Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1158 AM EDT Mon May 20 2024

Day 1
Valid 16Z Mon May 20 2024 - 12Z Tue May 21 2024

...THERE IS A SLIGHT RISK FOR PORTIONS OF EASTERN NEBRASKA AND WEST
CENTRAL IOWA...

...16Z Update...

...Western Corn Belt...

Following the special ERO issuance for the Florida Keys (see
below, which remains on track), the Marginal Risk in the Central
Plains and Mid Missouri Valley was expanded slightly farther south
and east. In addition, a Slight Risk was added from eastern NE on
east into west-central IA (in collaboration with OAX/FSD/DMX). By
06Z tonight, the region will be ideally placed beneath the left-
exit region of a robust 120kt 250mb jet streak located over NM.
This area will coincide at the nose as a 50kt LLJ that will overrun
a stalled frontal boundary position along and just south of the
Slight Risk area. NAEFS SAT for 06Z tonight shows an impressive
plume of 850-925mb mean specific humidity topping the 99th
climatological percentile over eastern NE and western IA, as well
as a growing footprint of >90th climatological percentile PWs
throughout the region. 14Z HRRR soundings along the NE/IA border
between 06-09Z tonight show highly saturated atmospheric soundings
along with as much as 1,000J/kg of MLCAPE and roughly 40kts of
sfc-6km shear to sustain blossoming clusters of thunderstorms. The
concern is for training along the front tonight in areas of
eastern NE and west-central IA where NASA SPoRT-LIS shows elevated
0-10cm volumetric soil moisture. This suggests soils are a little
more sensitive and may struggle to take in hourly rainfall rates
topping 1.5"/hr. Latest 12Z HREF sports 25-35% probabilities of
3-hr QPF > 3-hr FFGs between 06-12Z Tues within the Slight Risk
area where storms could produce hourly rainfall rates as high as
2"/hr. Given these factors, the Slight Risk was added for this
forecast cycle. With heavy rainfall likely to continue in parts of
the region into early Tuesday morning, morning commuters may have
to contend with flooded roads, particularly in lower lying areas
and along creeks/streams.

Elsewhere, given radar and satellite trends, opted to trim the
Marginal Risk out of northern IL given the fast storm motions in
the region. The MCV is advancing directly into southern WI, where
some additional thunderstorm development may ensue this afternoon
in areas already seeing thunderstorm activity this morning. This
led to maintaining the Marginal Risk in southern WI for the 16Z
update.

Mullinax

---1330Z Special ERO Issuance---

...Florida Keys...

It has been an active morning in the Florida Keys where rainfall
gauges have been filling up to as much as 4-7 inches thanks to
training convection aloft. Surface convergence oriented WSW-ENE and
healthy upper level divergence is working in tandem with roughly
1,500-2,000 J/kg of MLCAPE nearly 40 knots of surface-6km vertical
wind shear according to the 12Z KEY sounding. The 1km and 6km wind
barbs are out of the WSW and W respectively, which will keep the
orientation of line of heavy thunderstorms over the Keys for
several more hours. Hourly rainfall rates of 2-3"/hr will continue
through late morning but eventually, instability will be exhausted
and storms should dissipate later this afternoon. While the Keys
typically drain excessive rainfall well, high tide is approaching
and can affect drainage. Given these factors, a Marginal Risk was
issued for the Florida Keys.

Mullinax


---Overnight Discussion---

...Central Plains into the Mid Missouri Valley...
The amplifying mid to upper level trof pushing east across the
Central to Northern Rockies will ignite the next round of active
Central Plains convection late Monday afternoon, continuing into
the early hours of Tuesday. Overall favorable conditions for active
convection with strengthening of the low level flow, strengthening
boundary layer convergence and favorable upper difluence in the
vicinity of the frontal boundary forecast to stretch from the lee
of the Central Rockies, east northeastward into the Upper
Mississippi Valley. There is spread among the latest hi res
guidance as to whether this next round of convection will
consolidate into one distinct line as per the ARW, or have more
than one round of convection moving along the frontal zone as per
the NAMNEST, FV3LAM and ARW2. However, there is consensus that
either way, the convection will be fairly progressive to the east-
northeast, and continue to be a detriment to very heavy rainfall
amounts. We have kept the risk level at marginal given the expected
quick movement. This is reflected in HREF neighborhood
probabilities for 1"/hr amounts that are in the 20-45% range in the
marginal risk area, but much lower, mostly less than 15% and more
geographically spotty for 2"/hr amounts.

...Southeast Wisconsin into northeast Illinois...
The well defined MCV moving into the Lower Missouri Valley early
this morning is expected to remain intact as it pushes
northeastward into the Upper Mississippi Valley and Upper Lakes
region Monday into Monday night. While there may be weakening to
the current organized convection associated with this MCV early day
1, there is the possibility for additional convective enhancement
ahead of the MCV late Monday morning into afternoon across
southeast Wisconsin into northeast Illinois. No significant changes
made to the previous marginal risk area with concerns primarily
for urbanized runoff issues from Chicago, north through Milwaukee
and Green Bay.


Oravec

Day 2
Valid 12Z Tue May 21 2024 - 12Z Wed May 22 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE UPPER MISSISSIPPI VALLEY...

A strong vort ejecting northeastward on the eastern side of the
amplifying Rockies trof late day 1 into day 2 will become
increasingly negatively tilted as it pushes northeast across the
Central Plains into the Upper Mississippi Valley. This will help
deepen surface low pressure across the eastern portions of the
Central Plains early day 2, moving northeast into the Upper
Mississippi Valley/Upper Lakes region later day 2. There is pretty
good consensus for an axis of heavy comma head/deformation precip
from far eastern South Dakota, across large portions of Minnesota
and northern Wisconsin. Consensus is for two well defined areas of
precip to push northeastward across the Upper Mississippi Valley
into the Upper Lakes, comprised of a comma head/deformation precip
area on the northwest side and a more progressive convective
precip area on the southeast side. Similar to day 1, the expected
progressive convection on the southeast side of this system will be
a detriment to very heavy totals, while slower moving/pivoting
comma head precip should support potential for heavy total rainfall
amounts, although rates may not be very high. Stream flows as per
the National Water Model, remain high where the heavier comma
head/deformation precip is expected and correspond to the slight
risk area. There were only some minor changes made to the previous
slight risk area, decreasing it on the southeast side over
southeast Wisconsin into northeast Illinois where the convective
events are expected to be progressive.

Oravec

Day 3
Valid 12Z Wed May 22 2024 - 12Z Thu May 23 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM SOUTHEAST
OKLAHOMA, ACROSS THE LOWER ARKANSAS VALLEY INTO THE MID MISSISSIPPI
VALLEY...


The trailing frontal boundary from the low moving northeastward
from the Upper MS Valley into the Upper Lakes region day 2, will
slow significantly on the southern end from the Southern Plains
into the Lower Arkansas and Mid Mississippi Valley during day 3 as
it becomes aligned more parallel to the southwest flow aloft. PW
values expected to rise to 2+ standard deviations above the mean
along this slowing frontal zone, supporting potential for heavy
rainfall total from southeast Oklahoma, northeastward through the
Ozarks of Arkansas and Missouri and toward the Mid Mississippi and
Lower Ohio Valley regions. Shortwave energy embedded in the
southwest flow aloft will enhance uvvs in this anomalous PW axis
Wednesday afternoon continuing into the early hours of Thursday.
Training of cells parallel to the boundary is a concern for heavy
to locally excessive rainfall totals day 3 with the slight risk
drawn for the max qpf consensus and across regions where soil
moisture and stream flows are high. Changes to the previous outlook
were to narrow the slight risk area to better match current qpf
consensus and best region for potential training of precipitation
areas.


Oravec


Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: www.wpc.ncep.noaa.gov/qpf/99epoints.txt