FOUS30 KWBC 150809
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
409 AM EDT Tue Jul 15 2025

Day 2
Valid 12Z Wed Jul 16 2025 - 12Z Thu Jul 17 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
NORTHERN MID-ATLANTIC STATES AND THE CENTRAL PLAINS/UPPER MIDWEST...

...Gulf Coast...
A tropical wave (now up to 40% chance of development from NHC) will
continue to track slowly westward across the northern Gulf as the
mid-level vorticity center remains trapped beneath an elongated
ridge to the north. This will force the wave to track almost due
west on Wednesday, but there remains quite a bit of latitudinal
spread amongst the various global models as to where the vorticity
center will track. This causes lowered confidence in the QPF and
subsequent ERO for Wednesday, as the ridge to the north combined
with generally weak shore-parallel 850mb flow (at least until the
shortwave passes west of each area on the coast) could severely
limit the amount of rain that can penetrate onshore. However,
convection that does develop onshore will benefit from PWs above
2.25 inches (potentially approaching 2.5 inches) and MUCAPE of 2000
J/kg, supporting rainfall rates above 2"/hr. With some training
along the coast likely, this could produce 2-3" of rainfall in some
areas, although the exact placement of the heaviest rain is still
uncertain. At this time the MRGL risk remains for the Gulf Coast,
but if this system organizes more quickly or models form a
consensus for a more northward track, a SLGT risk may be needed
eventually for Wednesday.

...Central High Plains eastward through the Mid-Atlantic...
Broad and expansive mid-level ridge centered off the Carolinas will
continue to dominate the synoptic pattern with return flow around 
this ridge promoting a continuation of the exceptional moisture and
instability plaguing much of the CONUS. Wednesday will be no
exception as a moisture plume characterized by PWs of 1.5 to 2.25
inches stretches from Kansas through New England, coincident with
pools of MUCAPE exceeding 2000 J/kg. Within this axis of impressive
thermodynamics, a stationary front from Monday will dissipate as a
secondary front approaches from the Northwest late Wednesday night.
Additionally, within the mid-level return flow, multiple shortwave
impulses will rotate northeast around the ridge to provide
additional ascent. Where these impulses interact with the low-level
baroclinic boundary, enhanced rainfall from more organized 
convection will result.

This is most likely in two areas on Wednesday. The first is across
Pennsylvania/Ohio where a potent shortwave impulse is progged to 
track overhead during peak heating. This will not only drive 
locally enhanced ascent, but also accelerate bulk shear to 30-35 
kts forcing better storm organization. 850mb inflow across this 
area will also strengthen, potentially reaching 30-35 kts which 
will be 1.5 times the mean 0-6km wind, a signal for strong 
convergence. Although storms that develop should remain progressive
to the east, multiple rounds are likely, each one containing 
rainfall rates of around 2"/hr, leading to total rainfall of 2-4" 
in some areas. This falling atop saturated soils from recent heavy 
rainfall will likely result in instances of flash flooding.

The other area will be across the Central Plains and Upper Midwest,
especially focused from northern Kansas through Wisconsin where a 
multiple MCS driven by convectively enhanced shortwaves will move
across these areas with focused heavy rainfall. ALthough there 
continues to be some latitudinal spread with the placement of these
MCSs which will be the primary driver of the flash flood risk D2, 
a slowly veering 30-40kt LLJ emerging from the Southern Plains will
provide the focus for development through convergence and 
isentropic lift. Through this veering late Wednesday night, some
training/backbuilding is also anticipated (Corfidi vectors point 
directly against the mean flow by 06Z Thursday) suggesting enhanced
rainfall totals from rates that will be 1-2"/hr if not locally 
higher. The inherited SLGT risk was modified for the new guidance 
and positioned across the best overlap of REFS (where available), 
GEFS, and ECENS 3"/24hr probabilities exist

...Desert Southwest...
Increasing PWs on southerly low-level flow emerging from the Rio 
Grande and Mexico is expected again on Wednesday, with PWs surging
to as high as 1.5 inches over southern Arizona. This is higher and
more expansive than what is progged for D1. However, with the
expected widespread convective coverage continuing late Tuesday
night into Wednesday morning, re-destabilization may be limited,
which is reflected by degraded CAPE > 500 J/kg probabilities from
the GEFS that are below 40% except along the immediate NM/AZ
border. Additionally, forcing for ascent through shortwave activity
appears displaced a bit farther north and less intense as Tuesday,
so while any storms that develop Wednesday would be slow moving and
have brief 0.5+"/hr rates, the MRGL risk category was maintained.

Weiss

Day 3
Valid 12Z Thu Jul 17 2025 - 12Z Fri Jul 18 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE CENTRAL
GULF COAST...

...Gulf Coast...
A disturbance moving across the northern Gulf (NHC probability of
development 40%) will continue to drift westward on Thursday. While
there is still considerable temporal and spatial spread among the
various models, it is likely that a weakness will develop around
the Atlantic ridge to allow a slow NW drift of this feature towards
the LA coast on D3. With PWs expected to be 2.25 to 2.5 inches, and
forcing for ascent intensifying as the system consolidates, heavy
rainfall rates of 2+"/hr will be likely within convection/heavy
stratiform rain. Despite uncertainty as to how far north the
precipitation will spread by 12Z Friday due to subsidence/drier 
air to the north within the ridge, a consensus of the guidance 
still pushes some heavier focused rainfall into southern LA, 
prompting the SLGT risk from around Mobile Bay, AL to the LA/TX 
border. Additional adjustments are likely as the forecast becomes 
refined in the next few days.

...Central Plains through the Northeast...
Return flow around an elongated Atlantic ridge which will spread as
far west as the Southern Plains will maintain broad SW winds and a
moist/unstable environment across a large portion of the country.
Into this favorable environment, a cold front will slowly push
southeast, extending from New England to the Central High Plains by
the end of the period. Ascent along this front into the robust
thermodynamics will support at least scattered thunderstorms with
heavy rain rates. At this time the signals for any organized
convection leading to a higher flash flood risk is muted, so a
broad MRGL risk remains. However, with weak impulses moving through
the flow and interacting with the baroclinic zone, it is possible
some areas may be upgraded to SLGT risks with future issuances as
guidance converges on any locally enhanced rainfall threats.

...Southwest...
Elongated ridge from the Atlantic will spread westward into the
Southern Plains and Four Corners, while a closed low pushes north
from Baja into southern California. Between these two features,
southerly flow will become pinched, while weak impulses embedded
within the flow drift northward. PWs will climb to broadly above
1.25 inches, highest in AZ, but the environment at this time 
appears to be lacking sufficient instability for widespread
monsoons thunderstorms. Still, the southerly flow, high PW, and
weak impulses aloft will support convection, especially where flow
upslopes into terrain features. Storm motions will be slow, and
rain rates above 0.5"/hr are probable, so isolated flash flooding,
especially in vulnerable terrain or across burn scars/urban areas,
is possible.

Weiss
$$

 
--- SBBSecho 3.28-Linux
 * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (618:250/1)