FOUS30 KWBC 140830
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
430 AM EDT Wed Aug 14 2024

Day 1
Valid 12Z Wed Aug 14 2024 - 12Z Thu Aug 15 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
NORTH DAKOTA AND MIDDLE MISSISSIPPI VALLEY...

A bit greater uncertainty than normal with how convection evolves 
this morning across portions of IA/MO/AR, although things are
becoming a bit clearer as of 08z. The convection moving across NE 
(as of 08z) will likely be over portions of western IA and 
northwest MO by 12z this morning. It will likely align itself more 
northwest to southeast by this time, allowing for a greater 
training potential. However the longevity of this training is a bit
uncertain as the low level jet gradually weakens. Suspect that an 
isolated to scattered flash flood risk will exist, but not 
confident in anything more significant than that at this time. 

Another axis of training convection is likely this morning across
portions of southern MO into northeast AR. As of 08z starting to
see the beginning of this scenario starting to evolve, and expect
to see an uptick in organization and training between now and 12z.
NAM Nest runs have indicated localized 5-10" rainfall with this
event, and while not impossible, it does seem probable that 
amounts will stay below that level. However still thinking several
inches of rainfall is likely, locally higher, and thus do expect a
corridor of isolated to scattered flash flood risk to develop. 
This activity will likely continue into the morning hours before 
dissipating by later in the morning

By this afternoon into evening expect convection to develop across
far southeast NE into western IA and northwest MO, with the focus 
expected to be near a warm front slowly lifting north across the 
region. Stronger mid/upper forcing will move into the region, which
combined with strengthening low level moisture transport, should 
allow for organized convective development. Suspect that the 
convection this morning across NE/IA may help keep the warm 
frontal position a bit further south today. Given the heavier 
rainfall rates should generally be near and just south of the warm 
front, we were able to trim some of the northern areas (MN and 
northern IA) out of the Slight risk with this update...and this is 
generally supported by 00z HREF QPF probabilities and FFG 
exceedance probabilities. Area of heavy rainfall are still likely 
over northern IA into MN, but rates should stay low enough to keep 
any flash flood risk more localized in nature. Even over northern 
MO into central IA this late day and overnight convection will 
probably be rather quick moving, limiting rainfall total 
potential. However we could see some brief training near the warm 
front, and some of these areas will have also seen heavy rain from 
the morning round of convection. Thus do think at least some flash
flood risk will exist with this later convection as well.

Overall the broad Slight risk stretches from central IA to
northeast AR, and covers the risk from both the convection this
morning and also later today/tonight. The southern part of this
Slight risk is primarily for this mornings activity, with the
northern portion of the risk potentially getting both rounds of
storms.

A secondary rainfall maximum should occur over portions of ND this
morning into early afternoon along another slow moving convergence
axis. Hard to ignore the impressive 00z HREF signal over this 
area, with 3" neighborhood probabilities in the 50-80" range, and 
6hr FFG exceedance probabilities over 40%. There was a localized 
training cell that resulted in a flash flood warning last night 
over ND, and the pattern does appear favorable for that potential 
this morning as well, except probably a bit more convective 
coverage today. Thus think adding a Slight risk is the way to go, 
with isolated to scattered flash flooding a possibility.

Chenard


Day 2
Valid 12Z Thu Aug 15 2024 - 12Z Fri Aug 16 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
MISSOURI, ILLINOIS, INDIANA AND KENTUCKY...

The heavy rainfall threat shifts east into the Great Lakes and OH
Valley by Thursday. A bit more uncertainty with the details by
this time, but the overall ingredients remain in place for areas
of excessive rainfall. Expect one round of convection to be moving
across the region Thursday morning, but currently anticipating 
this activity to be on a downward trend by this time and thus 
should not be posing much of a flash flood risk. However with 
deeper troughing still upstream at this point, would expect to see
additional convective development by the afternoon hours. This 
convection is likely from portions of eastern MN and WI southward 
into portions of MO/IL/IN/KY. Convection across the northern half 
of this area will probably stay pretty quick moving off to the 
east, likely limiting the extent of the flash flood risk, keeping 
the threat more at Marginal risk levels.

The greater instability and moisture axis will likely end up 
further south from southeast MO into portions of IL/IN and KY. Even
here cell motions should be rather quick, however with this area 
further displaced from the forcing to the north and mean flow more 
parallel to the front, do expect that we could at least see some 
brief training of convection. Given the favorable thermodynamic 
environment for heavy rainfall rates, any training would likely 
result in at least some flash flood potential. This appears most 
likely along this corridor, and thus will carry a Slight risk here.

Chenard


Day 3
Valid 12Z Fri Aug 16 2024 - 12Z Sat Aug 17 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SOUTHEAST INTO THE GREAT LAKES...

On Friday the likelihood of convection shifts eastward stretching
from portions of the Southeast up into the Great Lakes region. The
probability of flash flooding by this time appears to decrease, 
but at least some risk likely persists. Stronger deep layer flow 
supports quicker cell motions and not really seeing much of a low 
level focus for training. However, the troughing moving overhead is
quite broad in nature, likely meaning the duration of lift will be
extended, potentially supporting a few rounds of convection. Also 
deep layer flow, while quick, is fairly unidirectional, which can 
support some brief backbuilding of convection at times. Thus while 
the overall setup for flash flooding and model QPF output are not 
all that impressive, there are enough favorable ingredients in 
place to suggest at least an isolated flash flood risk will exist. 
This warrants the continuation of the Marginal risk.

Chenard
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