FOUS30 KWBC 280827
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
427 AM EDT Sun Jul 28 2024

Day 1
Valid 12Z Sun Jul 28 2024 - 12Z Mon Jul 29 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE NORTHERN PLAINS AND THE OHIO/TENNESSEE VALLEYS...

...Northern Plains...
Heavy to potentially excessive rainfall is possible as a mid- and
upper system ejects northeastward from the Mid-Mississippi Valley
on Sunday into the western Great Lakes by early Monday morning. The
inherited Slight Risk was once again expanded towards the south and
east as moisture streaming northwards from the Mid-Mississippi
system interacts with the strengthening of mid-level deformation.
With model guidance showing 1 to 3 inches of rainfall...much of
which could be delivered in an hour or two...there is the potential
for flash flooding.

...Central Ohio/Tennessee Valley... 
Mid-level height falls approaching from the west should help focus
and support showers and thunderstorms capable of producing some 
downpours from the afternoon into the evening...mainly across the 
western portion of the Ohio and Tennessee Valley. Northerly flow to
the east and ridging to the east should slow the eastward progress 
of storms...at least initially. Overall...the GFS looks to be the 
most progressive while the ECMWF was on the slowest side of the
guidance...leaving the NAM as a compromise in terms of position and
its 1 to 2 inch amount as being reasonable given the amount of the
instability and moisture.

...Southwest...
The Marginal Risk in this area was extended east to once again
include the Sacramento Mountains of south-central New Mexico, given
the sensitivities of the soils in this area. Expect otherwise
typical afternoon convection in the monsoonal moisture to develop
once again, with several very sensitive areas into southern
Arizona and New Mexico. 

Bann


Day 2
Valid 12Z Mon Jul 29 2024 - 12Z Tue Jul 30 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE EASTERN OHIO/TENNESSEE VALLEY TO SOUTHERN APPALACHIANS...

The Slight Risk across this area was shifted eastward...again...largely
due to guidance shifts in the axis of heaviest deterministic rainfall,
which now appears to be focused along much of the central and 
southern Appalachians, but extending back across much middle and 
upper Ohio Valley. Given areas where flash flood guidance has
lowered values around 1.5 inches per hour or 1.9 inches per 3 hours
due to due to recent rainfall...the risk of excessive rainfall
persists given the amount of moisture still in place.  

Upper Midwest...
Rainfall should be diminishing in areal coverage and intensity as
deeper moisture and the better dynamics get shunted eastward.
Maintained a Marginal risk area here given some on-going rainfall
early that could overlap with flash flood guidance lowered by rains
on Sunday.

...Southwest US...
Steering flow in the mid and upper levels begin to weaken as
heights begin to rise while precipitable water values linger at or
above an inch (even 1.25 inches in places over the desert). This
warrants keeping the previously issued Marginal risk along/near the
Arizona/New Mexico border.

Bann


Day 3
Valid 12Z Tue Jul 30 2024 - 12Z Wed Jul 31 2024

...There is a Slight Risk of Excessive Rainfall Lingering Over
Portions of the eastern Ohio Valley to the Appalachians...

...Eastern US...
With lingering potential for rainfall over a region of lowered
Flash Flood Guidance in the eastern Ohio/Tennessee Valleys and
adjacent areas of the Appalachians...opted to maintain a Slight
Risk area in a similar position to that of the Slight Risk on
Monday. Model QPF is actually pretty modest...but the environment
remains capable of producing downpours on scales that are difficult
for models to handle this far out.

Surrounding the Slight Risk...kept a Marginal risk area in place
for areas to the north where the ECMWF and GFS show energy in the
northern stream making its way eastward across the eastern Great
Lakes. The model spread on timing this energy is large enough to
warrant a Marginal although an upgrade is certainly in the realm
of possibilities if the large scale pattern supports more
organization over a broader area...especially upstream.

...Southwest...
Steering flow in the mid and upper levels and persistent monsoonal
moisture will still be in place along the Arizona/New Mexico border
that begins to ease northward towards southern Colorado during the
period.

Bann
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