Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
729 PM EDT Thu Aug 27 2026
Day 1
Valid 01Z Fri Aug 28 2026 - 12Z Fri Aug 28 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
GULF COAST AND THE NORTHEAST/MID-ATLANTIC...
...Northeast/Mid-Atlantic...
The Slight Risk area was adjusted for recent radar reflectivity
trends and 18z HREF/12z RRFS guidance. Portions of NJ were
stabilized by convection earlier today, and the guidance suggests
that heavy rain concerns could continue as late as 06z in/near an
area of 1.5-1.75" precipitable water values, effective bulk shear
of 25-30 kts, and ML CAPE of ~2000 J/kg. Hourly rain amounts have
peaked at 2.5" at times per radar estimates. Local totals near 5"
remain possible until convection wanes.
...Southeast and Gulf Coast...
The tail-end of the longwave upper trough, with distinct shortwave
energy traversing the Gulf Coast, along with the stationary front,
will provide the dynamical focus for more organized (widespread)
areas of convection across this region through 04z or so before
waning for several hours overnight, and then picking up during the
early morning hours on Friday. Flood prone urban areas such as New
Orleans, Mobile, and some of the more interior areas of
Mississippi dealing with the latest of multiple days of heavy rains
could experience widely scattered instances of flash flooding.
...Southwest...
Convection across the Southwest should fade by roughly 04z, while
activity across the High Plains lingers into early Friday morning
as current thunderstorms across NE drop southward with time. In the
High Plains, ML CAPE is 1500-3000 J/kg presently and effective bulk
shear of 30-40 kts should allow for convective organization,
including mesocyclone formation. Coverage is not expected to be
broad, so the Marginal Risk category continues to suffice. Hourly
rain amounts to 2" are possible where storms merge, congeal,
mesocyclones form, or thunderstorms show some training character.
Roth
Day 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
729 PM EDT Thu Aug 27 2026
Day 2
Valid 12Z Fri Aug 28 2026 - 12Z Sat Aug 29 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE COASTAL
CAROLINAS INTO SOUTHEAST VIRGINIA...
...Southeast/Mid-Atlantic...
Minor expansion made to the previous Slight Risk in the Mid-
Atlantic based on the latest HREF and REFS guidance. The upper-
level trough continues to linger in place while continuing to dig.
A sharpening upper-level jet streak on the lee side of the upper
trough will enhance deep-layer ascent as well and low-level FGEN,
increasing the convective coverage and intensity during the peak
heating hours Friday afternoon and evening. This forcing combined
with PWs exceeding 2", 1000-2000 J/kg MUCAPE, and a stalled front
will likely lead to multiple rounds of heavy rainfall within the
Slight Risk area, with the strongest cells capable of hourly rain
rates of up to 3". Heavy rainfall is also expected in the surrounding
areas extending from the Gulf Coast to southern Virginia, in which
isolated instances of flash flooding are possible. 12Z guidance
has a signal for a weak circulation developing off of the Florida
Panhandle over the next 24 hours, which focuses convection near the
coastline. While FFGs in the are are high, a categorical upgrade
could be needed if the heavy rainfall advertised from this feature
trends further inland.
...Northern Plains...
A shortwave trough and associated surface warm front will quickly
punch through/somewhat flatten the upper ridge and move through the
eastern Dakotas and western Minnesota and Iowa. This will bring a
progressive complex of thunderstorms into the area, with some
potential for heavy rainfall. Healthy moisture and solid
instability will be present with PWs exceeding 1.5" and MUCAPE
exceeding 2000 J/kg, respectively. However, given the progressive
nature of thunderstorms, the Marginal risk area was maintained.
...Southwest and Portions of the Great Basin...
Monsoonal moisture continues the fuel diurnally drive convection
along the southwest. A plume of moisture will drive PWATs around
1.5-1.75" northward along much of the Southwest. This will lead to
renewed chances of isolated flash flooding, as has been similar for
much of the monsoon season.
Over the Great Basin, anomalous mid-level moisture, on the order
of 4-5 standard deviations above average, will stream northward
from Post-T.C. Iselle and get swept into a deep mid-level trough
propagating through the Pacific NW. MUCAPE values near 500 J/Kg
working their way north through the southern Great Basin/Sierra
should be enough to produce heavy rainfall and at least isolated
flash flooding out of the available moisture. The Marginal Risk
area was adjusted southward to reflect the highest HREF
probability of MUCAPE exceeding 500 J/Kg -- where rainfall rates
would be most intense.
Asherman/Hurley
Day 2 threat area:
www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
729 PM EDT Thu Aug 27 2026
Day 3
Valid 12Z Sat Aug 29 2026 - 12Z Sun Aug 30 2026
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE SOUTHWEST AND GREAT BASIN INTO THE ROCKIES AND FRONT RANGE,
NORTHERN PLAINS INTO THE UPPER MIDWEST, AND THE SOUTHEAST TO THE
CAROLINAS...
...Southwest and Great Basin into the Rockies and Front Range...
The upper high weakens/flattens across the Southern Plains as
troughing becomes increasingly established over portions of the
West and Northern Plains. Moisture gets funneled in between the two
features, which may have a connection to a future northeast
Pacific tropical cyclone. A Marginal Risk of excessive rainfall
remains for Sat-Sat night as above average moisture for the
monsoon fuels diurnally driven widely scattered to scattered
convection. The pattern would support isolated flash
flooding, with favored terrain, urbanized areas, box/slot canyons,
arroyos/dry washes, and burn scars being the most prone to
flooding.
..Southeast...
Showers and thunderstorms capable of producing heavy to excessive
rainfall each day should be focused in the vicinity of a quasi-
stationary front near the Gulf coast region and aforementioned
disturbance in the central Gulf, luring the dying front west as an
easterly wave Sunday and Sunday night. Increasing ridging poking
in from the western subtropical North Atlantic should lead to a
decreasing heavy rain threat across the Carolinas with time. With
precipitable water values of 2"+ expected, hourly rain amounts to
3" are possible where cells merge or train, particularly near the
Gulf Coast.
...Northern Plains into the Upper Midwest...
Pattern remains unsettled across this area during Day 3 (Sat-Sat
night), owing to shortwave energy (convectively-aided) riding along
the periphery of the upper ridge ("ring of fire"), along with the
persistent moisture anomaly in place (PWATs ~2 standard deviations
above normal). Still quite a bit of spread among the guidance QPFs;
however, given the degree of instability and moisture in play,
some of the stronger storms will be capable of intense short-term
rainfall rates and localized flash flooding.
Asherman/Hurley
...Southeast and Gulf Coast...
The tail-end of the longwave upper trough, with distinct shortwave
energy traversing the Gulf Coast as per the 00Z guidance, along
with the attendant (eventually quasi-stationary) surface front,
will provide the dynamical focus for more organized (widespread)
areas of convection across this region. While approaching and
eventually residing near the coast, the boundary will contend with
ample moisture moving northward from the Gulf, yielding multiple
rounds of storms with the potential for heavy rainfall. Flood prone
urban areas such as New Orleans, Baton Rouge, Mobile, and some of
the more interior areas of Mississippi dealing with the latest of
multiple days of heavy rains will all support widely scattered
instances of flash flooding. Rainfall and thunderstorms will bring
heavy rainfall to Alabama and Georgia as well, and the previously
mentioned trough encounters a similarly moist airmass being pushed
northeastward in the lower levels. This will support several rounds
of storms in the form of multiple lines across Georgia and parts
of South Carolina. The storms moving northeastward could allow for
some training as the cold front moves through, and could bring
widely scattered instances of flash flooding.
...Southwest...
As similarly stated in previous outlooks, a very gradual eastward
progression of the upper ridge across the Southwest should allow
for a shifting of the heaviest precipitation to the east out of
Utah and more into the central Plains. The ridge should largely
preclude much organization from the storms, so the Marginal
continues to focus on the areas where either terrain or prior
rainfall could locally increases the risk of flash flooding.
Hurley/Blanco-Alcala
Day 3 threat area:
www.wpc.ncep.noaa.gov/qpf/99epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
729 PM EDT Thu Aug 27 2026
Day 4 and Day 5
...A MARGINAL RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PARTS OF
THE WEST & NORTHERN TIER OF THE COUNTRY...
...Southwest/Great Basin/Intermountain West...
A warm core high shifts from the Southern Plains into the Southern
Appalachians as troughing becomes increasingly established over
portions of the West and the Northern Plains. Moisture gets
funneled in between the two features, which may have a connection
to Hurricane Karina. A Marginal Risk of excessive rainfall remains
for Sunday through Monday as above average moisture fuels
diurnally driven widely scattered to scattered convection across
the Southwest, portions of the Rockies, and parts of the Great
Basin. The degree of moisture expected could lead to hourly rain
amounts to 2" wherever cells sit, merge, or train. This could lead
to isolated flash flooding...with favored terrain, urbanized areas,
box/slot canyons, arroyos/dry washes, and burn scars being the
most prone to flooding.
..Southeast...
Showers and thunderstorms capable of producing heavy to excessive
rainfall each day should remain focused in the vicinity of a
dying front near the Gulf coast region and behind a retrograding
upper level disturbance moving through the central Gulf. The
combination of what becomes an easterly wave and an increasing
amount of ridging building westward from the western subtropical
North Atlantic should lead to the heavy rain threat being
constrained to portions of the northeast and north-central Gulf
coast. With precipitable water values of 2"+ expected, hourly rain
amounts to 3" are possible where cells merge or train. Depending
upon how much rain falls during the short range period, a Slight
Risk area may become necessary as we get closer in time.
...Upper Midwest/Northern Plains/Great Lakes...
There is a risk of excessive rainfall persisting Sunday and Monday
from the Northern Plains across the Great Lakes into the Northeast
over the top of an eastward shifting warm core ridge where the flow
becomes increasingly deep layer out of the west. Gulf moisture
sneaks in at the low levels, which across the Plains and Midwest
are more out of the southwest. Precipitable water values are
expected to be in the 1.5-2" range. Where cells train or merge,
hourly rain amounts to 2.5" are possible, which would be most
problematic in urban areas. Changes to continuity were made to
adjust to where the 12z guidance was most agreeable on a heavy rain
signal which removed portions of the Midwest from the risk area.
Based on the ingredients available, a Slight Risk will likely be
needed somewhere between the Great Lakes and Northeast as we get
closer in time.
Roth
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
729 PM EDT Thu Aug 27 2026
Day 4 and Day 5
...A MARGINAL RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PARTS OF
THE WEST & NORTHERN TIER OF THE COUNTRY...
...Southwest/Great Basin/Intermountain West...
A warm core high shifts from the Southern Plains into the Southern
Appalachians as troughing becomes increasingly established over
portions of the West and the Northern Plains. Moisture gets
funneled in between the two features, which may have a connection
to Hurricane Karina. A Marginal Risk of excessive rainfall remains
for Sunday through Monday as above average moisture fuels
diurnally driven widely scattered to scattered convection across
the Southwest, portions of the Rockies, and parts of the Great
Basin. The degree of moisture expected could lead to hourly rain
amounts to 2" wherever cells sit, merge, or train. This could lead
to isolated flash flooding...with favored terrain, urbanized areas,
box/slot canyons, arroyos/dry washes, and burn scars being the
most prone to flooding.
..Southeast...
Showers and thunderstorms capable of producing heavy to excessive
rainfall each day should remain focused in the vicinity of a
dying front near the Gulf coast region and behind a retrograding
upper level disturbance moving through the central Gulf. The
combination of what becomes an easterly wave and an increasing
amount of ridging building westward from the western subtropical
North Atlantic should lead to the heavy rain threat being
constrained to portions of the northeast and north-central Gulf
coast. With precipitable water values of 2"+ expected, hourly rain
amounts to 3" are possible where cells merge or train. Depending
upon how much rain falls during the short range period, a Slight
Risk area may become necessary as we get closer in time.
...Upper Midwest/Northern Plains/Great Lakes...
There is a risk of excessive rainfall persisting Sunday and Monday
from the Northern Plains across the Great Lakes into the Northeast
over the top of an eastward shifting warm core ridge where the flow
becomes increasingly deep layer out of the west. Gulf moisture
sneaks in at the low levels, which across the Plains and Midwest
are more out of the southwest. Precipitable water values are
expected to be in the 1.5-2" range. Where cells train or merge,
hourly rain amounts to 2.5" are possible, which would be most
problematic in urban areas. Changes to continuity were made to
adjust to where the 12z guidance was most agreeable on a heavy rain
signal which removed portions of the Midwest from the risk area.
Based on the ingredients available, a Slight Risk will likely be
needed somewhere between the Great Lakes and Northeast as we get
closer in time.
Roth
