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000
FOUS30 KWBC 152008
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
408 PM EDT Mon Apr 15 2024

Day 1
Valid 16Z Mon Apr 15 2024 - 12Z Tue Apr 16 2024

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS
OF NORTHEAST NEBRASKA INTO SOUTHEAST SOUTH DAKOTA...

16z Update:
Went ahead with a targeted Slight risk upgrade across portions of
northeast NE into southeast SD for tonight into Tuesday morning.
Convection is expected to periodically train/backbuild near and
just north of the stationary front across this region. Enough
instability is forecast to support hourly rain up to about 1.5"/hr,
with totals through 12z Tuesday locally exceeding 3" within/near
the Slight risk area. This should be enough to result in an
isolated to scattered flash flood threat. Given the dry antecedent
conditions and low streamflows this is likely a lower end Slight
risk...but excess runoff and flash flooding is possible in urban
areas. Even outside urban areas the dry ground and lack of green
vegetation does suggest that rainfall rates may overcome the
infiltration rate in areas...resulting in at least isolated
flooding concerns.

Chenard

...Previous Discussion...

A deepening low and amplifying jet will pivot into the Upper
Midwest creating broad areas of ascent while increasing low-level
southerly flow from the Gulf of Mexico. Precipitable water values
will surge to 1.25/1.5 over the Plains with moisture anomalies
peaking around 2.5 standard deviations. The environment will be
conducive for convection to develop ahead of the front that will be
capable of multiple hazards like very large hail, destructive
winds(possible tornadoes) and heavy rainfall. The nature of these
storms will be progressive which may somewhat limit the threat for
flooding however with the support of mixed-layer CAPEs of at least
1000-2000 J/Kg over the southern portion of the Marginal Risk area,
there will be a higher probability of more intense 1-3hr rainfall
rates compared to areas farther north. Higher FFGs (lower soil
moisture percentiles lower) over the southern portion of the
outlook area will maintain a Marginal or localized flash flood
risk.

Meanwhile, lower FFGs/wetter antecedent soils over the northern
portion of the outlook area, along with a better chance for more
prolonged activity along/north of the surface warm front, will be
offset by decided lower deep-layer instability (even elevated). So
even here, again the flash flood risk remains marginal.

Campbell/Hurley

Day 2
Valid 12Z Tue Apr 16 2024 - 12Z Wed Apr 17 2024

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS EASTERN
PORTIONS OF THE NORTHERN PLAINS SOUTHWARD THROUGH MUCH OF THE
MISSISSIPPI VALLEY...

20z Update:
We considered a Slight risk upgrade across portions of northeast IA
into southern MN and southwest WI. It is here where the heaviest
rainfall rates and totals are expected on Tuesday. Enough
instability is forecast to support 1-2"/hr rainfall over this area,
which will likely exceed 1hr FFG in spots. One limiting factor for
flash flooding will be the expected quick movement of cells,
limiting the duration of heavy rates. Some periodic
training/backbuilding is possible near the warm front, which
supports some areas exceeding 3" of total rainfall. However HREF 5"
exceedance probabilities drop to near 0, likely indicative of the
overall progressiveness of the system. Antecedent conditions and
streamflows are also generally below average across most of the
region. Thus despite the likelihood of some 1hr and 3hr FFG
exceedance, the coverage and magnitude of impacts are expected to
remain below Slight risk levels. The main threat will be to urban
areas, where the intense rates may lead to localized flash
flooding. Outside of urban areas the dry ground and limited green
vegetation may allow for some excess runoff, however these impacts
are expected to remain isolated as well. After coordination with
local WFOs, the consensus was to stick with just a Marginal risk
for now.

There may be an ongoing flash flood threat over southeast SD to
begin the period, but will let the day 1 Slight risk over that
area handle this threat for now. Pending observations later
tonight we may need to extend the Slight into this day 2 period.

Otherwise, did expand the Marginal risk into WV, where some slow
moving convection near a warm front may result in localized heavy
rains over an area that already has elevated soil saturation and
streamflows.

Chenard

...Previous Discussion...

The closed low and trailing cold front will advance from the High
Plains into the Upper Mississippi Valley while an west-east
orientated warm front extends across the Upper Midwest to the Mid-
Atlantic region. An axis of anomalous precipitable water values up
to 2 standard deviations above the mean, will persist along and ahead
of the well defined cold front while pooling along the warm front.
Organized convection will likely fire up in proximity to this
anomalous moisture axis. Like the previous day, the convection is
expected to be fairly progressive that continues to limit the
potential for heavy rainfall and associated areas of localized
flooding.

Locations along and north of the warm front have the greater
potential for organized precipitation to span from the Northern
Plains to the Upper Mississippi Valley/Upper Midwest. Heavy rain
will likely spread across locations north of the warm front and
maintain for longer durations as the moisture surging
northward from the Gulf of Mexico over the frontal boundary
persists. Consensus suggests broad areal averages of 1 to 2 inches
with locally higher amounts certainly possible. This region has
been relatively dry over the past few weeks, resulting in dry soils
and below average stream flows.

Campbell

Day 3
Valid 12Z Wed Apr 17 2024 - 12Z Thu Apr 18 2024

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF
THE GREAT LAKES, OHIO VALLEY AND WESTERN PARTS OF THE NORTHEAST...

20z Update:
Only minor changes to the inherited Marginal risk. Convection
moving across the OH Valley will be strong and capable of producing
high rainfall rates, but should generally be progressive in nature.
There is less instability with eastward extent, which should result
in lower rainfall rates. But given the above average soil
saturation and streamflows over portions of PA and NY the Marginal
looks good.

Chenard

...Previous Discussion...

Showers and thunderstorms will shift east as the closed low and
fronts advance through the Ohio Valley toward the Mid-Atlantic.
Much of the guidance has 1 to 2 inches for this period for parts of
the Great Lakes, eastern Ohio Valley and portions of the Northeast.
This part of the country has much lower FFG guidance from
antecedent rain thus are more sensitive to additional rainfall. A
Marginal Risk spans from Michigan/Wisconsin south to Kentucky and
east into western New York, Pennsylvania and Maryland.

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