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Excessive Rainfall Discussion
(Caution: Version displayed is not the latest version. - Issued 1604Z Sep 22, 2023)
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Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1204 PM EDT Fri Sep 22 2023

Day 1
Valid 16Z Fri Sep 22 2023 - 12Z Sat Sep 23 2023


16z Update:
With Potential Tropical Cyclone Sixteen, only minor adjustments
were made. The 12z 3km NAM and ARW2 members of the HREF appear to
be western outliers with the track of the system...with the 12z
HRRR/FV3/Gem Reg a much closer match to the NHC forecast and the
generally consistent global models. Thus ERO tweaks were made in
the direction of these solutions...which resulted in a minor
contraction of the Slight risk on the western and northern extent,
and a minor westward expansion of the MDT risk over coastal NC.
Guidance indicates the potential for a narrow swath of 4-8" of
rainfall near the track of the system, as well as just to it's
west (although a tight western gradient). This could be very close
to the previous western edge of the MDT risk, so nudged that a bit
further west to account for this risk.

The ERO areas over the central U.S. looked in pretty good
shape...however did go ahead and introduce a Marginal risk over
portions of northeast IA, southeast MN, southwest WI and northern
IL. Slow moving and periodically repeating convection should be a
threat over this corridor into tonight. The 12z HREF probabilities
have increased across this corridor...with 3" neighborhood
probabilities now exceeding 60% in spots. A localized flash flood
risk is possible this afternoon into tonight.


...Previous Discussion...

...Eastern North Carolina & Mid-Atlantic...
Potential Tropical Cyclone (PTC) Sixteen is forecast to strengthen
as it makes its way over the Gulf Stream and approaches the North
Carolina coast Friday evening. The storm will not only have the
Gulf Stream at its disposal for strengthening, but it will reside
just north of a negatively tilted 250mb trough axis, thus
fostering excellent upper level divergence aloft. Rain bands
associated with PTC Sixteen will be making their way ashore Friday
afternoon as 850mb theta-e advection increases. Moisture will be
funneled into the Mid-Atlantic Friday afternoon and into Friday
night thanks to a robust 750-1000 kg/m/s integrated vapor
transport (IVT) that is surpassing the 97.5 climatological
percentile from the Mid-Atlantic coast to as far inland as the
VA/NC Piedmont. PWs are set to be highest east of I-95 in NC and
southeast VA where 1.5-1.75" PWs will be most common, while the NC
Outer Banks and Virginia Beach have the best odds of observing 2"
PWs late Friday. As the storm makes its way ashore Friday evening,
the axis of the heaviest rainfall will likely reside on the
northern periphery of its circulation. 00Z HREF suggests 1,000
J/kg of MUCAPE will be located to along and east of PTC Sixteen's
circulation and provide thunderstorms with a suitable
thermodynamic profile aloft to generate Excessive Rainfall rates
across eastern NC. Lastly, warm cloud layers from eastern NC on
north to southeast VA are likely to be at least 12,000ft deep
Friday night and in some cases could be as deep as 14,000ft.

The 00Z HREF 24-hr probabilistic footprint is honing in on the
lower DelMarVa Peninsula, southeast VA, and eastern NC that
feature at least 60% odds of receiving >3" of rainfall through
early Saturday morning. Of greater concern, there is a 70-90% area
in eastern NC with rainfall totals >5". In collaboration with the
Morehead City WFO, went ahead and upgraded to a Moderate Risk
given their ideal placement along and just east of PTC Sixteen's
path. Locally significant flooding is possible from North Topsail
Beach on east to the Outer Banks Friday afternoon and Friday night.

...Northern Rockies across the Northern Plains...
A closed low over the northern Rockies will slowly track east into
the northern High Plains by Friday. Guidance remains in good
agreement on the heaviest rainfall setting up to the north of the
500mb low, where PWs are >99th climatological percentile
throughout central MT. These highly anomalous moisture values are
a result of IVT levels over the Dakotas that are above the 90th
climatological percentile. Easterly flow within the 850-700mb
layer are also below the 1st climatological percentile. These
percentiles being referenced are to show just how anomalous the
setup is with exceptional moisture and the transport delivering it
into the region. The frontal forecast shows a stationary front is
likely to position itself over South Dakota where southerly 850mb
moisture flux will intersect the boundary and aid in the formation
of heavy showers and thunderstorms. The 00Z HREF shows central SD
is likely to see the best available instability that combined with
PWs up to 1.25" could result in 1.5-2"/hr rainfall rates.
Meanwhile, MUCAPE of 500-1,000 J/kg and >1" PWs over central MT
would support hourly rates as high as 1.5", which happens to be at
or above 1-hr FFGs. For these reasons, the Slight Risk remains in
place here while the Marginal Risk as far south and west as
northern UT remains in place given the slower storm motions and
ample atmospheric moisture present.

...Central Plains to the ArkLaTex...

Not much change to the Marginal Risk as a stationary front will be
the catalyst for scattered thunderstorms today. Prolonged
southwesterly 850mb moisture flux will intersect the stationary
front oriented north-south and act as a steady source of low-level
inflow through the morning hours. There will also be 500-1,000
J/kg of MUCAPE at these storms disposal. Rainfall rates could
reach as high as 1.5"/hr, and with the 00Z HREF hinting at
multiple hours worth of Excessive Rainfall possible, 3-hr rainfall
totals could surpass 3-hr FFGs that are as low as 2-2.5" in some
locations. Storms should dissipate by Friday afternoon, although
the front and resurgent LLJ Friday night could trigger additional
storms over eastern OK and western AR late Friday night.


