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Excessive Rainfall Discussion
(Caution: Version displayed is not the latest version. - Issued 1806Z Sep 20, 2023)
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Geographic Boundaries -  Map 1: Color  Black/White       Map 2: Color  Black/White

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
206 PM EDT Wed Sep 20 2023

Day 1
Valid 16Z Wed Sep 20 2023 - 12Z Thu Sep 21 2023


...16Z Update...


No major changes were made for the Ozarks region of eastern
Oklahoma and western Arkansas. An MCS is expected to redevelop
tonight over many of the same areas that saw heavy rain over the
past couple days. While coverage will be reduced and overall storm
strength weaker tonight as compared with last night's round of
convection, the threat for isolated flash flooding remains
anywhere storms can repeat over the same areas. This is most
likely from the northeast corner of OK southeast into central AR.


A stalled out frontal boundary with a few passing upper level
disturbances will combine with ample atmospheric moisture in an
environment supported by PWATs of over 2 inches. Convection
currently out over the Gulf will organize into a line of strong
and moisture-laden storms this afternoon as the aforementioned
upper level disturbances approach. Meanwhile, convection
associated with the typical sea-breeze and afternoon instability
will also form over the Peninsula. The two areas of convection
will merge over the northern part of the peninsula before shifting
northeastward into northeast FL and the GA coast. Any urbanized
and flood-sensitive areas, including the Orlando and Jacksonville
metros are at a higher risk for localized flash flooding as the
storms are likely to locally generate 3 inches or more of rain,
based on the latest 12Z HREF neighborhood probabilities.


A new Marginal Risk area was introduced with this update across
southeast WA and northeast OR. A heavily dynamically driven upper
level low and associated jet streams will track southeastward
across WA and into OR this afternoon into tonight. Wraparound
rainfall will develop starting in Idaho and then shift westward
across WA and OR. While there will be little in the way of
instability, low FFGs and a steady moderate rain could cause
localized flash flooding issues, especially tonight after 06Z.


...Previous Discussion...

...Eastern Oklahoma into western Arkansas...
An upper vort expected to move slowly east northeastward from
central Missouri into central Illinois during Wednesday.  While
convection has been active in the vicinity of this vort during
Tuesday, the latest hi res runs are not showing much in the way of
convection along the track of this vort across south central
Missouri into southwest Illinois during the day 1 period.  At the
moment, this lessening trend looks reasonable given very little in
the way of instability along and ahead of the track of the vort
across Missouri into Illinois.  With precip below average across
these regions over the past few weeks and soil moisture low, the
previous marginal risk area was removed from this region.  Better
instability is expected to persist on the southwest side of this
vort across the Southern Plains .  Latest hi res guidance is
showing potential for additional convection to fire in the
vicinity of eastern Oklahoma/western Arkansas along the position
of a stationary frontal boundary.   850-700 mb moisture flux
continues to be anomalous, 1 to 2 standard deviations above the
mean, into this front day 1.  This will support potential for
additional convection to push east southeast from eastern Oklahoma
into western Arkansas/far northeast Texas.  HREF neighborhood
probabilities are high for 1"+ amounts across this area, 40-70%,
decreasing to 20-50% along the Oklahoma/Arkansas border area. 

...Central to Southeast Florida...
No changes made to the previous broad marginal risk area across
central to southeast Florida.  PW values will remain high, 2"+ in
the vicinity of the stationary front currently across the central
portion of the peninsula.  An upper trof will begin to amplify day
1, supporting another round of organized convection across the
central to southern Florida Peninsula Wednesday afternoon.  Low
confidence in any qpf details, but localized runoff issues
possible, especially in urbanized regions.  HREF neighborhood
probabilities are high for 1, 2 and 3"+ amounts in the upcoming
day 1 period in the marginal risk area.


Day 2
Valid 12Z Thu Sep 21 2023 - 12Z Fri Sep 22 2023


...1830Z Update...

No major changes were made with this update. For the Slight Risk
area, the biggest change made was to expand the area north to
include the KS/MO/OK/AR junction region. This area was hard hit
with heavy rain last night, and the potential for additional heavy
rainfall, while not as high as areas further south along the OK/TX
border, may still cause additional flash flooding overnight
Thursday night.

The Marginal Risk in IL was expanded southward a bit as a front
draws storms northward with the potential for repeating as they
track parallel to the frontal interface. While most of the
rainfall is likely to be west of Chicago, there remains enough
potential in the city itself when adding the lower FFGs there to
maintain the Marginal Risk.

The Marginal Risk for the intermountain west remains largely the
same, with the best potential for heavy rain on the east facing
slopes of the mountains of central/western MT due to upsloping.
This risk area was separated from the one over the Dakotas, where
an isolated flash flooding risk remains. The portions of eastern
MT and WY between the 2 areas have largely dried out in recent
runs, so the previous Marginal risk was separated into two.


...Previous Discussion...

...Southeast Oklahoma, far northeast Texas, far southwest
A stationary frontal boundary expected to remain oriented west
northwest to east southeastward from the Southern Plains toward
the Lower Mississippi Valley.  Additional shortwave energy moving
west to east across the Southern Plains and along this boundary
will help to strengthen the low level inflow into this front,
supporting potential for additional convection to ride along the
front across eastern Oklahoma, northeast Texas, into western
Arkansas.  There will be potential for training of cells parallel
to the boundary and potential for overlap of day 1 heavy rains
along this front with day 2 amounts.  Over this potential training
and overlap region, a slight risk was maintained. 

...East coast of Florida into coastal Georgia...
A marginal risk was maintained along the east coast of Florida and
extended slightly northward from the previous issuance into
coastal Georgia.  The bulk of the heavy rainfall will likely be
offshore day 2 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 will support slow
moving areas of rain, while another round of convection may form
across inland South Florida Thursday 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
rainfall amounts of 1-2" are possible day 2.

...Great Basin into the Northern Rockies
The strong closed low forming over the northern Great Basin day 1
is expected to move little during the day 2 period.  There is good
model agreement on an axis of moderate to heavy rainfall in the
slow moving region of above average PW values stretching from
northeast Nevada, across northern Utah, eastern Idaho, northwest
Wyoming into western Montana.  No significant changes made to the
previous marginal risk across these areas with areal average .50"+
amounts possible and locally heavy totals in the upslope area of
western Montana where easterly low level flow will support some
localized totals of 2"+

...Portions of the Northern High Plains...
Shortwave energy expected to push north northeastward day 2 on the
eastern side of the Great Basin closed low across portions of the
Central to Northern High Plains.  Strong upper difluence
associated with these height fall in an axis of instability
stretching across the Central to Northern High Plains will support
organized convective potential Thursday from central to northwest
Nebraska, northward into central South Dakota and central North
Dakota.  Only some small changes made to the previous marginal
risk area, trimming the eastern portion over South Dakota into
Nebraska to better fit the overlap of heavy rain potential and
lower ffg values.

...Northeast Illinois into southeast Wisconsin...
A small marginal risk area was added for day 2 across northeast
Illinois into southeast Wisconsin associated with the northward
moving vort across these areas.  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. 
Recent heavy rains over northeast Illinois into southeast
Wisconsin have increased soil moisture and lowered ffg values,
with a marginal risk denoted across these lower ffg values areas
where additional locally heavy rains are possible.


Day 3

The Day 3 outlook will be updated by 2030Z.

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