Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
855 PM EDT Sun May 11 2025
Day 1
Valid 01Z Mon May 12 2025 - 12Z Mon May 12 2025
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SOUTHEAST...
Isolated to scattered flash flooding will remain a concern
overnight across portions of southeast GA and southern SC. Some of
this area has already seen training convection and some flash
flooding earlier today, and additional convection is streaming
northward back into the region. The latest SPC mesoanalysis depicts
an area of instability offshore, and with easterly low level flow,
this will likely advect inland and help sustain convection across
these areas. Expect periodic backbuilding/training to persist, and
some additional flash flooding is likely. The REFS has been
favoring this corridor for heavy rainfall with both its 12z and
18z run, but the HREF and recent HRRR runs have not been as
aggressive. However the 23z HRRR run did make a pretty significant
jump towards a wetter scenario...and based on observational
trends...it does appear like something closer to the wetter
guidance may indeed pan out. Opted against a MDT risk given some
lingering uncertainty on the location and coverage of higher
amounts, along with much of the coastal plain having high FFG...but
do now consider this a higher end Slight risk and localized higher
end impacts are possible.
We also maintained a Slight risk across portions of central AL
into northern MS. Convection to the east of the deep layered low
will continue to pose an isolated to scattered flash flood risk.
Some recent indications that an uptick in activity may be starting
to occur across portions of central AL. Guidance indicates some
maintenance of this convection through the evening hours, so do
anticipate some increase in the flash flood potential.
A Marginal risk covers much of FL. Convective evolution is less
certain here...but do expect to see scattered to numerous showers
and thunderstorms. The combination of low confidence in model
solutions over FL and the high FFG keeps the risk at Marginal. It
is possible that none of the convection organizes enough or trains
long enough to cause flash flooding overnight...but can not rule
out some training segments hanging up and resulting in isolated
flash flood concerns.
Chenard
Day 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
855 PM EDT Sun May 11 2025
Day 1
Valid 01Z Mon May 12 2025 - 12Z Mon May 12 2025
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SOUTHEAST...
Isolated to scattered flash flooding will remain a concern
overnight across portions of southeast GA and southern SC. Some of
this area has already seen training convection and some flash
flooding earlier today, and additional convection is streaming
northward back into the region. The latest SPC mesoanalysis depicts
an area of instability offshore, and with easterly low level flow,
this will likely advect inland and help sustain convection across
these areas. Expect periodic backbuilding/training to persist, and
some additional flash flooding is likely. The REFS has been
favoring this corridor for heavy rainfall with both its 12z and
18z run, but the HREF and recent HRRR runs have not been as
aggressive. However the 23z HRRR run did make a pretty significant
jump towards a wetter scenario...and based on observational
trends...it does appear like something closer to the wetter
guidance may indeed pan out. Opted against a MDT risk given some
lingering uncertainty on the location and coverage of higher
amounts, along with much of the coastal plain having high FFG...but
do now consider this a higher end Slight risk and localized higher
end impacts are possible.
We also maintained a Slight risk across portions of central AL
into northern MS. Convection to the east of the deep layered low
will continue to pose an isolated to scattered flash flood risk.
Some recent indications that an uptick in activity may be starting
to occur across portions of central AL. Guidance indicates some
maintenance of this convection through the evening hours, so do
anticipate some increase in the flash flood potential.
A Marginal risk covers much of FL. Convective evolution is less
certain here...but do expect to see scattered to numerous showers
and thunderstorms. The combination of low confidence in model
solutions over FL and the high FFG keeps the risk at Marginal. It
is possible that none of the convection organizes enough or trains
long enough to cause flash flooding overnight...but can not rule
out some training segments hanging up and resulting in isolated
flash flood concerns.
Chenard
Day 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
855 PM EDT Sun May 11 2025
Day 2
Valid 12Z Mon May 12 2025 - 12Z Tue May 13 2025
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SOUTHEAST AND SOUTHERN MID-ATLANTIC...
20z Update: Continue to be very close to needing a MDT risk
upgrade across the upslope areas of the southern Appalachians, but
we will hold at a higher end Slight risk for now. Orographically
enhanced rainfall and embedded convective elements periodically
pushing rainfall rates over 1"/hr will likely drive at least a
minor flash flood risk across this portion of the western
Carolinas and southwest VA. However confidence on seeing more
significant higher end impacts remains lower. With easterly low
level flow in place over this corridor the better instability may
tend to stay farther south over GA/SC and over the eastern
Carolinas. Probabilities of exceeding 1"/hr in the HREF peak
around 40-50% in the 12z HREF and REFS, likely indicative of some
exceedance, but probably not widespread. It will likely take these
higher rates of 1-2"/hr or greater to drive more significant
impacts.
