Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
822 PM EDT Wed May 21 2025
Day 1
Valid 01Z Thu May 22 2025 - 12Z Thu May 22 2025
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE CENTRAL APPALACHIANS/EASTERN OHIO VALLEY AND FAR SOUTH TEXAS...
01Z Update...
Ohio Valley through the Mid-Atlantic States...
Removed the Slight Risk area entirely and trimmed the western
portion of the remaining Marginal Risk areas of western
Pennsylvania behind the wave/impulse that led to locally heavy/excessive
rainfall earlier in the day. Did not remove all of the area in the
wake of the wave due to additional showers and thunderstorms
upstream that was poised to move across areas rained upon earlier
in the day. The presumption is that there could still be a
(presumably low) risk for excessive rainfall this evening despite
meager rainfall rates.
...South Texas...
Maintained the Marginal Risk area in South Texas as convection was
growing in coverage and intensity over Mexico as it prooagated
eastward. The FV3 and more recent runs of the RRFS both paint
scenarios where this covection results in some heavy...if not
excessive...amounts. The northern boundary of the Marginal Risk
area was along the gradient of low level equivalent potential
temperature shown by the high resolution CAMs and supported by a
quick look at the 22/00Z soundings.
Bann
...Ohio Valley through the Mid-Atlantic States...
A series of shortwaves will interact and consolidate into a large
upper low centered over the Great Lakes by the end of D1 /12Z
Thursday/. During this evolution, impulses/shortwaves rotating
around the amplifying low will spin northeast from the Ohio Valley
into the Mid-Atlantic and Northeast, providing periods of enhanced
ascent. This lift will encounter favorable thermodynamics
characterized by PWs as high as 1.25 inches, overlapped by a plume
of modest MUCAPE reaching above 500 J/kg, especially across western
PA. Low-level flow at 850mb will vary in direction, but there is
likely to be an area of enhanced convergence across the Central
Appalachians as far north as northern PA where westerly flow south
of the primary gyre meets locally backed SE flow from the Atlantic
spreading inland from the NJ/DE coasts. This will result in
enhanced convergence and moisture confluence, combining with at
least subtle upslope flow to enhance lift across the region. Where
temporally this combines with the greatest instability, which will
likely peak within a narrow warm sector downstream of a surface low
moving near Lake Erie, showers and thunderstorms with heavy rain
are expected.
Although activity is ongoing this morning, with clouds and some
weak subsidence behind the primary surge of moisture will delay the
convection this aftn at least a few hours, but the high res CAMs
support a resurgence of convection with rain rates of 1-2"/hr at
times this aftn. This activity is progged to congeal into a cluster
or MCS moving across OH, PA, and WV this evening as convection
becomes organized through 0-6km bulk shear of 30-40 kts, and lapse
rates steepen beneath the developing upper low. Although this
cluster, or clusters, will move rapidly on mean cloud-layer winds
of around 25 kts, some short duration training is possible along
the line to enhance the temporal length of these rain rates.
Additionally, showers and thunderstorms blossoming behind the
primary line and beneath the steepening lapse rates may produce
repeated rounds of heavy rainfall across the same areas into
tonight. FFG across this region is only 1-2"/3 hrs, which has a
10-20% chance of exceedance according to the REFS and HREF,
supporting the inherited SLGT risk which was modified only
cosmetically for recent guidance.
Farther east towards the coasts of DE/NJ and the eastern shore of
MD, the 24-hr ensemble probabilities, including rain occurring
this morning, feature a low end risk (10%) for 3" of rainfall in a
few rounds through tonight. However, instability is minimal which
will generally limit the potential for any heavy rainfall rates.
For this reason, the recently trimmed MRGL risk for these areas
remains left out despite some areas likely picking up 2-3" of
rainfall today.
...South Texas and Gulf Coast...
The tail end of a cold front draped along the Gulf Coast will serve
as the impetus for convection today, some of which has already
developed across Louisiana. Plentiful ascent along this front
through convergence combined with impressive PWs and high
instability will support rainfall rates in excess of 2"/hr at
times, which has already been occurring within an FFW over SW LA.
