Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
403 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...

...20Z Update...

Main change in the D2 MRGL risk was to trim a bit of the southern
edge across the lower Ohio Valley given the latest trends in the
HREF blended mean QPF footprint and associated EAS probabilities
for at least 1". We've seen a dip in the probabilities from the
previous forecast issuance with a 1-5% probability in-of south-
central OH with nothing now showing up across northeastern KY. This
is likely due to the slight shift in the progression of the low and
a further north push of the warm sector into central and northern
OH where the better QPF signal and convective rates are being
depicted. Low FFG threshold across eastern OH into western PA are
the primary factors for the MRGL risk being maintained anywhere as
the QPF maximum will likely only top out between 1.25-2". EAS
probabilities for 2" rainfall totals are solidly 0%, so the risk
is capped overall. That is where we will maintain continuity, as
well as the eastern extent over eastern PA into NJ where multiple
rounds of convection throughout the period will lead to totals
pushing over 1" within areas of low FFG indices, especially in the
3 and 6-hr FFG intervals.

A non-zero threat also exists over portions of the Deep South as
well tomorrow afternoon and evening. Some weak low-level
convergence was noted on CAMs with a few members going more robust
on the convective threat in eastern MS over into central AL during
peak diurnal instability. There was not enough consistency for one
to prompt an additional risk area. Also, the rates expected would
not threaten current FFG intervals. Unless we get some significant
upgrade in the potential, this will remain outside any additional
MRGL issuance's.

Kleebauer

...Previous Discussion...

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...

...20Z Update...

Limited changes necessary to the previous issuance of the D3 ERO as
guidance remains consistent within the overall synoptic progression
of the expected SLP and cold frontal passage over the central
plains to Mid-Mississippi Valley. A solid convergence signal ahead
of a cold front will lead to locally heavy rainfall over portions
of MO down through northeast TX during the period in question.
There's a bit more emphasis across MO in the latest model suite
with some guidance indicating a bit more organization within the
confines of the front bisecting an area of relatively unstable air
based on the latest theta-E forecast. This should promote some
local totals exceeding 1" with potential of up to 2" based on the
recent deterministic. The southern end of the MRGL may have the
best threat for higher impacts due to prominent MLCAPE and
elevated PWATs ahead of the tail end of the cold front promoting
higher rate potential. This should be enough for localized flood
concerns during any convective initiation over areas between
Austin/DFW and points northeast. Will be monitoring that area
closely, along with the Mid-Mississippi Valley for possible
upgrades pending future CAM outputs.

Kleebauer

...Previous Discussion...

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 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