FOUS30 KWBC 030808
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
308 AM EST Sun Nov 3 2024

Day 1
Valid 12Z Sun Nov 03 2024 - 12Z Mon Nov 04 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
SOUTHERN PLAINS AND THE OZARKS...

...Southern Plains and the Ozarks...

In coordination with TSA/Tulsa, OK; LZK/Little Rock, AR; and
SGF/Springfield, MO forecast offices, the inherited Moderate Risk
area was downgraded to a Slight with this update. The surrounding
Marginal was trimmed out of Wisconsin, Minnesota, and much of Iowa. 

There was a notable south and eastward shift in the guidance as to
the axis of heaviest rainfall expected across Oklahoma and Arkansas
today into tonight. A longwave trough over the Southwest will shift
eastward today. Embedded shortwave disturbances as well as a
developing 90 kt jet streak will move over the region through
tonight. A well-established low level jet (LLJ) will continue to advect
plentiful Gulf moisture northward into the area. The result will be
multiple rounds of storms with embedded training cells in OK/AR and
into MO. Much of the area has been very dry lately, though
yesterday some rain impacted the area, especially in Oklahoma and
into southern Missouri. Multiple "streaks" of much heavier rainfall
are anticipated where cells of much heavier rain train over the
same areas. This is likely to cause widely scattered to scattered
instances of flash flooding. 

Antecedent conditions will be the primary limiting factor keeping
the flooding impacts in check. Very dry conditions prior to this
event has led to drought conditions developing with very low river
and stream levels. Thus...much of the rain expected today will be
beneficial and work to return those rivers and streams to much more
normal levels. However, given the plentiful moisture, training
storms, and some flood prone areas, the heavy rain could cause
localized flash flooding. The eastward shift in the guidance has
also introduced the Ozarks into the highest risk area. The
topography of the area will help to focus the heavy rainfall into
the valleys...which could result in rapid river, stream and creek
rises. This also locally increases the flash flooding threat as
compared to the same amount of rain falling over relatively flat
terrain.

While forecast rainfall amounts haven't changed appreciably as
compared with previous forecasts, the shift of the heaviest rain
eastward and into many areas that did not receive appreciable rain
today has reduced the likely flood impact threat as well as
coverage. When added to the dry antecedent conditions, this should
keep flooding impacts from becoming widespread enough to reach
Moderate Risk levels. Hence, while flash flooding remains likely as
amounts could locally exceed 5 inches, the impact and coverage was
determined to be in the Slight Risk category...hence the downgrade.

...Upper Midwest...

In the upper Midwest, the eastward shift in the guidance and a more
west to east movement of the expected convection should limit how
much instability and moisture reach the upper Mississippi Valley.
It's likely any remaining convection will be in a much weakened
state by the time the rain reaches this area. Since similarly dry
conditions and drought have impacted the upper Great Lakes in
recent weeks, the largely stratiform nature of the rain should mean
all of the rain will be beneficial with no flooding concerns noted.
Thus, the Marginal was trimmed towards the south.

Wegman


Day 2
Valid 12Z Mon Nov 04 2024 - 12Z Tue Nov 05 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM NORTHEAST
TEXAS THROUGH PORTIONS OF WESTERN LOWER MICHIGAN...

Few changes were made or needed to the Slight Risk area across the
middle of the country.

A deep longwave trough will continue move eastward but begin to
weaken and lift northward back into the jet stream on Monday. As it
does so, associated jet streaks will intensify downstream of the
trough, which will support its deamplification. The LLJ that has
been supplying Gulf moisture will also elongate towards the north
reaching as far north as lower Michigan by Monday. This will cause
the resultant moisture to focus in a SSW to NNE corridor from Texas
to Michigan. Unidirectional SSW flow will support training
thunderstorms. The strongest storms are expected from Texas through
Missouri, which will have access to the greatest Gulf instability
and moisture. The storms will gradually weaken the further north
you go, but are expected to maintain their integrity into Wisconsin
and Michigan. The topography of the Ozarks will further locally
enhance rainfall totals due to some upslope support. 

Due to overlap with the rainfall footprint Monday with that 
expected today, there is some potential for a Moderate Risk upgrade
from western Arkansas into southern Missouri. However, this will be
highly dependent on how much rain falls in that area today/Sunday.
Total rainfall amounts will be somewhat lower on Monday as compared
to Sunday, hence the importance of rain falling over the same areas
as Sunday to enhance the flooding threat to Moderate Risk levels. Thus,
the potential for an upgrade will continue to be evaluated.

Wegman


Day 3
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...

By Tuesday, a progressive cold front pushing across the Mississippi
Valley and associated upper level trough and shortwave will
separate...with the upper shortwave ejecting northeastward into the
Great Lakes, leaving the front behind. The combination of steady
forward movement of the storms through the day followed by the
rapid diminishing of any and all upper level support by Tuesday
night will rapidly diminish the flooding threat as storm coverage
and intensity also diminish. With some supply of Gulf moisture
continuing on Tuesday, the storms that form through the day will
still have enough punch to cause locally heavy rainfall. However,
the eastward progression of the front should allow this rain to
fall largely over areas that remain dry today/Sunday and Monday.
Antecedent dry conditions will preclude much of the flooding threat
by Tuesday, so only isolated flash flooding in urban and flood
prone areas is anticipated. The Marginal Risk is largely unchanged
from previous forecasts.

Wegman

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