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Excessive Rainfall Discussion
(Latest Discussion - Issued 0100Z Sep 22, 2023)
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Abbreviations and acronyms used in this product
Geographic Boundaries -  Map 1: Color  Black/White       Map 2: Color  Black/White

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
859 PM EDT Thu Sep 21 2023

Day 1
Valid 01Z Fri Sep 22 2023 - 12Z Fri Sep 22 2023


...Portions of the Northern and Central Plains...
Shortwave energy continues to eject east from a closed-low located
over the Great Basin, crossing the north-central Plains today.
This will prolong an ongoing heavy rainfall event over Nebraska
through the South Dakota border today which should expand south
into Kansas tonight. Strong upper-level diffluence associated with
height falls in an axis of instability (1000-3000 J/kg MUCAPE)
that extends north from the southern Plains through western
Nebraska. 12Z HREF CAMs are mightily struggling with this
activity, but recent HRRRs do have some semblance of reality and
feature repeating activity along the central Neb/SD border into
this afternoon which warrants maintenance of the Slight Risk in
that area.

Late afternoon through tonight, further impulses which should
track a bit south of the current one is progged by most 12Z HREF
CAMs to allow development in the increasing southerly LLJ pumping
moisture and instability up the Plains. Hedged between guidance
and expanded the Marginal Risk over eastern KS through the KC
metro which connects to the southern Plains Marginal Risk. There
is a fair amount of uncertainty with this activity with a risk for
some high rain rates, but much of this area is in high drought
categories, so a Marginal Risk should suffice for now.

...Southern Plains...
A quasi-stationary frontal boundary is expected to remain oriented
west-northwest to east-southeastward from the Southern Plains
toward the Lower Mississippi Valley today. Additional shortwave
energy moving from west to east across the Southern Plains will
help to strengthen the low-level inflow into this front again
today, likely supporting additional convection along the front
across eastern Oklahoma, northeast Texas, into western Arkansas
and far southeast Kansas and southwest Missouri (with localized
rainfall totals of 2-4" having fallen yesterday). There will be
potential for training of cells parallel to the boundary, with the
best chance of overlap with prior days rainfall occurring across
southeast Oklahoma and into portions of North Texas. Over this
potential training and overlap region, a Slight Risk was

...East Coast of Florida through Coastal Georgia and South
The Marginal Risk was ticked north to include the rest of the SC
coast per regional radar this morning. The bulk of the heavy
rainfall will likely be offshore today as a surface low forms off
the Southeast coast in response to the closing off mid- to
upper-level low along the Southeast coast. Easterly low-level flow
from the central east coast of Florida, northward into coastal
Georgia and South Carolina will support slow moving areas of rain,
while another round of convection may form across inland South
Florida this afternoon and push eastward to the southeast coastal
regions. Concerns for any runoff issues will continue to be
primarily over urban areas where short-term, localized rainfall
amounts of 2-4" are possible (per 00z HREF 40-km neighborhood
exceedance probabilities as high as 30-60% for the 3" threshold).

...Parts of Central-Eastern Montana into North-Central and
Northeast Wyoming...
0100 UTC Update -- Upslope easterly flow with axis of above-normal
TPW, MUCAPEs of 500-1000 J/Kg, and difluent flow aloft (persistent
left-exit region upper jet forcing east of the closed upper low)
will favor scattered-numerous showers and thunderstorms through
much of the overnight period, even well after the  loss of diurnal
heating. Based on the latest guidance trends, including the 18Z
HREF exceedance probabilities, any short-term runoff issues will
likely be localized, and as such the Marginal Risk was shifted
eastward toward the MT-WY border with SD.

...Illinois and St. Louis...
The Marginal Risk area was focused more over central IL with
maintenance over the St. Louis metro. This area is in association
with the northward moving vorticity maxima, and some weak
convective activity is ongoing across the region early this
morning. Strengthening southerly low-level flow in the vicinity of
this vort may support some locally heavy rains as training of
cells in a south to north direction occur. The bulk of the area
remains quite dry from a soil moisture perspective, leading to
mostly beneficial rainfall, with just a localized threat for
excessive rain.


