252
FOUS30 KWBC 152027
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
427 PM EDT Tue Jul 15 2025

Day 1
Valid 16Z Tue Jul 15 2025 - 12Z Wed Jul 16 2025

...THERE IS A MODERATE RISK OVER A PORTION OF THE MID-ATLANTIC
REGION...

16Z Update...
Introduced a moderate risk of excessive rainfall over portions of
southeast Virginia in proximity to the overlap of 10 to 15 percent
neighborhood probabilities shown by the 12Z HREF and areas with
flash flood guidance now under half inch per hour/1 inch per 3 hour
rates in places. The ensemble agreement supports...in
general...the placement. 12Z upper air analysis showed the axis of
deepest moisture in the area with satellite derived precipitable
water values above 2 inches roughly coincident with the most
hydrologically sensitive areas...enough ingredients were in place
to warrant a targeted upgrade. Shifts in the synoptic pattern
allowed for a subtle southward shift in the Slight Risk area away
from the Mason Dixon line. Elsewhere...the on-going ERO did not
require more than a few minor adjustments based on radar and
satellite trends,

Bann

...Florida...
Potent and intensifying mid-level trough positioned just east of
the FL peninsula tonight will translate slowly westward beneath a
ridge positioned to its north. As this feature tracks back over the
Gulf on Tuesday, a surface reflection is likely to develop (even
if not Tuesday, eventually, and NHC has a 40% chance of
development) which will enhance ascent. This feature will be
accompanied by a pool of exceptional PWs of 2.25 to 2.5 inches,
approaching +3 standard deviations above the climo mean, and
elevated MUCAPE of 1000-2000 J/kg. As this tracks across the
peninsula Tuesday, convection is expected both in the vicinity of
the mid-level center, but also along any convergence bands that
occur within the pivoting 850mb inflow. Due to the robust
thermodynamics, rainfall rates of 2-3"/hr are expected (50-70%
chance of 2+"/hr according to HREF neighborhood probabilities),
which through generally slow and chaotic storm motions will result
in axes of 3-5" of rain, highest along the western coast or beneath
the core of the mid-level center. This rain will occur atop areas
that are more vulnerable due to heavy rainfall on Monday, and the
SLGT risk remains with only modest cosmetic adjustments.


...Northern Rockies through the Upper Midwest...
Expanding trough digging across the Northern Rockies and Northern
Plains will drop steadily southward Tuesday in response to a potent
shortwave digging out of Alberta, Canada. As this impulse gets
embedded into the more pinched westerlies downstream, it will shed
spokes of vorticity across the region to help enhance ascent
already intensifying through height falls. This evolutions will
also drag a cold front southward, but this front will oscillate
north and south through D1 in response to the aforementioned
shortwaves/vorticity spokes interacting with its baroclinic
gradient. In fact, the latest guidance suggests that two distinct
waves of low pressure will develop along this front and track
eastward, leading to more focused heavy rain from convection within
otherwise scattered thunderstorms.

Although the broad MRGL risk remains, there appears to be two
focused areas for heavier rainfall. The first is in a SW to NE
oriented axis from Nebraska through the Arrowhead of Minnesota.
Here, southerly 850mb inflow will surge out of the Central Plains at
20-25 kts and impinge into the front itself. This will additionally
draw impressive thermodynamics (MUCAPE above 2000 J/kg and PWs above
1.75 inches, or the 97th percentile according to NAEFS) northward
into the front. Where the maximum ascent occurs atop these
thermodynamics, widespread convection with heavy rainfall rates of 1-
2"/hr are expected, and these storms have a high potential to train
SW to NE along the front due to 0-6km mean winds that are
progressive at 10-20 kts, but parallel to the boundary. Where these
elements train, HREF 3-hr FFG exceedance probabilities peak above
40%, and the inherited SLGT risk remains.

A second region of heavier rainfall is expected across MT. Here,
persistent westerly mid-level flow atop the front combined with
sloped frontogenesis will drive nearly continuous periods of
rainfall. PWs in the region will be elevated above the 90th
percentile according to NAEFS (0.75 to 1 inch), but accompanying
instability is minimal. This suggests that most of the rainfall
rates should be 0.5"/hr or less, so despite the impressive ECMWF EFI
signal for heavy rain (EFI > 0.9), the excessive rainfall risk
remains MRGL even if a few locations approach 3 inches of total
rainfall.


...Mid-Atlantic and Southeast...
Broad mid-level ridging centered off the Carolinas will continue
to extend back to the west as far as the Southern Plains. This will
persist the moist and unstable environment that has been plaguing
much of the east for several days now, resulting in another day of
scattered to numerous thunderstorms. On Tuesday, the focus for
convection will likely be along the decaying front which will drag
slowly southeast as it wanes, reaching from the Ozarks to the Mid-
Atlantic states, although with some oscillation north-south
expected at times. Along this boundary, generally SW flow emerging
from the Southern Plains will drive periodic impulses northeast,
interacting with the front and providing locally enhanced focused
ascent for convection. With PWs consistently above 2-2.25 inches
and MUCAPE 1000-2000+ J/kg, storms that develop will be capable of
producing efficient rain rates in excess of 2"/hr pretty much
anywhere across the region.

