Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
253 PM EST Mon Nov 4 2024

Day 1
Valid 16Z Mon Nov 04 2024 - 12Z Tue Nov 05 2024

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS SOUTHEAST
OKLAHOMA THROUGH THE OZARKS OF ARKANSAS AND MISSOURI...

...Texas to the Great Lakes...

16Z Update: Highly amplified flow ahead of a potent closed
low/upper trough axis moving through the Western CONUS will
continue to usher in well-above normal moisture anomalies from the
Gulf Coast up through the Great Lakes during the period. Heavy
rains overnight across OK into the AR/MO Ozarks have crashed FFG
indices regionally with cases of hourly FFG's now down <1"/hr
necessary to garner flash flood concerns. A widespread areal
average of 2-4" with locally as high as 7" has fallen over the
above area, saturating top layer soils up into the 70-85th
percentile as indicated via the latest NASA SPoRT output for the
0-10cm layer. Expectation is for the pattern to continue evolving
as such that another wave of heavy precip will initiate upstream
over OK as the nose of a 110kt 500mb jet streak rounds the base of
the mean trough over NM and ejects into the Southern Plains. Large
scale ascent will be maximized over within the time frame of 21z
through the remainder of the period allowing for a large cluster of
showers and storms to breakout and pivot east-northeast, aided by
the mean flow. A theta-E tongue is well documented over Eastern TX
through the Lower Mississippi Valley up through the the Missouri
Ozarks lending to favorable instability continuing through the
afternoon and evening time frames.

CAMs have become aggressive with the QPF output given the
aforementioned variables lending to a repeated pattern of heavy
rainfall over areas that saw copious amounts of rain overnight.
12z HREF signals a high end probability for at least 3" within the
neighborhood probability fields (70-90+%) for the Ozarks down into
the Southeast corner of OK and is correlated well with a 25-50% EAS
prob field for 3" with a max prob of 60% centered over South-
Central MO to the east of Springfield. HREF blended mean output is
elevated with 3-5" of rainfall over those areas with a max of up to
7", all of which is additional to what has transpired. Considering
the expected additional rainfall with rates generally 1-2"/hr at
peak intensity likely breaching the current FFG indicators for
1/3/6 hr periods, and in coordination with the TSA/LZK/SHV/SGF WFOs
impacted, a Moderate Risk ERO was introduced to cover that area
from Southeast OK up through North and Northwest AR into South-
Central MO.

Further south into TX, there is a growing consensus of heavy
thunderstorm genesis on the southern edge of the flanking line
along the axis of convergence settling within the western edge of
the theta-E gradient later this evening, mainly after the
development of the nocturnal LLJ. The jet streak on the base of
the mean trough under the closed low will be juxtaposed to the
northwest of Central TX allowing for increasing RER dynamics within
the confines of the I-35 corridor extending from DFW down into the
Austin/San Antonio metros. There is consensus on the development
of the flanking line, but exactly where the heaviest will occur
depends on the CAM output. Some place the heavy core of precip near
New Braunfels to San Antonio. Others place it near Austin to Waco.
Regardless, the instability fields will be prime for any
convective initiation with PWAT anomalies sufficiently above 2
deviations above normal. These types of setups are sneaky with
regards to potential, leading to an expansion of the SLGT risk to
include much of that I-35 corridor and adjacent areas in the Hill
Country to the west of the interstate.

Consensus for locally heavy rainfall north of the Missouri Valley
continues up into the Great Lakes as the low pressure progresses
into the Midwest with the amplified flow ahead of the potent
longwave trough. Lack of sufficient surface based instability caps
the potential for significant rainfall totals to more of 1-2" with
locally up to 3", but this would be enough to concern some urban
flash flood threats for those areas extending through Illinois into
Southeast WI into Western MI. The previous SLGT risk was maintained
as a result with a MRGL risk encompassing.

Kleebauer

...New Mexico...

16Z Update: Very little change necessary for the previous MRGL risk
over New Mexico as CAMs deviated very little from the overnight
output. Heaviest rainfall concerns will be located within that
northern periphery of the closed upper low pivoting eastward
through the area as ascent remains maximized within the diffluent
area north of the closed reflection, textbook synoptic scale
evolution. Higher terrain will actually resolve as snow, so the
main areas of concern will be the valleys and locales along the
I-40 corridor.

Kleebauer

..Previous Forecast..

A leeside low currently over south central New Mexico will track
northeast as several mesolows work to consolidate into a dominant
single low over Texas and Oklahoma late today. With peak heating,
some wraparound moisture and modest instability over an area with
modestly depressed FFG values, steady rain with some convective
cells ongoing over the area by this afternoon could result in an
isolated flash flooding threat, particularly over any burn scars or
other flood sensitive areas. Thus, a small Marginal Risk area was
added with this update. Precipitation will be ongoing further north
and into the mountains of southern Colorado but temperatures there
will be cold enough for most if not all of the precip to fall as
snow, precluding any flash flooding risk.

Wegman

Day 2
Valid 12Z Tue Nov 05 2024 - 12Z Wed Nov 06 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE MISSISSIPPI AND OHIO VALLEYS...

20Z Update: The previous MRGL risk was relatively maintained as the
overall synoptic pattern if forecast to decay somewhat, limiting
the magnitude of heavy rainfall with regards to both rates and
totals. A core of heavy rainfall is still anticipated, but the
limiting factor of rates generally 1-1.5"/hr at peak intensity over
an area that has seen very little rainfall for close to a month
will inhibit significant flash flood concerns and relegate the
setup to more of the isolated flash flood variety. The current
ensemble means indicate the heaviest QPF footprint within the
confines of the Lower to Mid-Mississippi Valley inflection areas
encompassing Northeastern AR through Southeast MO and Western TN.
Some of the CAMs are a bit bullish over areas like Memphis which
could garner some flash flood interest even with the lower rates
just due to the urbanization factors. Even still, FFG markers for
the 1-hour time frame are running between 2.5-3"/hr over the area,
so the threat will be mainly within the MRGL threshold and perhaps
just missing out on the Slight Risk prospects with the dry
antecedent conditions being the deciding factor. ARI exceedance
probabilities are pretty tame with a 5-10% output within the latest
HREF. This isn't exactly one that moves the needle to an upgrade,
in agreement with the impacted WFO's. As a result, only minor 
adjustments were made to expand the risk area a touch further west 
and north, and to nudge a bit more in the Southeast flank as 
convective activity is likely over areas near Baton Rouge and 
points west. 

Kleebauer

..Previous Forecast.. 

The upper level trough responsible for the past few days of heavy
rain across the southern and central Plains will finally retreat
back into the broader jet into Ontario and Quebec. The upper level
energy supporting the corridor of storms will continue supporting
storms across the Mississippi and Ohio Valleys Tuesday and into
Tuesday night respectively. As the energy lifts north, it will mark
the end of persistent heavy rain and storms from south to north.
Thus, the flooding threat will be more in the morning in Texas, and
more towards the night for the Ohio Valley. Training lines of
storms will move across the Mississippi Valley for much of the day.
These storms will be capable of heavy rains which may cause
isolated flash flooding, especially in any urban areas such as
Memphis. By Tuesday night, weakening of the storms and the shift
well away from any prior days' rain footprint will effectively end
the flooding threat.

Wegman

Day 3
Valid 12Z Wed Nov 06 2024 - 12Z Thu Nov 07 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR CENTRAL AND
EASTERN GEORGIA INTO PORTIONS OF WESTERN SOUTH CAROLINA...

20z Update: Guidance continues to trend upwards with regards to the
evolving heavy rain forecast across GA into the Western portions of
SC for Wednesday. Deep tropical moisture advection will funnel out
ahead of now Tropical Depression 18 (TD18), which will be a
hurricane likely by the time of this forecast evolution. The
moisture flux is caused by the proxy of the disturbance within the
western fringes of a broad surface ridge located over the Western
Atlantic providing a "highway" for the enhanced tropical moisture
plume to funnel northward into parts of the Southeastern U.S. To
the west, our longwave trough and closed low pattern will become
sheared out with the cold front slowing and eventually becoming a
quasi-stationary boundary once it enters the Ohio and Tennessee 
Valleys. A 130kt jet streak will be located over the Great Lakes 
into the Northeastern U.S providing a broad upper forcing pattern 
to transpire within the RER of the jet. This will help in 
initiation of precip across parts of GA into SC with some modest 
rainfall developing. 

As we work into Wednesday evening the boundary to the north will
slowly drift into the Southeast with a developing convergence axis
located over Central and Eastern GA into parts of the SC Piedmont
leading to a broadening axis of steady precip with some increasing
convective elements thanks to the unstable airmass that is
advecting into the area. Several members of the global ensemble
suite along with the bias corrected ensemble forecast indicate a
substantial area of rainfall with totals >4" becoming focused
within the greatest convergence axis along the slow frontal
progression. With the high pressure stable in the Atlantic, a
persistent easterly component to the wind field will only add to
the convergence regime providing a prime focal point for slow-
moving convection over the course of the overnight hours through
Thursday morning. This setup can be characterized as a Predecessor
Rainfall Event (PRE) that can develop well out ahead of an
approaching tropical disturbance as it moves closer to CONUS. The
key in all this is the tropical moisture advection that enhances
the environment for more effective warm rain processes that aren't
common within more mid-latitude cyclones and patterns. The
interaction of the front and surface ridging will make all the
difference on the specific locations that see the most rainfall. In
this case, there's a growing consensus over Central and Eastern GA
extending to the GA/SC line near Augusta. 

Considering the variables at play and the trend towards a more
prolific rainfall event, a Slight Risk was introduced across the
areas above with a MRGL risk extending through portions of the
Southeast, including Florida where the proximity to the tropical
disturbance will allow for some heavier convective prospects over
the Peninsula. There is still some uncertainty on the exact
location of the heaviest precip and PRE setup, so be sure to
monitor the situation closely as the location of the higher risk
could change, as well as an upgrade to a higher risk if the signal
maintains magnitude and/or higher confidence in the potential.

Kleebauer

Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt