave trough and embedded 
shortwave energy slowly moves eastward. It will likely be an active 
day/night with a broad region where flash flooding will be possible. 
Not much change was needed to the inherited risk area, with a Slight 
risk stretching from northeast TX into southern IA. Expect 
convection to again develop Monday afternoon along the dryline and 
approaching cold front from TX into NE. The northern extent of the 
Slight risk will be near the warm/stationary front extending east 
across northern MO and southern IA. Strong forcing overrunning this 
boundary will likely result in an expansive area of convection, with 
some training/backbuilding possible. The southern portion of the 
Slight risk from TX into eastern OK and AR covers initial dryline 
development and possible upscale growth. While the low pressure 
driven stronger convergence will be farther north over IA/MO, there 
are indications that a corridor of enhanced low level moisture 
transport could evolve from TX into AR aided by shortwave energy at 
the base of the longwave trough. Several pieces of guidance thus 
depict a secondary max QPF swath over these areas from possible 
training convection. 

Exactly where the focus of highest QPF ends up remains a bit of a 
question, but generally anywhere within the Slight risk is in play. 
There are meteorological reasons (described above) for two maximum 
rainfall swaths, one farther north near the warm front over northern 
MO and southern IA, and another from northeast TX into AR. However
getting these details right at this lead time is difficult. So 
whether or not we end up seeing two separate areas of training 
convection or one more consolidated area, the main story is that 
the ingredients will be in place for training/backbuilding 
convection leading to excessive rainfall. Some of these areas will 
also likely see heavy rain on day 1 as well, and so hydrologic 
conditions could be more sensitive by this day 2 time frame. Thus 
do consider most of the Slight area as a higher end Slight risk at
this time, and think areas of flash flooding are likely. It seems 
possible (maybe even probable) that an embedded MDT risk will 
eventually be needed somewhere within the broad Slight area. 
However uncertainty in both the convective details and hydrologic 
conditions (pending what rainfall happens on day 1) leads to low 
confidence in any MDT risk location at this time. Thus we will hold
with a higher end Slight and continue to monitor trends.

Chenard


Day 3
Valid 12Z Tue May 20 2025 - 12Z Wed May 21 2025

The system described in the day 1 and 2 discussions begins moving 
east on Tuesday. The overall setup for heavy rainfall amounts is 
generally not as good by this time as it was on previous days 
farther west. The large scale forcing is more progressive by Tuesday 
and the degree of instability is not as robust. Thus not surprising 
that the global model QPFs are generally lower than what is forecast 
farther west on day 2. With that said, there is likely still an 
excessive rainfall risk Tuesday. While the system is becoming more 
progressive by this time, there will be a stationary front extending 
from IL into KY through, with enough moisture transport and synoptic 
ascent to support a broad area of convection along this front. This 
setup thus could allow for either multiple convective rounds near 
the front, or even a more prolonged period of repeat/training 
convection. The main question will be whether we are able to 
maintain enough instability to support prolonged intense convection 
and also whether the mid/upper level forcing is enough to allow for 
a persistence of stronger convection. It is also worth noting that 
portions of KY and WV are more sensitive to additional rain, with 
higher soil saturation and streamflows in place.

Overall think this is a solid Slight risk, with isolated to 
scattered flash flooding possible anywhere from IA to 
KY/TN/WV...generally along and just south of wherever the 
stationary/warm front ends up by this time. The potential for 
greater flash flood coverage and higher impacts is 
uncertain...noting the aforementioned uncertainties with 
forcing/instability and the general downtick in model QPFs by this 
time. However the orientation of the forcing and stationary front 
does suggest a potential for training and more significant rainfall 
totals, which could open the door for a corridor of more widespread 
and/or higher impacts. The 00z RRFS depicts a swath of strong 
convection Tuesday, but generally shows things progressive, keeping 
most areas in the 1-2" range...likely not enough for widespread 
flash flooding. However it does show some swaths over 3", especially 
over portions of KY and TN where a bit more training occurs. So we 
will continue to monitor trends, and pay close attention to areas 
that may be more sensitive to additional rainfall, such as KY and WV.

Chenard

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