The currently forecast 3-7" of rainfall, combined with
embedded rates of 0.5"-1"/hr should be enough to drive at least
scattered instances of minor flash flooding, with localized more
significant impacts still possible. For now the expectation of
scattered minor flash flood and localized higher end impacts falls
more into the high end Slight risk category. If confidence in
greater coverage of higher rainfall rates increases, that would in
turn increase the potential for higher end impacts and justify a
MDT risk upgrade. Will thus continue to closely monitor.
The southeastern FL urban corridor is another area of concern for
flash flooding Monday. Neighborhood probability of exceeding 8"
are over 50% in both the 12z HREF and REFS and some deterministic
members are over 10". Overall seems like a favorable synoptic
setup for training convection and thus higher end urban impacts
are possible. Not confident enough yet to go with a MDT risk
upgrade, however do consider this a higher end Slight risk and the
potential is certainly there for significant urban impacts. We
will continue to monitor observational and model trends.
Chenard
...Previous Discussion...
The nearly stationary upper low that impacted the Southeast over
the weekend will finally gain some momentum and shift north towards
the Tennessee Valley on Monday, pulling the surface low pressure
system along with it. The trailing cold front will gradually push
towards the Southeast Atlantic Coast while the warm front lifts
into the southern Mid-Atlantic, and another round of widespread
showers and storms are forecast Monday into Monday night in the
warm sector of the system. Deep moisture will surge north ahead of
the cold front, extending from Florida through the Carolinas to
southern and central Virginia. PWATs are anticipated to reach
1.5-2+ inches in these areas with even higher values possible over
the Florida Peninsula. Strong south-southeasterly mean flow will
remain in place, which will likely result in two convergent zones,
one along the Southeast Coast and one along upslope regions of the
southern Appalachians where high QPF totals will be possible.
Strong diurnal convection over the Florida Peninsula will also
contribute to high QPF values over portions of Central and South
Florida.
The largest flooding concerns will come with repeat/training
activity, which could result in high QPF totals over a relatively
short period of time, potentially reaching or exceeding the 1 and 3
hour FFGs in these regions. To account for this flash flood risk,
there is a Slight Risk area stretching from Florida up the
Southeast Coast to North Carolina and southern Virginia and also
for the upslope regions of the Appalachians from Georgia to
Virginia. A Marginal Risk area extends even further into the Mid-
Atlantic where moderate rain rates may contribute to localized
flash flooding and into the Tennessee and Mid-Mississippi Valleys
where enough moisture will likely wrap around the surface low to
create locally heavy rainfall in showers and storms.
Upslope regions of the southern Appalachians are particularly
sensitive to heavy rain, especially in areas that were impacted by
Hurricane Helene. Impacts from heavy rain on Monday will likely
fall into the high-end Slight Risk category, and ERO upgrades may
be needed in the future if high QPF trends hold,. However,
instability may be a limiting factor as the better instability is
expected to reside along the coast. After coordinating with local
forecast offices, it was decided to hold off on any upgrades at
this time. By this afternoon, the CAMs will go out far enough to
cover the entire Day 2 period and will likely give us a better idea
of the magnitude of potential flooding impacts.
Dolan
Day 2 threat area:
www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
403 AM EDT Mon May 12 2025
Day 3
Valid 12Z Wed May 14 2025 - 12Z Thu May 15 2025
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE NORTHERN PLAINS AND MID-ATLANTIC...
...Northern Plains...
A deep upper level trough will push across the Northwest to the
Plains on Wednesday, resulting in a strengthening low pressure
system in the northern Plains. Stratiform rain with likely some
embedded convection on the northern and western side of the low are
forecast to produce 0.5-1 inches of rain, with locally higher
amounts possible, in eastern Montana and northeastern Wyoming. The
main threat will be due to the persistent nature of the
precipitation in this area, starting up Tuesday night and
continuing through Wednesday into Wednesday night. Rainfall totals
will likely come close to the 6 hour FFGs in eastern Montana.
For the Dakotas, convective precipitation is forecast to develop
ahead of the low along a strengthening frontal boundary. Models are
in good agreement on the axis of heaviest precipitation setting up
over central North Dakota and eastern South Dakota. PWAT values
are expected to be at least 2 standard deviations above normal
(1-1.5 inches), and there should be enough instability (MUCAPE >
1500 J/kg) during the day on Wednesday to support at least an
isolated flash flood threat.