Day 2
Valid 12Z Sat Sep 23 2023 - 12Z Sun Sep 24 2023


...Mid-Atlantic into Coastal New England...

PTC Sixteen is forecast to track through eastern NC on Saturday
delivering copious amounts to areas that stretch from the central
Appalachians to the southern New England coast. The Excessive
Rainfall threat is driven by an impressive easterly IVT that NAEFS
shows values that surpass that 97.5 climatological percentile. PWs
in southeast VA will likely top 2" to start the day, but values
above 1.75" will be common from the central NC Piedmont north and
east to Long Island. The highest concentration of CAPE will reside
on the eastern flank of PTC Sixteen's track, where freezing layers
will also be higher and result in warm cloud layers up to 13,000ft
deep. When factoring the core of where the tropical moisture,
instability, and forcing is most likely to reside, this would
include central VA on east across southern MD, the DelMarVa
Peninsula, and into southern NJ. There is the potential for a
Moderate Risk being necessary somewhere within this region, but
given lingering uncertainty in where the best confluence at 850mb
sets up and how much instability will be available, have chosen to
hold off on a Moderate Risk upgrade this forecast cycle.

North and west of I-95 from the DC/Baltimore metro on north and
east to southern New England, instability will be harder to come
by and rainfall rates may struggle to surpass 1"/hr. Still, warm
cloud layers up to 12,000ft deep in some cases will be plenty deep
enough to support efficient rainfall processes, and localized
flash flooding cannot be ruled out Saturday and into Saturday
night. The Slight Risk encompasses southern coastal New England
where some meager instability values may be present, while the
Marginal Risk extends as far north as the Poconos, Catskills, and
central Massachusetts.

...Northern Plains & Lower Missouri River Valley to eastern
A closed low traversing the Northern Plains will provide healthy
upper level divergence over the Upper Midwest while, farther
south, brisk 850mb moisture flux will be directed at a stalled
frontal boundary over the Lower Missouri River Valley. Starting up
north, a 50-60 knot 850mb jet will supply plenty of anomalous
moisture into the Northern Plains. PWs will surpass 1.5" as far
north as the Red River of the North, and as moisture wraps around
the closed 500mb low, can result in PWs topping the 97.5
climatological percentile according to NAEFS. Farther south, while
there is not as much synoptic-scale forced ascent aloft, there is
an even greater reservoir of CAPE (up to 3,000 J/kg available) and
PWs (reaching 2" in parts of central OK and western AR). Some
portions of northeast OK, southwest MO, and northwest AR have been
wetter than normal in recent days. This is depicted on AHPS where
as much as 300-400% of normal rainfall has occurred in the past
7-days. There is the concern for repeating rounds of storms,
especially in the Northern Plains as mean steering flow winds
parallel the orientation of the occluded front Saturday evening.
There is also the concern for the comma head on the western flank
of the 500mb low to produce a slow moving, pivoting axis of storms
over the central ND where 3-hr FFGs are <1.5" in some areas. Given
these reasons, the Slight Risk remains in place from the Dakotas
on south to eastern Oklahoma. Given the lingering upslope flow
into the Bighorns along the MT/WY border and soils growing more
saturated in wake of Friday's rainfall, have maintained the
inherited Marginal Risk for that region.


Day 3
Valid 12Z Sun Sep 24 2023 - 12Z Mon Sep 25 2023



Latest NHC forecast shows that by 12Z Sunday, PTC Sixteen is
likely to be positioned somewhere near the Chesapeake Bay. This is
a still a decent spread in ensemble guidance with regards to the
storm's center and has implications on both where the storm's
heaviest axis of rainfall sets up and its duration. However, there
is a growing consensus for a persistent easterly 850mb moisture
transport to be directed at the Lehigh Valley, Poconos, and
Catskills. The low will be weakening and the expansive cloud cover
aloft should also limit instability. The primary concern, however,
is the persistent upslope flow into the higher terrain of eastern
PA, northern NJ, and upstate NY that is set to continue atop
increasingly saturated soils. The Slight Risk was expanded to
include more of southern New England given recent QPF trends.
Southern New England continues to sport overly saturated soils
thanks to 14-day rainfall totals that are as much as 300-400% of
normal during that span according to AHPS precipitation analysis.

...Arkansas River Valley & ArkLaTex...

The Arkansas River Valley & ArkLaTex region will reside within the
warm sector ahead of an approaching cold front from the west. A
progressive 500mb disturbance ejecting east out of the southern
Rockies will coincide within a diffluent 250-500mb flow overhead
that fosters favorable upper level divergence overhead. At low
levels, global guidance is on board with a steady diet of
southwesterly 850mb moisture flux into the region with 2" PWs
expected by Sunday afternoon and into the overnight hours. There
also remains an expansive channel of southerly 850mb moisture flux
as far north as the Upper Mississippi Valley, where there is also
the potential for Excessive Rainfall rates. MLCAPE is currently
forecast to reach ~2,000 J/kg and the persistent 850mb flow
overnight could lead to additional rounds of storms ahead of the
cold front or via upsloping into the Ouachita Mountains. Given the
available instability and moisture content, there is a scenario
where storms could produce up to 3"/hr rainfall rates from east TX
into southern AR and northern LA. The Slight Risk remains in
place, but was adjusted more to the south where the better axis of
instability is currently expected to setup.


Day 1 threat area:
Day 2 threat area:
Day 3 threat area: