Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
508 AM EDT Fri Aug 8 2025
Day 1
Valid 12Z Fri Aug 08 2025 - 12Z Sat Aug 09 2025
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER CENTRAL AND
NORTHERN MINNESOTA INTO FAR NORTHWESTERN WISCONSIN...
...Northern Plains through Upper Mississippi Valley...
An anomalous 500 mb low will track eastward from the
Montana/Canada border today with strong height falls moving into
the Dakotas later today and tonight. At the surface, low pressure
over South Dakota will track eastward toward Minnesota today before
advancing into southern Canada overnight as a cold front becomes
more defined to its south. Strong to extreme instability is
forecast again with this system today, ahead of the low/front over
central/southern Minnesota along with highly anomalous moisture
over central and northern Minnesota (+2 to +4 standardized
anomalies via 00Z GFS). The front will steadily track eastward
between 00-12Z Saturday but potential will exist for high rainfall
rates given the environment.
An MCS from early Friday morning is expected to continue eastward
into the northern half of Minnesota early in the period but with a
weakening trend as the low level jet weakens and veers. While this
first round of rainfall is not expected to pose much in the way of
a flash flood threat prior to 18Z, high short term rates and
perhaps an inch or so of rain may prime soils for a second round
Friday night. A second round of thunderstorms is expected to form
over eastern North Dakota by late evening with convection
orienting from NNE to SSW with the advancing cold front. Numerous
thunderstorms are expected ahead of this boundary for Friday night
as low level forcing combines with increasing diffluence/divergence
aloft ahead of a jet max east of the upper low. Should line
orientation briefly match the mean steering flow, allowing for
training, high rain rates of perhaps 2+ in/hr can be realized as
the convective axis shifts eastward into northwestern Wisconsin by
Saturday morning. A Slight Risk was introduced for the potential
for flash flooding and potential for 2-4 inches in central to
northern Minnesota. The Marginal Risk surrounds this region for
lower confidence in excessive rainfall potential.
...Southeastern Coast into Northern and Central Florida...
High moisture with precipitable water values near 2 inches (a bit
higher over central/northern Florida) will be in place again today
in the vicinity of a lingering stationary front that extended from
offshore of the Carolinas into northern Florida. Daytime heating
should allow MLCAPE values to rise into roughly the 1000-2000 J/kg
range. Weak mid-level ridging over Florida will be accompanied by
weak deeper layer mean flow, supportive of slow storm motions. 00Z
models support the potential for some slightly stronger low level
flow over the western Peninsula, northeast of an inverted trough
axis in the eastern Gulf, which could increase potential for slow
cell movement/backbuilding. The environment could support rain
rates of 2 to 4 in/hr, but the threat appears rather isolated to
widely scattered, only supporting a Marginal Risk.
Otto
Day 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
508 AM EDT Fri Aug 8 2025
Day 2
Valid 12Z Sat Aug 09 2025 - 12Z Sun Aug 10 2025
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER IOWA INTO
SOUTHERN WISCONSIN AND NORTHWESTERN ILLINOIS...
...Midwest...
A cold front will enter the upper Great Lakes to central Plains and
stall on Saturday as the core of an anomalous 500 mb low tracks
northeastward from south-central Canada. Precipitable water values
of 1.5 to 2.0 inches are forecast by the model consensus to pool
along the front and moderate to strong instability values are
likely to develop with daytime heating. While there may be some
lingering convective activity along the front Saturday morning
across the Upper Midwest, the main flash flood concern is expected
to develop later in the day on Saturday. Backing low level flow is
forecast to setup east of a surface low in Kansas with 30-40 kt at
850 mb forecast by the 00Z model consensus (some guidance is near
50 kt Saturday night). Flow aloft will be divergent and diffluent
to the south of an upper jet max over the upper Mississippi Valley,
enhanced by an upstream shortwave trough forecast to advance
through the central Rockies Saturday night. Increased forcing for
ascent along the front with mean steering flow parallel to the
synoptic front should favor areas of training with heavy rain
likely. While the details are a bit uncertain, the most likely
areas to see heavy rain potential (3 to 5 inches) will be near the
front from Iowa into southern Wisconsin and far northwestern
Illinois where a Slight Risk was introduced with this update.
...Central Plains...
In the wake of a cold front moving into the southern High Plains,
low level upslope into western Nebraska/Kansas and eastern Colorado
will favor increased moisture into the region and sustaining of
thunderstorms forecast to move off of the higher terrain by
Saturday afternoon. MLCAPE is forecast to be at least 1000-1500
J/kg over the High Plains and when combined with sufficient shear
aloft, some organized storms will be possible. There will be
potential for isolated flash flooding late Saturday evening into
the overnight as a low level jet develops, aiding convergence in
the vicinity of the frontal boundary, forecast to return north as a
warm front.
...Southeastern Coast into Northern and Central Florida...
A similar setup across the Southeast will set up for Saturday as
was in place for Friday with high precipitable water values over 2
inches in place from the eastern Gulf coast into coastal South
Carolina near a lingering front. Portions of the outlooked area
will have weak steering flow in place yet again and low to mid-
level winds from the east to southeast will favor scattered
thunderstorms during peak heating with potential for brief
training/backbuilding and high rates of 2-4 in/hr. At this time,
coverage of flash flood potential looks to be low enough to
warrant only a Marginal Risk.
...Southwest...
The mid to upper-level ridge that will be in place from Friday into
Saturday will begin to break down as upper level troughing
amplifies into the region from the north. Moisture values are
expected to return to near seasonal levels for early August across
Arizona while remaining slightly below average in New Mexico.
Thunderstorms are expected to form with solar insolation along the
higher terrain by 18Z, ramping up in intensity and coverage through
21Z. Given the increase in moisture compared to prior days, storms
should have a better shot of producing 1 to 2 in/hr rates,
especially across the southern half of Arizona and New Mexico where
weaker steering flow will be present as 00Z model forecasts show
850-300 mb mean wind speeds of 10 kt or less.
Otto
Day 2 threat area:
www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
508 AM EDT Fri Aug 8 2025
Day 3
Valid 12Z Sun Aug 10 2025 - 12Z Mon Aug 11 2025
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER FROM EASTERN
NEBRASKA/KANSAS INTO SOUTHERN WISCONSIN/NORTHERN ILLINOIS...
...Central Plains into Upper Midwest...
A quasi-stationary front will be in place from Sunday into Monday,
extending from the Great Lakes into the southern High Plains. The
base of an upper trough axis will be moving overhead through Monday
morning with southwesterly flow downstream along/over the front. An
enhanced low level flow regime will be in place much of the day
across the southern High Plains into the Midwest with potential
for mid-level impulses within the southwesterly flow aiding with
lift across the Plains to Midwest. The parallel nature of the
steering flow and frontal boundary will again favor the potential
for repeating and training of thunderstorms. Multiple rounds of
thunderstorms will be possible with peak flash flood potential
occurring late Sunday evening and overnight as convection moving
off of the higher terrain meets with strengthening low level flow
ahead of the upper trough. Precipitable water anomalies will
generally be +1 to +2 over the Plains with higher values toward the
Great Lakes. Questionable instability values over the Great Lakes
region will limit flash flood potential as the better consensus for
instability will be over southern locations. Model QPF values
seemed to agree with at least 3 to 5 inch potential from the global
guidance and locally higher from the 00Z RRFS and regional CMC.
...Southern Mid-Atlantic Coast to Northeastern Gulf Coast...
A lingering frontal boundary over northern Florida and high
pressure anchored over the northern Mid-Atlantic will maintain
low level southeasterly winds into the southeastern U.S. on Sunday.
Precipitable water values of 2.0 to 2.3 inches over the Southeast
and Florida will again support intense tropical downpours as
thunderstorms increase with daytime heating. Low level
southeasterly winds and confluent axes will combine with sea breeze
convergence and other mesoscale boundaries to support scattered
thunderstorms with isolated flash flood potential from 2-4 in/hr
rainfall rates.
...Southwest...
Continued advancing of the base of an upper trough from the central
Rockies into the central Plains and an elongated ridge over
California will favor northwesterly mid-level flow into the
Southwest. While this may reduce moisture availability across the
Four Corners region, low level southerly flow is expected to be in
place over the northern Gulf of California into Arizona, with
westerly flow advecting low to mid-level moisture downstream toward
New Mexico. Meanwhile, the front over the High Plains from
Saturday night is forecast to again sink down to the south with an
upslope flow regime setting up over northeastern New Mexico,
helping to increase low level moisture into the region. Moisture
values for much of central/southern Arizona and New Mexico are
expected to be near average and monsoon thunderstorms are expected
during the late afternoon and evening hours with perhaps greater
coverage than previous days. Typical potential for high short term
rain rates (1-2 inches in an hour or less) will set up with
isolated to widely scattered flash flood potential.
Otto
Day 3 threat area:
www.wpc.ncep.noaa.gov/qpf/99epoints.txt
Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
252 AM EDT Fri Aug 8 2025
A cold front over the Great Lakes (on Monday) back to the Plains
will be a focus for some locally heavy rain Monday into Tuesday as
it slowly pushes to the east and southeast into the Midwest. To the
south, a lingering stationary boundary draped along the I-10
corridor amid higher than normal moisture levels will also act as
an impetus for some heavier rain. Both of these areas, for Monday
and Tuesday, are denoted with a Marginal risk of flash flooding
(excessive rainfall). In addition, much of New Mexico and into
southeastern Arizona may see some localized modest rainfall amid
sensitive terrain. A Marginal risk exists there for Monday. For the
rest of the week, rainfall chances will increase over the eastern
1/3 of the country as the northern frontal boundary wavers over the
region.
Temperatures will warm into the Northeast to near record levels
Mon-Wed with highs into the 90s. This will push HeatRisk levels
into the Moderate (level 2) to Major (level 3 out of 4) category
after a relatively cool/comfortable start to August. Lingering heat
over the Southwest will slowly subside through the period though
temperatures will remain slightly above normal.
On the flip side, increased troughing into the Pacific Northwest
will cool temperatures rather smartly from 5-15 degrees above
normal Monday to 5-10 degrees below normal by Thursday (WA/OR). To
the south through coastal CA, temperatures will hover near to below
normal as the cooler than normal summer carries on.
Fracasso
Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
252 AM EDT Fri Aug 8 2025
A cold front over the Great Lakes (on Monday) back to the Plains
will be a focus for some locally heavy rain Monday into Tuesday as
it slowly pushes to the east and southeast into the Midwest. To the
south, a lingering stationary boundary draped along the I-10
corridor amid higher than normal moisture levels will also act as
an impetus for some heavier rain. Both of these areas, for Monday
and Tuesday, are denoted with a Marginal risk of flash flooding
(excessive rainfall). In addition, much of New Mexico and into
southeastern Arizona may see some localized modest rainfall amid
sensitive terrain. A Marginal risk exists there for Monday. For the
rest of the week, rainfall chances will increase over the eastern
1/3 of the country as the northern frontal boundary wavers over the
region.
Temperatures will warm into the Northeast to near record levels
Mon-Wed with highs into the 90s. This will push HeatRisk levels
into the Moderate (level 2) to Major (level 3 out of 4) category
after a relatively cool/comfortable start to August. Lingering heat
over the Southwest will slowly subside through the period though
temperatures will remain slightly above normal.
On the flip side, increased troughing into the Pacific Northwest
will cool temperatures rather smartly from 5-15 degrees above
normal Monday to 5-10 degrees below normal by Thursday (WA/OR). To
the south through coastal CA, temperatures will hover near to below
normal as the cooler than normal summer carries on.
Fracasso