Storm motions are progged to remain 10-15 kts along the front to
the east, but will be aligned to the Corfidi vectors, and with
upstream development likely, this will result in at least short-
term training for parts of the region. Despite dry antecedent
conditions and high FFG, training of these rates could support
isolated flash flood instances today. After coordination with the
metwatch forecasters and the Gulf Coast WFOs, a MRGL risk has been
expanded from the Lower Texas Coast through eastern Louisiana.
Farther SW, the setup is favorable for convection to blossom
rapidly across the Sierra Madre of Coahuila, Mexico as a modest
shortwave lifts eastward and interacts with extreme thermodynamics
this afternoon and evening. PWs as high as 2+ inches and MUCAPE
above 3000 J/kg will drift westward as modest 850mb inflow from
the Gulf pushes onshore the lower Texas Coast and across the Rio
Grande Valley. This will effectively upslope into the Sierra Madre,
providing additional impetus for convective development in the
terrain. With 0-6km bulk shear progged at 30-45 kts, this will
quickly organize into clusters or an MCS which will then follow the
moisture eastward into South Texas. Mean 0-6km winds of just 5-10
kts combined with aligned Corfidi vectors of 15-20 kts suggests
some training is likely as this MCS pivots east, and with rain
rates likely exceeding 2"/hr, this will result in areas of QPF of
more than 3", with both the HREF and REFS probabilities indicating
a low-end potential (10-15%) for 5+ inches along the Rio Grande
Valley. This area has been dry recently (7-day rainfall less than
5% of normal according to AHPS) which will limit the flash flood
risk, but these intense rain rates could still locally result in
instances of flash flooding.
Weiss
Day 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
822 PM EDT Wed May 21 2025
Day 2
Valid 12Z Thu May 22 2025 - 12Z Fri May 23 2025
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE NORTHEAST, AS WELL AS SOUTHEAST FLORIDA...
...Northeast...
Closed mid-level low centered over the Great Lakes will elongated
and become aligned west to east in response to tugging via multiple
spokes of energy /shortwaves/ rotating around the primary gyre. One
of these shortwaves will intensify as it lifts from the Mid-
Atlantic coast early Thursday, featuring a negative tilt as it
moves near Cape Cod Thursday evening/night. In response to this
evolution, a surface low will develop and deepen into a Nor'easter
moving just east of the New England coast, with surface pressure
drops aided by diffluence in the LFQ of an upper jet streak
pivoting through the trough axis. The deepening low pressure will
result in intensifying 850mb inflow onshore, reflected by NAEFS
850mb U-winds reaching below the minimum recorded in the CFSR
database Thursday aftn. Although PWs will be modest, only
exceeding 1" along the Cape and the Islands, this intense onshore
flow will result in moisture flux that may exceed 3 standard
deviations into Long Island and eastern New England.
This impressive moisture advection will efficiently converge into
the region, offsetting what will be modest instability (MUCAPE 500
J/kg or less). This will still support rainfall rates that could
reach 1"/hr (10-20% probabilities on the HREF/REFS). Additionally,
Corfidi vectors are progged to become increasingly aligned against
the mean 0-6km winds, suggesting training/backbuilding of even the
more modest rainfall rates as cells continue to rotate onshore.
This suggests that 3" of rain on D2 is likely (HREF and REFS
probabilities > 70%), with locally as much as 5" possible (10%
chance). This could result in isolated instances of flash flooding
on Thursday, primarily across urban areas along I-95 between New
London, CT and Portsmouth, NH. For this reason, a MRGL risk has
been added and coordinated with the local WFOs.
...Southeast Florida...
The tail of a cold front will sink into the southern Florida
peninsula Thursday evening, driven southward by an expanding trough
across the east, additionally pushing a jet streak southeast into
the vicinity. The overlap of strong convergence along the front and
RRQ diffluence in the pivoting jet streak will produce plentiful
ascent into favorable thermodynamics resulting from PWs of more
than 1.75 inches and MUCAPE above 2000 J/kg. The CAMs are in
generally good agreement with thunderstorms blossoming during the
aftn and then shifting slowly east across the Peninsula through the
evening as 0-6km mean winds remain steady at around 10 kts to the
east. Strong ascent into these thermodynamics will support rainfall
rates in excess of 2"/hr (HREF probabilities above 40%), and some
slowly of convection is likely where cells merge with the sea
breeze, especially along the Gold Coast where rainfall
probabilities are most significant. With slow moving storms moving
atop the urban Gold Coast producing locally 3-5 inches of rain, a
MRGL risk for excessive rainfall has been introduced.
Weiss
Day 2 threat area:
www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
822 PM EDT Wed May 21 2025
Day 3
Valid 12Z Fri May 23 2025 - 12Z Sat May 24 2025
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM EASTERN
KANSAS AND OKLAHOMA THROUGH THE OZARKS...
Broad closed low over the Northeast and an expanding mid-level
ridge blossoming across the Southern Plains will sandwich the
Central Plains and Mid-Mississippi Valley between them. This will
result in pronounced W/NW flow across the region on D3, within
which embedded shortwaves will track quickly, leading to periods of
enhanced ascent. At the same time, a jet streak digging around the
trough to the northeast will at least peripherally place its RRQ
across the region, especially late Friday/Friday night, leading to
enhanced deep layer lift over the area.
This deep layer ascent will impinge upon improving thermodynamics
as return flow from the Gulf surges PWs to above 1.5" Friday night
as the 850mb LLJ reaches as high as 40 kts. This will additionally
transport elevated instability northward, and the overlap of this
CAPE with the moisture and lift will lead to increasing
thunderstorm coverage, especially Friday night. With Corfidi
vectors aligned perpendicular to the mean 850mb flow, and 0-6km
bulk shear forecast to exceed 45 kts, convection should rapidly
grow upscale into an MCS with training of echoes from WNW to ESE
through the night. This will likely result in axes of at least 2-3"
of rain, with locally higher amounts possible.
At this time, there remains enough spread, both temporally and
spatially, to preclude any upgrades to a SLGT risk. However, the
MRGL risk was pulled a bit SW from inherited to best match the
latest guidance, and to overlap regions of higher instability where
convection may develop first the latter half of D3. It is possible
a SLGT risk will be needed eventually, as this setup (continuing
into D4) appears favorable for flash flooding.
Weiss
Day 3 threat area:
www.wpc.ncep.noaa.gov/qpf/99epoints.txt
Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
231 PM EDT Wed May 21 2025
A heavy rainfall pattern will be in place by Saturday as ample
Gulf moisture and instability stream north due to return flow from
surface high pressure over the Southeast and in response to a
shortwave/right entrance region of the upper jet coming into the
north- central Rockies and High Plains. Rain and storms are likely
to overlap with Friday's (Day 3 ERO period) convection where there
will be wetter antecedent conditions. A Slight Risk remains
centered over the Ozarks (which can be sensitive to flash flooding
anyway) in the Day 4/Saturday ERO. Convection ongoing from Saturday
night could continue east into Sunday across the Mid-Mississippi
Valley, with uncertainty in if/where it could persist into the
Tennessee or Ohio Valleys. Then additional convection is forecast
to form farther southwest in the southern Plains/Lower Mississippi
Valley as well on Sunday. A fairly large Slight Risk from
northeastern Texas across parts of the Lower and Middle Mississippi
Valley continues for the Day 5/Sunday ERO to cover both areas.
Into the workweek, the front will press south farther into the
southern tier and east into the Appalachians and eventually the
Eastern Seaboard. Areas of heavy rainfall are possible with details
to be refined with time.