...Mid-Atlantic...
Soils are expected to be thoroughly saturated across the Mid-
Atlantic after heavy rain on Monday and Tuesday, which may create a
low end flood threat with lingering showers on Wednesday. The
slow-moving surface low is forecast to track right over the Mid-
Atlantic with scattered precipitation, and there should be enough
instability near the low to support some convection capable of
producing locally heavy rain. Therefore, a Marginal Risk area was
introduced for portions of the Mid-Atlantic.
Dolan
Day 3 threat area:
www.wpc.ncep.noaa.gov/qpf/99epoints.txt
Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
258 AM EDT Mon May 12 2025
The region with heaviest rainfall going into the second half of the
work week will be across North Dakota and into Minnesota. Although
rainfall rates are not expected to be all that high, multiple
hours of moderate rainfall with rates of half an inch per hour in
some cases may just be enough to cause some minor flooding issues
in vulnerable areas, and therefore the Marginal Risk area has been
maintained for Thursday (Day 4) from eastern North Dakota into
central/northern Minnesota. Going into Friday (Day 5), showers and
thunderstorms increase in coverage and intensity across the Mid-
South in the general vicinity of a quasi-stationary front that will
be intercepting advection of deep moisture from the south. A
Marginal Risk area is now valid from Arkansas to eastern Kentucky
where the model QPF is strongest for organized convection.
Elsewhere across the nation, the Storm Prediction Center is
monitoring the potential for some severe storms on Thursday across
portions of the Midwest. Out West, it will seem more like March
across
the northern Rockies with heavy snow for the highest ranges of
southwestern Montana and northern Wyoming, with the potential for
6-12 inches of accumulation in some cases through mid-week, and a
second round of snow for the central and northern Rockies Sunday
into Monday as the next upper trough and shortwave moves into the
region. Lighter snows are likely elsewhere across the higher
terrain of the northern Rockies and Intermountain West.
Very warm weather will be commonplace across the eastern half of
the country to close out the work week, with highs well into the
80s and even lower 90s from the Ohio Valley and Mid-Atlantic
southward to the Gulf Coast. Very hot conditions are expected
across southern Texas for the entire forecast period with highs
exceeding 100 degrees near the Rio Grande, and HeatRisk reaching
the major to extreme category for some of these areas. In
contrast, chilly conditions are likely for the Intermountain West
and Northern Rockies with highs running 5-15 degrees below average
with the upper level trough and increased cloud cover.
Hamrick
Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
258 AM EDT Mon May 12 2025
The region with heaviest rainfall going into the second half of the
work week will be across North Dakota and into Minnesota. Although
rainfall rates are not expected to be all that high, multiple
hours of moderate rainfall with rates of half an inch per hour in
some cases may just be enough to cause some minor flooding issues
in vulnerable areas, and therefore the Marginal Risk area has been
maintained for Thursday (Day 4) from eastern North Dakota into
central/northern Minnesota. Going into Friday (Day 5), showers and
thunderstorms increase in coverage and intensity across the Mid-
South in the general vicinity of a quasi-stationary front that will
be intercepting advection of deep moisture from the south. A
Marginal Risk area is now valid from Arkansas to eastern Kentucky
where the model QPF is strongest for organized convection.
Elsewhere across the nation, the Storm Prediction Center is
monitoring the potential for some severe storms on Thursday across
portions of the Midwest. Out West, it will seem more like March
across
the northern Rockies with heavy snow for the highest ranges of
southwestern Montana and northern Wyoming, with the potential for
6-12 inches of accumulation in some cases through mid-week, and a
second round of snow for the central and northern Rockies Sunday
into Monday as the next upper trough and shortwave moves into the
region. Lighter snows are likely elsewhere across the higher
terrain of the northern Rockies and Intermountain West.
Very warm weather will be commonplace across the eastern half of
the country to close out the work week, with highs well into the
80s and even lower 90s from the Ohio Valley and Mid-Atlantic
southward to the Gulf Coast. Very hot conditions are expected
across southern Texas for the entire forecast period with highs
exceeding 100 degrees near the Rio Grande, and HeatRisk reaching
the major to extreme category for some of these areas. In
contrast, chilly conditions are likely for the Intermountain West
and Northern Rockies with highs running 5-15 degrees below average
with the upper level trough and increased cloud cover.
Hamrick
