FOUS30 KWBC 120828
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
428 AM EDT Tue Aug 12 2025

Day 1
Valid 12Z Tue Aug 12 2025 - 12Z Wed Aug 13 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
SOUTHERN APPALACHIANS AND PORTIONS OF THE CENTRAL GULF COAST...

...SOUTHERN APPALACHIANS...
There continues to be a strong model signal for heavy rains in the
vicinity of the Southern Appalachians day 1. PW values remain well
above average from the Central Gulf coast, northeastward into the 
Southern Appalachians and then eastward through the Carolinas and 
Southern Mid-Atlantic. Convection will be enhanced through this 
anomalous PW axis by a stream of mid to upper level vorts forecasts
to rotate across this region, around the peripheries of a weak mid
to upper level ridge centered just off the northeast Florida 
coast. While locally heavy rains are possible anywhere in this high
PW axis, one strong model signal is across the Southern 
Appalachians where moist upslope flow and enhanced uvvs from the
embedded vorts will support heavy precip. The slight risk area was
not changed much from the previous issuance and fits well with the
latest axis of high neighborhood probabilities from the HREF and 
RRFS mean for 2"+ amounts. The RRFS mean has a more defined 3"+ 
high probabilities than the HREF, but both mean still emphasize the
area in the vicinity of the Southern Appalachians for heavy rains day 1.

...Central Gulf Coast...
A slight risk was continued along portions of the Central Gulf
Coast from in the vicinity of Pensacola, west to New Orleans.
Consensus from the latest hi res guidance is for the potential for
two periods of heavy rains affecting the immediate coastal region
in association with the mid level vorts rotating north
northeastward off the Gulf. HREF probabilities for .50 to 1"+ 
amounts begin to increase prior to 1200 UTC Tuesday and continue to
be high until 1500 UTC across the Central Gulf coast. A second 
period of high hourly HREF probabilities for .50-1"+ then occurs 
again in the early hours of Wednesday, beginning around 0600 UTC
and continuing to just after 1200 UTC. Isolated runoff issues
possible from each heavy precip max period, especially in more
urbanized regions.

...Great Lakes, Ohio Valley, Mid Mississippi Valley into the
Southern Plains...
A broad marginal risk area was maintained across a large region
from the Great Lakes, south and southwestward into the OH Valley,
Mid MS Valley and Southern Plains. PW values will remain high
across these regions, 1.5 to 2"+, with the latest suite of models
showing at least localized heavy rain potential in this high PW
axis. While HREF and RRFS neighborhood probabilities for 1"+ totals
are high across these areas, the probabilities do drop 
significantly for higher totals. The HREF EAS probabilities for 1"+
are low across much of this region, depicting poor overlap with 
the hi res heavy rain areas. One exception is over northeast IL
into far northern IN and southwest L.P. of MI where EAS 1"+
probabilities are as high as 25-30%, showing better hi res heavy
rain overlap. The higher EAS probabilities are to the southeast of
where the heaviest rains have occurred recently. This and the
expected progressive nature of the convection, supports keeping 
the risk level as marginal. 

...Southwest...
No changes made to the previous marginal risk area from southeast
AZ into much of NM. Weak shortwaves progged to rotate around the
mid to upper level ridge building eastward into the Great Basin day
1, enhancing large scale uvvs in an axis of seasonable PW values. 
Scattered convection across this area will support potential for 
localized heavy rainfall amounts and isolated runoff issues, 
especially across more vulnerable terrain features or burn scars.

Oravec


Day 2
Valid 12Z Wed Aug 13 2025 - 12Z Thu Aug 14 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS MUCH OF
THE EASTERN AND SOUTHERN UNITED STATES, AS WELL AS FOR PORTIONS OF
THE NORTHERN PLAINS AND SOUTHWEST...

No changes of note made to the very broad marginal risk area day 2
stretching northeastward from the Central Gulf coast, into the
TN/OH Valleys, Southern to Central Appalachians, Mid-Atlantic into
NY State and northern New England. A broad axis of above average PW
values, 1.5 to 2+ standard deviations above the mean will stretch
along and ahead of the primary surface front moving west to east
across the northern tier from the Lower Lakes/OH Valley region,
east into the Mid-Atlantic/Northeast. The above average PW values
across the Gulf coast into the Southeast/Southern Mid-Atlantic
during day 1, will also continue into day 2. There is general
agreement in the models for the potential for widespread scattered
convection in this high PW axis, with areal average moderate totals
depicted and likelihood of locally heavy totals. Confidence is low
with respect to where the heavier, more concentrated convection may
be. Subsequently, we have not changed the risk level from previous
issuances, maintaining it at marginal for the time being.

A few areas of concern for potential higher threat level day 2 in
subsequent issuances.

Southern Appalachians: Early morning heavy rains late day 1 across
the Southern Appalachians may linger into the post 1200 UTC
Wednesday period across eastern TN/western NC. The HREF 12 hour 
neighborhood probabilities ending 0000 UTC Thu Aug 14 are high for 
1 and 2"+ amounts.

Mid-Atlantic: Convection likely to begin to become more organized
ahead of the above mentioned frontal boundary pushing eastward.
HREF 12 hour neighborhood probabilities ending 0000 UTC Thu Aug 14
are high for 1 and 2"+ amounts from northern VA, northward across
much of PA into the Southern Tier of NY State. 

Central Gulf Coast: HREF hourly precip probabilities for .50-1"+
amounts remain high early day 2 from the expected second round of
heavy rains to affect the Central Gulf coastal region during the
day 1 period, that may extend into the day 2 period.

...Northern Plains...
There continues to be a lot of model spread day 2 with potential
convective development ahead of the next front pushing into the
Northern High Plains 0000-1200 UTC Thursday. The low level 
southerly flow expected to strengthen after 0000 UTC Thursday 
across the Northern Plains ahead this front, supporting increasing
convective potential. Several of the global models do show
potential for heavy amounts late day 2, but continue to show large
amounts of spread with respect to placement, leading to continued
low confidence. Given this, the risk level was maintained at marginal.

...Southwest...
No significant changes to the overall flow pattern for the
Southwest day 2. Additional weak vorts rotating around the Great
Basin/Southwest mid to upper high will support another round of 
scattered convection. Low confidence on any qpf details, but 
potential will continue for localized heavy rains and isolated 
runoff issues, especially across more vulnerable terrain features 
or burn scars.


Day 3 
Valid 12Z Thu Aug 14 2025 - 12Z Fri Aug 15 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM THE LOWER
MISSISSIPPI VALLEY, TENNESSEE VALLEY, SOUTHERN APPALACHIANS INTO 
THE SOUTHEAST AND ACROSS THE SOUTHWEST AND THE UPPER MISSISSIPPI VALLEY...

...Southeastern portion of the CONUS...
The cold front moving across the northeast tier of the CONUS day 2
will keep the axis of above average PW values, 1.5 to 2+ standard
deviations above the mean, concentrated across the southeast
quarter of the CONUS from the Lower MS Valley, east across the TN
Valley, Southern Appalachians and into the Southeast. There is
general model consensus for widespread scattered convection across
these areas day 3, with locally heavy rains likely where convection
becomes more organized. Large scale uvvs will continue to be
enhanced by shortwaves rotating around the mid to upper level ridge
that is expected to build westward day 3 across North Florida into
the northeast Gulf. Models are showing a lot of spread with qpf
details day 3, keeping the risk level, at the moment to marginal
across these areas.
 
...Southwest...
Similar to the day 2 period, no significant changes to the overall
flow pattern for the Southwest expected on day 3. Additional weak 
vorts rotating around the Great Basin/Southwest mid to upper high 
will support another round of scattered convection. Low confidence 
on any qpf details, but potential will continue for localized heavy
rains and isolated runoff issues, especially across more 
vulnerable terrain features or burn scars.

..Upper Mississippi Valley...
The frontal boundary moving into the Northern Plains day 2 will
continue to push eastward into the Upper MS Valley day 3. There may
be two rounds of precip affecting this area day 3. An initial 
surge of warm advection rains early day 3, followed by potential 
frontal convection late day 3. Continued low confidence in any qpf 
details at this range, warranting only a marginal risk. 

Oravec
$$
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