In general, coverage should be scattered and not support more than a
MRGL risk of excessive rainfall. However, some locally focused
convection beneath an impulse and aided by upslope 850mb inflow
could produce heavier rainfall from the Central Appalachians into
the Mid-Atlantic states. The guidance has continued to trend just a
bit south overnight with the axis of heaviest rainfall, but with the
front wavering through the day, and a general noted northward bias
in the convection on Monday (compared to progs), changes to the
inherited SLGT risk were modest and focused over more vulnerable
areas from recent heavy rainfall.


...Southwest...
The monsoon will become more active today as the elongated mid-
level ridge over the Desert Southwest weakens in response to a
potent vorticity maxima/shortwave drifting around its periphery.
This feature will traverse westward beneath the ridge, crossing
from the High Plains of NM through southern Arizona, and act upon
an increasingly moist and unstable environment. PWs are progged to
surge to as high as 1.25-1.5 inches, highest in southern AZ,
coincident with a ribbon of MUCAPE of 1000 J/kg lifting northward
as low-level flow surges out of the Rio Grande Valley and Mexico.

The high-res CAMs are generally modest with convective coverage on
Tuesday, but the recent HRRR and RRFS are much more aggressive with
simulated reflectivity, and are likely the better solutions
conceptually for the environment. As storms develop, generally in
the higher terrain first, they will be slow movers with rain rates
of 0.5-1"/hr. However, during peak instability, storms will congeal
and feature at least subtle organization beneath the shortwave as
bulk shear rises to 20-25 kts and storms coming off the terrain will
move generally west-southwest and feature rain rates that may
briefly exceed 1"/hr. The inherited SLGT risk is maintained and
adjusted subtly.


...Central Texas...
850mb LLJ surging to 20-25 kts from the south and up through the
Rio Grande Valley will again support overnight convection
persisting into the new D1 period (after 12Z Tuesday). This will be
accompanied by a shortwave/vorticity maxima lifting northward and
favorable instability of more than 1000 J/kg to support at least
scattered thunderstorms. The high-res CAMs are not aggressive with
convective coverage after 12Z Tuesday, but even isolated activity
will contain rain rates that could exceed 2"/hr, and Corfidi
vectors that are aligned anti-parallel to the mean wind, combined
with favorable upslope into the Hill Country on southerly winds
could result in backbuilding/nearly stationary storms at times.
With the area remaining extremely vulnerable due to repeated rounds
of heavy rain in the past week, (FFG as low as 0.25"/3hrs) the
SLGT risk was tailored but continued into Tuesday.


...Mid-Mississippi Valley...
Weak impulses/MCVs embedded within the mean SW to NE mid-level
flow will lift out of Texas and interact with a weakening
stationary front over the Mid-Mississippi Valley. ALthough in
general, forcing for ascent will be modest, it will be locally
enhanced where these impulses move atop the surface convergence
associated with the front itself. Intense thermodynamics will
remain across the area reflected by PWs approaching or exceeding 2
inches collocated with MUCAPE above 1000 J/kg to support rainfall
rates of 1-2"/hr in convection. Mean winds of 10-15 kts suggest
individual storms will be progressive, but some weak organization
and some training along the front could enhance the duration of
heavy rainfall. This appears most likely in the vicinity of
southern IN where moisture confluence maximizes, creating a locally
greater excessive rain risk, but the MRGL risk remains for
isolated impacts today.


Weiss

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...

2030Z Excessive Rainfall Discussion Update...
Primary change with this issuance was the southward expansion of
the Slight Risk area across the Central Plains to the western High
Plains. The NCEP deterministic guidance showed the southward
adjustment...but the ensembles tended to be less unanimous with
the NMM core runs and ARW core runs clustering amongst themselves
at somewhat different latitudes. Therefore...a somewhat broader
corridor for the Slight and Marginal risk areas was preferred, Made
a subtle westward shift compared with the previous outlook across
the Gulf coast based primarily on the model guidance. Few changes
needed elsewhere.

Bann

Previous Excessive Rainfall Discussion...

...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...

2030Z Excessive Rainfall Discussion Update...
Primary change was to shift the Slight Risk a bit to the west in
concert with the large scale guidance. Given uncertainty in
timing...remain reluctant to upgrade to a Moderate risk this cycle
but it is not entirely out of the question towards the end of the
Day 3 period. Except for a few minor adjustments elsewhere...the
on-going outlook seemed to be in good shape.

Bann

Previous Excessive Rainfall Discussion

...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


Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt