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FOUS30 KWBC 210058
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
858 PM EDT Mon May 20 2024

Day 1
Valid 01Z Tue May 21 2024 - 12Z Tue May 21 2024

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

...01Z Update...

...Midwest states...

The existing Slight Risk area was expanded some to the south to
include more of southeastern Nebraska and into southwestern Iowa,
with the latest CAM guidance, especially the HRRR and NAM conest
suggesting the main QPF axis could be a little southeast of earlier
expectations. The Slight Risk area was also extended into far
southern Minnesota to account for current radar trends with slow
moving cells southeast of the greater Minneapolis-St. Paul metro
area. The potential exists for 2 to locally 4 inches of rainfall
through 12Z within the main moisture convergence axis, and much of
this could fall within a two hour time period where cells train
over the same area.

...Western Great Lakes...

A Marginal Risk is in place for eastern portions of the Lower
Peninsula of Michigan into northern Indiana with a broken line of
thunderstorms slowly tracking eastward. The best potential for
training convection based on current radar trends is across
portions of northeast Illinois (south of Chicago) and into northern
Indiana as the flow becomes more parallel to the surface boundary.
The latest HRRR runs are likely underdone with QPF across this
region, and the NAM conest and FV3 on the higher end of the
guidance.

Hamrick


---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...

...2030Z Update...

Today`s 06/12Z ensemble and deterministic guidance has placed the
axis of heaviest QPF a little farther north and west than where
the 12Z HREF mean has its heaviest QPF. Recent trends have been
for a more amplified solution, even on recent CAMs, that would
suggest a slightly more westerly orientation of the heavier QPF.
As the storm deepens Tuesday, the IVT will reach highly anomalous
levels for late May, surpassing 750 kg/m/s at 18Z over western IA.
PWs over western and central MN will top 1.5" between 18Z Tues -
00Z Wed and sampled soundings in the area sport warm cloud layers
deeper than 9,000ft. While instability on the northwest flank of
the storm will be hard to come by, the efficient warm rain
processes involved can still produce steady 1-1.5"/hr rainfall
rates beneath the deformation zone over multiple hours Tuesday
afternoon.

A similar situation occurred last week in southern New England
where instability was lacking, but highly saturated/deep warm
cloud layer environments still supported efficient rainfall rates
that caused flooding. In that same case, over 4" of rainfall
occurred in southern New England and those kind of totals are
being modeled in both CAMs and deterministic guidance. The 12Z HREF
does depict low chances (20-35%) for localized rainfall totals on
Day 2 that could surpass 5" in portions of central MN. Should
probabilistic guidance increase further in the coming shifts, there
is a scenario where a Moderate Risk may need to be considered. At
the moment, given most soils are not highly saturated and there
remains some uncertainty on which areas see the heaviest rainfall,
maintained a Slight Risk as the highest threat level. Did extend
the Slight Risk south to include more of northern IA where their
soils will be more sensitive following the thunderstorms rolling
through Monday night and early Tuesday morning and another round of
storms ahead of an approaching cold front could bring more flash
flooding opportunities Tuesday afternoon and evening.

Mullinax

---Previous Discussion---

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...

...2030Z Update...

Adjustments to the threat areas this forecast cycle were to expand
the Slight and Marginal Risks areas farther south where greater
moisture content (up to 2" PWs) and instability (1,000-2,000 J/kg
MLCAPE) looks to be present. This setup also takes place beneath
unusually strong WSW 250-500mb mean winds aloft, along with an IVT
above the 90th climatological percentile that is intersecting the
aforementioned frontal boundary Wednesday afternoon and into
Wednesday night. With modeled soundings also depicting warm cloud
layers as deep as 12,000-13,000ft Wednesday afternoon, this setup
has a higher ceiling for potential flash flooding and will be
closely monitored in subsequent forecast cycles. For now, no
consideration of a Moderate Risk today given the lingering
uncertainty on QPF and the axis on where the heaviest amounts
occur.

Mullinax

---Previous Forecast---

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