Precipitation, mostly rain except for snow in the highest peaks,
is possible across the north-central Rockies and Plains into
Saturday underneath upper troughing and as frontal systems move
through. The details are uncertain based on the model spread with
the supporting energies. But the shortwave providing dynamical
support, above normal PWs, and the cusp of the instability gradient
could provide some heavy rain and a low end flash flood risk in
the north-central High Plains. A Marginal Risk remains in place
there for the Day 4/Saturday ERO.
Elsewhere, light showers could continue across the Northeast but
gradually lessen in magnitude through the weekend as the upper and
surface lows are slow to pull away there. Summertime scattered
storms are likely across the Florida Peninsula through the period.
The Northwest may see renewed precipitation chances early next
week.
Warmer than normal temperatures will be in place across the
southern Plains on Saturday underneath a central U.S. ridge.
Temperatures well into the 90s to around 100 will support areas of
Major HeatRisk in parts of Texas and Louisiana this weekend. Then
as the mean upper ridge rebuilds farther west, the Great Basin and
northern/central Rockies can expect temperatures generally around
10 to 15 degrees above average. Meanwhile, cooler than normal
temperatures by 5 to 15 degrees will take hold over the north-
central to eastern U.S. into the weekend underneath upper
troughing. The trough will allow for temperatures to cool in the
southern tier during the workweek compared to the hot weekend, but
overall temperatures should gradually trend closer to normal as
next week progresses.
Santorelli/Tate
Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
231 PM EDT Wed May 21 2025
A heavy rainfall pattern will be in place by Saturday as ample
Gulf moisture and instability stream north due to return flow from
surface high pressure over the Southeast and in response to a
shortwave/right entrance region of the upper jet coming into the
north- central Rockies and High Plains. Rain and storms are likely
to overlap with Friday's (Day 3 ERO period) convection where there
will be wetter antecedent conditions. A Slight Risk remains
centered over the Ozarks (which can be sensitive to flash flooding
anyway) in the Day 4/Saturday ERO. Convection ongoing from Saturday
night could continue east into Sunday across the Mid-Mississippi
Valley, with uncertainty in if/where it could persist into the
Tennessee or Ohio Valleys. Then additional convection is forecast
to form farther southwest in the southern Plains/Lower Mississippi
Valley as well on Sunday. A fairly large Slight Risk from
northeastern Texas across parts of the Lower and Middle Mississippi
Valley continues for the Day 5/Sunday ERO to cover both areas.
Into the workweek, the front will press south farther into the
southern tier and east into the Appalachians and eventually the
Eastern Seaboard. Areas of heavy rainfall are possible with details
to be refined with time.
Precipitation, mostly rain except for snow in the highest peaks,
is possible across the north-central Rockies and Plains into
Saturday underneath upper troughing and as frontal systems move
through. The details are uncertain based on the model spread with
the supporting energies. But the shortwave providing dynamical
support, above normal PWs, and the cusp of the instability gradient
could provide some heavy rain and a low end flash flood risk in
the north-central High Plains. A Marginal Risk remains in place
there for the Day 4/Saturday ERO.
Elsewhere, light showers could continue across the Northeast but
gradually lessen in magnitude through the weekend as the upper and
surface lows are slow to pull away there. Summertime scattered
storms are likely across the Florida Peninsula through the period.
The Northwest may see renewed precipitation chances early next
week.
Warmer than normal temperatures will be in place across the
southern Plains on Saturday underneath a central U.S. ridge.
Temperatures well into the 90s to around 100 will support areas of
Major HeatRisk in parts of Texas and Louisiana this weekend. Then
as the mean upper ridge rebuilds farther west, the Great Basin and
northern/central Rockies can expect temperatures generally around
10 to 15 degrees above average. Meanwhile, cooler than normal
temperatures by 5 to 15 degrees will take hold over the north-
central to eastern U.S. into the weekend underneath upper
troughing. The trough will allow for temperatures to cool in the
southern tier during the workweek compared to the hot weekend, but
overall temperatures should gradually trend closer to normal as
next week progresses.
Santorelli/Tate
