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Message   Mike Powell    All   HVYRAIN: Excess Rain D2-3   August 2, 2025
 6:20 PM *  

FOUS30 KWBC 022000
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
400 PM EDT Sat Aug 2 2025

Day 2
Valid 12Z Sun Aug 03 2025 - 12Z Mon Aug 04 2025

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF
THE SOUTHEAST/GULF COAST AND SOUTHERN PLAINS...

...Southeast/Gulf Coast... 
The cold front from Saturday will continue to waver along the Gulf
Coast as a stationary front Sunday in response to subtle 
additional height falls as the trough extending from the Northeast 
continues to subtly amplify. The trough axis may reach the Gulf 
Coast Sunday evening, but flatten at the same time in response to 
an elongated ridge draped across the Gulf. Moisture pooled along 
this front will remain impressive as PWs remain above 2.25 inches 
with deep column saturation noted via moist-adiabatic lapse rates 
through the depth of the column indicating tall-skinny CAPE which 
is progged to reach above 1000 J/kg during peak heating. 
Convergence along the front into these thermodynamics will support 
another day of slow moving convection noted by 0-6km mean winds of 
just 5-10 kts pointed to the east, with rainfall rates likely 
eclipsing 2"/hr at times (HREF 50-60% chance) supported by warm 
cloud depths nearing 15,000 ft to produce efficient warm-rain 
collision processes. 

Late in the aftn and into the evening, a shortwave is progged to 
rotate beneath the trough and pivot E/NE from the FL Panhandle into
eastern GA, coincident with a surge in bulk shear up to 35 kts. 
This could support some more widespread convection with 
organization into clusters/multi-cells. With weak storm motions in 
place, any collisions/organization during this time could briefly 
pulse up rain rates even more significantly, while also leading to 
longer temporal duration or repeating rounds of these rain rates. 
Where this occurs, the HREF and REFS both indicate a 70-90% chance 
of 3"/24hrs and locally a 40-70% chance of more than 5"/24hrs. This
rain will be falling atop areas that will likely receive 
significant rainfall on D1 as well, further enhancing the threat 
for flash flooding. Although this region generally takes a lot of 
rainfall to flood, the setup appears to support a higher than 
typical SLGT risk especially from the FL Panhandle through coastal GA.

Otherwise, scattered thunderstorms with intense rain rates could
produce instances of flash flooding on Sunday. The guidance has a
secondary area of maximum rainfall potential along a westward
advancing surface trough through northern AL into central TN which
may require an upgrade in the ERO with later issuances since
3"/24hr probabilities peak at 40-50%, but this correlated with 3-hr
FFG exceedance that is just 10-15%. At this time opted to maintain
the MRGL risk and re-evaluate with new guidance for a possible
upgrade overnight or on the D1 Sunday.

...Plains...
The elongated stationary front aligned north to south from MT
through TX will weaken Sunday, but leave at least a weak
convergence boundary as it decays. Mid-level flow upstream of this
feature will gradually become more W/NW as it gets squeezed between
an amplifying ridge over the Southwest and a trough across Canada.
Within this pinched flow, multiple shortwaves/vorticity impulses
will shift eastward, interacting with the front to cause another
day of convective development across the High Plains and into the
Plains, with several MCSs possible beneath any of the stronger impulses.

While there is considerable spread and thus uncertainty in the
placement of any of these MCSs, each of them will likely contain
intense rain rates of 1-2"/hr (locally higher) in response to
meridional 850mb flow drawing elevated thermodynamics northward.
From TX to MN a ribbon of PWs of 1.25 to 1.75 inches will overlap a
plume of MUCAPE above 2000 J/kg and bulk shear of 25-35 kts to
support the intense rainfall rates. While the CAMs are generally
suggestive of forward propagating features that will limit the
duration of heavy rainfall, locally any of these could produce some
short-term training, especially where any MCVs and lingering
boundaries can track. Uncertainty is high in where this is most
likely, but the REFS and HREF suggest a slightly higher potential
from north Texas into central KS, as well as across portions of
eastern SD/ND into Minnesota. 

The large inherited SLGT risk for TX/OK/KS was cosmetically 
adjusted, but further refinements are likely (with a reduction of 
category also possible) as confidence in MCS placement becomes more
clear. Across MN, a targeted SLGT risk was considered where HREF 
and REFS probabilities for 5"/24 hrs peak above 15% as storms 
slow/regenerate beneath a deformation axis downstream of a mid- 
level closed low. Confidence is modest across this area, but 24-hr 
LPMM from the HREF is 3-4", so FFG exceedance potential appears 
higher than surrounding areas. However, there is still a lot of
placement uncertainty among the available models, and some concern
that instability won't be sufficient to support multiple rounds of
heavy rain. While a SLGT risk is still possible with later
issuances, after coordination with WFO DLH opted to maintain the
MRGL risk at this time.

Weiss


Day 3
Valid 12Z Mon Aug 04 2025 - 12Z Tue Aug 05 2025

...THERE ARE MARGINAL RISK AREAS FOR THE NORTHERN ROCKIES, NORTHERN
PLAINS/MINNESOTA, AND THE SOUTHEAST...

...Southeast...
Moisture streaming northeast ahead of the longwave trough axis will
maintain unsettled conditions and widespread showers/thunderstorms
from southern AL through GA and coastal SC. There is quite a bit of
uncertainty in the placement of the heaviest axis of QPF on D3
owing to uncertainty in accompanying mid-level impulses progged to
lift out of the Gulf on Monday, and this limits confidence from
upgrading the inherited MRGL risk to a SLGT risk at this time.
However, with PWs likely continuing at 2-2.25 inches overlapping
MUCAPE of more than 1000 J/kg, rainfall rates of 1-2"/hr or more
are likely within any convection that develops and then tracks
northeast. Some repeating rounds are possible, but SREF/GEFS/ECENS
probabilities for 3"/24hrs are all quite modest at this time, and
the EC/EC-AIFS is north of the other guidance which could allow the
heaviest rain D3 to not overlap the heaviest rainfall footprint
from D1-2. For these reasons the MRGL risk was maintained and
adjusted, but a SLGT risk may be needed with later issuances.

...Northern Plains...
The closed mid-level low responsible for areas of heavy rainfall
Sunday will continue to lift northeast and weaken on Monday. The
weakening of this feature will result in less ascent, but a
lingering boundary in its wake could still produce enhanced low-
level convergence, primarily across Minnesota. The threat for 
heavy rainfall appears lower D3 than D2, as both ascent and 
thermodynamics are reduced, but locally rainfall rates in excess of
1"/hr are possible, with a secondary surge in convection possible 
late in response to 850mb winds reaching 20-30 kts converging into 
the region. With FFG likely to be lowered from antecedent rainfall 
D1 and D2, isolated excessive rainfall impacts will again be 
possible on Monday.

...Northern Rockies...
A shortwave moving northeast across ID/MT will help amplify a
trough shifting into the Northern High Plains on Monday. This will
drive enhanced ascent into the already broad synoptically forced
lift, helping to spawn a weak wave of low pressure tracking into
the High Plains the latter half of D3. Low-level southeasterly flow
will draw elevated PWs and MUCAPE into the region aiding a
moistening column from high-level SW flow emerging from the
Pacific. The result of this will likely be showers and
thunderstorms moving progressively eastward from the Northern
Rockies through the northern High Plains. With PW anomalies +1 to
+1.5 sigma above the climo mean, rainfall rates may reach 1"/hr at
times, which despite the anticipated progressive nature of these
cells, could produce isolated impacts due to runoff.

Weiss
$$
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