Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
418 AM EDT Thu Aug 27 2026
Day 1
Valid 12Z Thu Aug 27 2026 - 12Z Fri Aug 28 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
CENTRAL GULF COAST AND THE NORTHEAST/MID-ATLANTIC...
...Northeast/Mid-Atlantic...
The Slight Risk area inherited from yesterday's Day 2 ERO was
expanded a bit southwest to include the DC metro region. Deep-layer
ascent ahead of the broad upper trough/surface front will get a
boost from a developing 70-90kt upper jet streak on the lee side of
this trough, adding to the frontogenetic support during peak
heating (max diurnal destabilization) later this afternoon into
early evening. Mixed layer CAPEs of 1000-2000+ J/Kg with PWATs
approaching 1.75" will support locally intense rainfall rates (at
least 2" within an hour) under the most intense cells. Simulated
reflectivity guidance from multiple high-res CAM runs show the
potential for multiple rounds of convection (at least 2, including
the main pre-frontal QLCS), especially within the Slight Risk
area. Probabilities of exceeding 3"/24hr ending 12Z Fri per the 00Z
HREF peak between 40-70% within the Slight Risk area, with a
similar range advertised by the REFS (though with the highest probs
over northern NY into Upstate NY). The 00Z REFS in fact has a small
area of 40-50% probabilities of exceeding 5" across portions of
Update NY.
...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 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
418 AM 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...
Some changes were made to the Slight Risk area inherited from
yesterday's Day 3 ERO, most notably to extend the northern portion
to include SE VA based on the latest guidance and (in particular)
the bump in REFS 24hr QPF exceedance probabilities. The upper-
level trough mentioned in the Day 1 discussion continues to linger
in place while continuing to dig. A sharpening upper-level jet
streak on the lee side of the upper trough will boost deep-layer
ascent as well as low-level FGEN, increasing the convective
coverage and intensity during the peak heating hours Friday
afternoon into the evening hours. PWATs meanwhile area expected to
range between 2.00-2.25" per the ECMWF and GFS. High PWATs, decent
dynamical support, and a stalling surface 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". As
such, scattered instances of flash flooding will be possible along
the coast. 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.
...Northern Plains...
A plume of upper-level energy by way of 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 potential for heavy rainfall.
Healthy moisture and solid instability will be present, with PWATs
exceeding 1.5", and MUCAPE exceeding 2000 J/kg. 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.
A Marginal Risk was retained over portions of the Sierra Nevada up
into the Great Basin for Day 2 (Friday-Friday night). Anomalous
mid-level moisture, on the order of 4-5 standard deviations above
average, will stream northward from Tropical Storm Iselle and get
swept into a deep mid- level trough propagating through the
Northwestern U.S. on Friday. MUCAPE values between 250-500 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.
Hurley/Kebede/Blanco-Alcala
Day 2 threat area:
www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
418 AM 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 behind a
retrograding upper level disturbance moving through 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.
Hurley/Roth
Day 3 threat area:
www.wpc.ncep.noaa.gov/qpf/99epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
418 AM EDT Thu Aug 27 2026
Day 4 and Day 5
Valid 12Z Sun Aug 30 2026 - 12Z Tue Sep 01 2026
...A MARGINAL RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PARTS OF
THE WEST, SOUTHEAST, PLAINS, UPPER MIDWEST, & GREAT LAKES...
...Southwest/Great Basin/Intermountain West...
A warm core high weakens across the Southern Plains 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 a future northeast Pacific
tropical cyclone. A Marginal Risk of excessive rainfall remains
for the latter part of the weekend into early part of the upcoming
workweek as above average moisture fuels diurnally driven widely
scattered to scattered convection. 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
quasi- stationary front near the Gulf coast region and behind a
retrograding upper level disturbance moving through the central
Gulf. The combination of the surface front weakening and an
increasing amount of ridging building westward from the western
subtropical North Atlantic should lead to a decreasing heavy rain
threat across the Carolinas and a portion of the Southeast US. With
precipitable water values of 2"+ expected, hourly rain amounts to
3" are possible where cells merge or train, particularly near the
Gulf Coast.
...Upper Midwest/Northern Plains/Great Lakes...
There is a risk of excessive rainfall persisting through the latter
part of the weekend and into the early portion of the upcoming
workweek across parts of the Upper Midwest/Northern Plains/Great
Lakes. A mid- and upper-level low entering the Pacific Northwest on
Sunday will result in backing mid- and upper level flow and
increasing divergence aloft over the Northern Plains to the lee of
the Rockies. At the same time...locally heavy rainfall will develop
over parts of the Upper Midwest into the Great Lakes region as
moisture flux increases between the subtropical high to the east
and lowering surface pressure to the west. The threat of heavy
rainfall moves north of the International border by Monday while
the risk across the Great Lakes persists where isolated maximum rainfall
rates of 1 to 2 inches per hour will be possible.
Bann
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
418 AM EDT Thu Aug 27 2026
Day 4 and Day 5
Valid 12Z Sun Aug 30 2026 - 12Z Tue Sep 01 2026
...A MARGINAL RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PARTS OF
THE WEST, SOUTHEAST, PLAINS, UPPER MIDWEST, & GREAT LAKES...
...Southwest/Great Basin/Intermountain West...
A warm core high weakens across the Southern Plains 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 a future northeast Pacific
tropical cyclone. A Marginal Risk of excessive rainfall remains
for the latter part of the weekend into early part of the upcoming
workweek as above average moisture fuels diurnally driven widely
scattered to scattered convection. 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
quasi- stationary front near the Gulf coast region and behind a
retrograding upper level disturbance moving through the central
Gulf. The combination of the surface front weakening and an
increasing amount of ridging building westward from the western
subtropical North Atlantic should lead to a decreasing heavy rain
threat across the Carolinas and a portion of the Southeast US. With
precipitable water values of 2"+ expected, hourly rain amounts to
3" are possible where cells merge or train, particularly near the
Gulf Coast.
...Upper Midwest/Northern Plains/Great Lakes...
There is a risk of excessive rainfall persisting through the latter
part of the weekend and into the early portion of the upcoming
workweek across parts of the Upper Midwest/Northern Plains/Great
Lakes. A mid- and upper-level low entering the Pacific Northwest on
Sunday will result in backing mid- and upper level flow and
increasing divergence aloft over the Northern Plains to the lee of
the Rockies. At the same time...locally heavy rainfall will develop
over parts of the Upper Midwest into the Great Lakes region as
moisture flux increases between the subtropical high to the east
and lowering surface pressure to the west. The threat of heavy
rainfall moves north of the International border by Monday while
the risk across the Great Lakes persists where isolated maximum rainfall
rates of 1 to 2 inches per hour will be possible.
Bann
