Excessive Rainfall Discussion...UPDATED
NWS Weather Prediction Center College Park MD
226 PM EDT Tue Apr 16 2024

Day 1
Valid 1830Z 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...

...1815Z Special Excessive Rainfall Discussion...
Slow-moving convection had evolved into a complex that was
producing 1 to 2.5 inch rainfall per hour rates over portions of
northeast Nebraska into far western Iowa as of early this
afternoon. The rates...combined with the fact that the convection
has already persisted for a couple of hours and the proximity to
mid/upper level cold temperatures...has raised the potential for
flash flooding in the area. Have issued a Slight Risk area to cover
that potential. Refer to Mesoscale Precipitation Discussion 0159
for additional details.

Bann

...16Z Update...

Previous forecast was maintained with only minor modifications to
the expansive MRGL risk area extending from the northern Plains
into the Mississippi Valley over into the central Appalachians.
Current radar/sat composite shows our broad surface reflection with
SLP center located over central NE. Convective points of interest
include the current convective bands located over SD/NE and MO/IA
that are producing efficient rates of 1-2"/hr within the banded
structures (More info in MPD #0158). This trend will continue as
warm front progression lifts north and portions of the central
Midwest see an increase in convective coverage after 18z with a
progressive propagation to the northeast. The previous discussion
outlines the reasoning perfectly as the combo of progressive storm
motions and drier antecedent conditions over the impacted area will
limit flooding in an areal extent and lean more locally within one
or two cells that could over perform given the favorably evolving
environment. Generally max QPF of 3-3.5" is the top end forecast
given the latest probability fields from both NBM and HREF with
majority seeing closer to 1.25-2.25" as per the latest mean(s).

Over the south, increased diffluent pattern downstream of the mean
trough will initiate a line of convection beginning in the Lower
Mississippi Valley over AR, spreading northeast into the Ohio Valley
by later in the period. This line will not have as favored an ascent
pattern as areas in the Midwest, so the threat for flash flooding
leans more on the lower end of MRGL comparatively, but certainly
non-zero. Another smaller area over WV has a low-end probability
for flash flood concerns as an area of mid-level vorticity advects
northward into the region during the time of peak diurnal
instability. MLCAPE around 1000-1500 J/kg are forecast in-of
central and eastern WV which is enough to spur some local
thunderstorm activity within the terrain. Chances again are low,
but non-zero with some CAMs being more aggressive with the
opportunity, so maintained continuity.

Kleebauer

...Previous Discussion...

Showers and thunderstorm are expected to align near and north of
an east-west orientated warm frontal boundary that is stretched
across the eastern portions of the Plains and the Ohio Valley. The
highest rainfall rates and accumulation will likely be concentrated
along the Minnesota/Iowa border where instability will be most
supportive to produce 1-2"/hr rainfall over this area, which will
likely exceed 1hr FFG in spots. One limiting factor for flash
flooding will be the swift movement of cells, limiting the duration
of heavy rates. Training/backbuilding near the warm front could
help accumulations to reach or exceed 5 inches.

Although there may be 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. A Marginal Risk area spans from the
eastern Dakotas, Upper/Central Mississippi Valley and across parts
of the Mid-Mississippi Valley.

Campbell

Day 2
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...

Showers and thunderstorms will track eastward across the Ohio
Valley and some of these storms will be strong and capable of
producing high rainfall rates. Overall, the storm motion will be
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 Pennsylvania and New York the
Marginal looks good.

Campbell


Day 3
Valid 12Z Thu Apr 18 2024 - 12Z Fri Apr 19 2024

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF
THE SOUTHERN PLAINS AND MISSISSIPPI VALLEY...

Another low pressure system will develop across the
Southern/Central Plains and tap into the return flow from the Gulf
of Mexico. Convection is expected to fire up ahead of the
approaching cold front with areal averages of 1 to 2+ inches across
the Missouri Ozarks/Mid-Mississippi Valley and trailing southwest
to central Texas. Recent rains will have lowered some of the FFG
across this region and may have an increased sensitivity to
additional rain/heavy rain. A Marginal Risk area covers part of
northeast Texas to southern Missouri and western Kentucky.


Campbell


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