Day 2
Valid 12Z Fri Sep 22 2023 - 12Z Sat Sep 23 2023


Improved agreement among 12Z guidance for the potential tropical
cyclone track north-northwest from off the Southeast coast to
eastern North Carolina Friday night. Strong low-level moisture
flux in advance of the low spreads up the Mid-Atlantic coast and
enhanced upper diffluence on the north side of the mid- to
upper-level closed low will support an axis of heavy rains across
eastern North Carolina, northward into southeast Virginia and the
southern Delmarva Peninsula. A Slight Risk for excessive rainfall
has been maintained and trimmed east a bit given confidence
increase from 12Z guidance. Areal average QPF of 2-4", local 6",
is expected by 12Z Saturday in this region which is on the
upper-half of the Slight Risk probabilistic spectrum. The broad
Marginal Risk was trimmed a bit east/south given the non-CMC
preferred solution for timing and intensity of QPF. 

...Northern Rockies across the Northern Plains...
The closed-low over the Great Basin ejects east across the
northern Rockies Friday night. Good agreement with the evolution
of this system remains with model consensus for a broad axis of
heavy rain from central MT through central SD which is where the
PW anomaly is around 3 standard deviations above the mean. The
heavy rains will be comprised of a well-defined comma
head/deformation precip area across western to central Montana,
and another round of convective rains across portions of the
northern plains from northern Nebraska, through much of South
Dakota, southwestern North Dakota, northeast Wyoming into eastern
Montana with strong low level moisture convergence present with
sufficient instability. The Slight Risk was expanded southeast to
the central SD/Neb border where heavier QPF would fall over areas
that have received heavy rain today. A broad Marginal Risk extends
across a larger portion of the surrounding northern Rockies and
Plains with a trim west over MN/IA based on latest frontal progs.

...Southern Plains to Ozarks...
The Marginal Risk was expanded north a bit into KS which is in the
vicinity of the stationary frontal boundary expected to remain
oriented west-northwest to east-southeast across portions of the
Southern Plains toward the Lower Mississippi Valley Friday.
Additional anomalous low-level moisture flux into this boundary
will support potential for additional moderate to heavy rainfall
amounts from eastern Oklahoma into western Arkansas, northeast
Texas and northern Louisiana. There is potential for a targeted
upgrade to a Slight Risk, depending on the evolution of heavy
rains through tonight.


Day 3
Valid 12Z Sat Sep 23 2023 - 12Z Sun Sep 24 2023


...Mid-Atlantic into New England...
PTC 16 is forecast to track north just inland from the
Mid-Atlantic coast/over the Chesapeake Bay Saturday/Saturday
night. The leading trough will bring the tropical plume of
moisture across the northern Mid-Atlantic/southern New England
coast with increasing ascent on the northern and western flank of
the low will aid in widespread moderate to heavy rainfall. PWATs
of 1.75" to 2" (1.5 to 2 standard deviations above normal) will
spread east of the Appalachian front within the corridor of
maximized ascent. 12Z guidance has slowed the tropical low a bit
more allowing a more expansive Slight Risk over the central
Mid-Atlantic, but limited to the southern New England coast. There
is a risk of continued heavy rain near the tropical low center
which is currently eastern NC and eastern VA which is where a
future Moderate Risk may be needed, but for now is considered a
higher end Slight Risk.

...Plains into Mississippi Valley...
The upper low will slow as it crosses the northern Plains Saturday
with increasing difluence over the Dakotas down through the
central Mississippi Valley. Deep, anomalous moisture transport up
through much of the central US will be key in the heavy rain
potential on Sat/Sun with NAEFS standardized anomalies a solid 1-2
deviations above normal for the Mississippi Valley up through the
central Midwest and 2-3 deviations above normal for areas of the
Dakotas, mainly across ND into MT thanks to dynamic influences
coupled with the moisture advection regime. This area is receiving
heavy rain today and the continued western Gulf connection will
make it possible for repeating widespread heavy rain. Organized
convective activity is likely within the corridor of deep moisture
transport and upper forcing as shortwaves pivoting around the mean
trough and ULL will create a large footprint of well-above normal
rain chances and totals. Periods of high intensity rain rates that
would exacerbate flash flooding concerns, particularly for
repeating areas such as near the OK/KS/MO border and over the
Dakotas. A broad Slight Risk remains across this area with a bit
of an expansion south per the 12Z consensus.

In addition, continued terrain enhanced rain near the comma head
on the north slopes of the Bighorn Mtns along the MT/WY border
into Saturday warranted a Marginal Risk addition.


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