Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
847 PM EDT Thu Jun 19 2025
Day 1
Valid 01Z Fri Jun 20 2025 - 12Z Fri Jun 20 2025
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE CENTRAL AND EASTERN UNITED STATES...
01Z Excessive Rainfall Discussion...
Maintained several of the Marginal Risk areas in the Upper
Midwest...southern Plains and the Northeast US where there was
stronger dynamics and greater instability. Elsewhere...enough
stabilization has occurred following the passage of a cold front in
the eastern part of the country combined with the loss of daytime
heating to remove territory from the Marginal risk. There was a
slight broadening of the Marginal risk area in the Upper Midwest
where the latest runs of the high resolution guidance was moving
convection more southward towards the better instability. Shifted
the Marginal westward a bit given radar trends this evening for
ongoing convection to build westward into Oklahoma from near the
Oklahoma/Arkansas state line. Finally...maintained the Marginal
Risk area in parts of northern New England as a convective line was
approaching from the west with the potential for local rainfall
rates approaching 2 inches per hour. Elsewhere...the risk of
excessive rainfall was diminishing fairly quickly but felt it too
early to remove the risk areas at this point.
Bann
Previous Excessive Rainfall Discussion...
...Central Appalachians, Mid-Atlantic, and the Northeast...
A potent shortwave will amplify as it tracks northeast from the
Great Lakes to along the northern New England/Canadian border,
potentially becoming negatively tilted by tonight. This will help
to deepen a surface low moving in tandem with its parent trough,
which will in turn drive the accompanying cold front rapidly to the
east. By 12Z Friday, this front should be offshore New England and
the Mid- Atlantic, but before that occurs scattered thunderstorms
with heavy rain are likely along it.
The downstream environment from this front is extremely favorable
for thunderstorms containing heavy rain. PWs will surge northward on
SW 850mb flow of 30 kts, reaching 1.75 to 2 inches, or higher, from
the Mid-Atlantic into New England, which will be above daily records
according to the SPC sounding climatology. This intense PW plume
will combine with a narrow corridor of MUCAPE progged to exceed 2000
J/kg from NC through ME, to support heavy rainfall within
convection. Although the CAMs are generally modest with their
coverage today, expect scattered to numerous thunderstorms to
develop during peak heating, with modest bulk shear of 25-30 kts
supporting at least isolated clusters in an otherwise pulse-type
environment. Due to the impressive PWs and tall skinny CAPE
profiles, max hourly rainfall could reach 2-3" according to the REFS
and HREF guidance, which would be sufficient to cause runoff and
isolated flash flooding.
Although the flash flood risk today is generally isolated due to
fast storm motions and minimal training, there is a slightly higher
risk for impacts across the Central Appalachians, generally in WV
and PA, where antecedent conditions (7-day rainfall more than 400%
of normal according to AHPS) have resulted in severely compromised
FFG as low as 1"/3hrs. Despite progressive cell motion, HREF
exceedance probabilities rise to 15-25% in this area, and it is
possible that even fast-moving heavy rain producers could cause
impacts, so the SLGT risk has been maintained. Additionally, after
coordination with WFO LWX, a targeted SLGT risk has been added for
generally the I-95 urban corridor of northern VA and MD where the
high-res CAMs suggest some local focusing of thunderstorms this
afternoon with heavy rain which could produce as much as 3" of
rainfall atop sensitive areas which received heavy rain on
Wednesday.
Farther north into New England, mean 0-6km winds remain quick at 20-
30 kts, but are more aligned parallel to the Corfidi vectors and the
front, suggesting an enhanced training risk. However, instability
and PWs are more modest, so rain intensity will be decreased as
forecast by lower 1"/hr rain rate probabilities from the HREF.
Although total rainfall could reach 3" in some areas from northern
VT through northern ME, the excessive rain risk remains in the MRGL
category.
...Southeast...
The same front that will race across the Mid-Atlantic and
Northeast, will trail back into the Southeast today. Although this
front is likely to decay by this evening, the residual convergence
zone will linger as it becomes embedded within more zonal flow
south of the primary trough. Convection ongoing early this morning
from AR to AL will weaken, but the accompanying outflow may merge
with the weakening front to provide additional impetus for
convective development thanks to robust thermodynamics
characterized by PWs of around 1.75 inches and MUCAPE of 2000-3000
J/kg. This will create scattered thunderstorms this aftn/eve with
rainfall rates of more than 2"/hr. Although there is some
uncertainty into how the weakening front will evolve (some CAMs
suggest the outflow will surge all the way to the Gulf Coast), any
storms that do form have the potential to train as Corfidi vectors
collapse to around 5 kts so storms could move west to east and
repeat through the evening. Both the HREF and REFS guidance
indicate a 30-40% chance of 3" of rain, which could produce
isolated flash flooding today.
...Florida Peninsula...
Bermuda-type ridge off the Southeast Coast will produce weak
easterly flow over the FL Peninsula today. As afternoon heating
commences and instability climbs above 2000 J/kg, scattered
afternoon thunderstorms will develop. Initially, these will occur
on the sea breeze, but as the Atlantic coast sea breeze pushes
westward and a weak jet streak develops downstream of a longwave
trough dropping across the Southeast, additional development is
likely along this boundary and due to storm mergers/boundary
interactions. This will likely be most prevalent near the SW coast
of FL where the Gulf Coast sea breeze will be pinned, fueling
multiple rounds of slow moving thunderstorms. With PWs progged
around 1.75", rainfall rates of 2-3"/hr are expected, producing
hourly rainfall as much as 3" according to both the RRFS and HRRR
models, and total rainfall over 5" possible (20-40%) in localized
areas. Should this rain occur atop more urban areas, especially
along the Sun Coast, instances of flash flooding could result.
...Upper Midwest...
A cold front dropping out of Canada today will slow and stall
across Minnesota this evening in response to flattening 500mb flow
aloft as a ridge build atop the Southern Plains. Return flow
circling this expanding ridge will amplify tonight, with 850mb
winds potentially reaching above 50 kts on S/SW flow emerging from
the Central Plains and surging into the front. This LLJ will
resupply favorable thermodynamics into the region as reflected by
an overlap of PWs above 1.25 inches and MUCAPE exceeding 2000 J/kg
to support thunderstorm development. Storms that develop along this
boundary may train from WNW to ESE as mean winds parallel the
front, and with rain rate probabilities reaching 40-60% for 1"/hr,
2-3" of rain is possible as noted by the HREF PMM and 3"/24hr
probabilities reaching 20-30. Despite relatively high FFG, training
of these rain rates where they produce the locally higher stripes
of rain could cause instances of flash flooding tonight.
Weiss
Day 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
847 PM EDT Thu Jun 19 2025
Day 2
Valid 12Z Fri Jun 20 2025 - 12Z Sat Jun 21 2025
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PARTS OF THE
UPPER MIDWEST...
...Upper Midwest...
An amplifying mid-level trough deepening into an impressive closed
low (500mb heights below the 1st percentile according to NAEFS)
will dig across the Pacific Northwest Friday into Saturday.
Downstream of this feature, resultant strong deep layer lift will
spread through divergence and height falls into the Northern
Rockies and Northern Plains, with vorticity lobes shedding from the
primary gyre to enhance downstream ascent. At the surface, a
wavering front will remain draped from Montana through Minnesota,
and the interaction of this baroclinic zone with the mid-level
ascent will cause surface cyclogenesis in the High Plains, and this
low will race northeastward, reaching the western Great Lakes by
the end of D2.
Downstream of the primary trough axis, low-level 850mb flow will
accelerate to as much as 50 kts in response to a tightening
gradient, and push a plume of impressive PWs as high as 1.75 inches
(above the NAEFS 97th percentile) into the Upper Midwest. This
plume will then be pulled northward and westward by backing 850mb
winds, accompanied by an instability surge (MUCAPE) of more than
3000 J/kg. This setup will support increasing convection Saturday
evening/night along the elevated baroclinic zone where moisture
confluence and isentropic lift overall, leading to widespread
shower and thunderstorm development. Storms that blossom along this
boundary will likely train west to east along the front, and
organize into clusters or an MCS in response to intense 0-6km bulk
shear of 50+ kts. In the extreme thermodynamics, rainfall rates
have a high chance (>70%) of exceeding 1"/hr, with locally more
than 2"/hr rates possible (20% chance). Where training occurs, this
may result in corridors of 3" of rainfall or more, with both the
HREF and REFS focusing the greatest risk from eastern ND through
the western U.P of MI. Despite elevated FFG in this area, these
intense rates could overwhelm soils to cause instances of flash
flooding, but the anticipated filtration capacity of the soils due
to recent dryness limit the risk category to just a MRGL at this
time.
However, there appears to be a locally higher threat around Lake
Superior and into the Arrowhead of Minnesota. Here, recent rainfall
earlier this week was significant, which has resulted in 7-day AHPS
rainfall that is more than 300% of normal. Despite that, FFG has
remained elevated and exceedance probabilities from the HREF are
just around 15%, but the REFS is much higher (30-40%). While the
REFS has been a bit overdone with recent events, the uptick in QPF
noted in model trends atop sensitive areas is concerning for a
slightly higher risk for flash flooding despite generally fast
storm motions. After coordination with WFO DLH, a targeted SLGT
risk was added to enhance the messaging for potential impacts.
...Northern Rockies...
The same anomalous upper low closing over the Pacific Northwest
will help deepen a surface low over WY through Saturday. Intense
downstream southerly flow over the Plains will surge elevated PWs
and instability northward, with backed flow invof the low pulling
these thermodynamics NW into the Northern Rockies. This will result
in the accompanying theta-e axis lifting NW into the region as a
modest TROWAL, helping to enhance elevated instability which would
otherwise be confined to the east across the High Plains. The
easterly flow around this low pressure will favorably upslope into
the terrain as well, providing additional ascent, which in the
presence of a ribbon of PWs above 1 inch will result in expanding
precipitation with rainfall rates likely eclipsing 0.5"/hr at
times. Slow moving and repeating showers and thunderstorms,
embedded within a broader stratiform rain shield, are expected,
especially immediately adjacent to the higher terrain. This will
produce an axis of 1-3" of rainfall which may result in runoff and
isolated instances of flash flooding.
Weiss
Day 2 threat area:
www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
847 PM EDT Thu Jun 19 2025
Day 3
Valid 12Z Sat Jun 21 2025 - 12Z Sun Jun 22 2025
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
NORTHERN GREAT LAKES, INTERIOR NORTHEAST, AND NORTHERN ROCKIES...
...Northern Great Lakes through Northern New England...
A stationary front draped from ND through Michigan will transition
to a warm front and lift northeast on Saturday. This evolution will
be driven by an expanding mid-level ridge centered over the TN VLY,
which will result in rising heights and developing SW flow from the
Plains into the Great Lakes. This synoptic change will push the
boundary northeast as a warm front, along which a shortwave
ejecting from the amplifying closed low in the west will track east
from Minnesota to Vermont in the pinched and progressive flow. This
overlap of ascent will drive increasing convection which will
likely organize into clusters due to 30-40 kts of bulk shear, and
then track progressively around the amplifying ridge. Storm motions
will be quick to the east, but favorable thermodynamics to support
heavy rain rates of 1-2"/hr could result in isolated instances of
flash flooding, primarily where any repeating convection occurs.
...Northern Rockies...
The closed and anomalously strong 500mb low across the interior
Pacific Northwest will close off near southern Alberta, at least
briefly, before opening and beginning to pivot the entire trough
eastward by Sunday morning. The surface low pressure over WY on
Saturday will respond by lifting northeast beneath the core of this
mid-level low, resulting in an axis of accelerated low-level flow
to draw moisture and instability northwest into the Northern
Rockies. The impressive deep layer ascent through height falls
combined with upslope ascent on easterly flow will result in
widespread moderate to heavy rain, with embedded convection
producing locally 0.5"/hr rainfall rates, especially beneath the
elevated instability/TROWAL pivoting southward. With
repeating/training cells with moderate to heavy rain moving across
areas just east of the higher terrain expected, this could result
in axes of as much as 3" of rain as forecast by 10-20%
probabilities from the ECENS and GEFS. The inherited MRGL risk
remains in place, with only minor adjustments tailored for updated
guidance.
Weiss
Day 3 threat area:
www.wpc.ncep.noaa.gov/qpf/99epoints.txt
Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
256 PM EDT Thu Jun 19 2025
For Sun/Mon, subtropical moisture will start to push northward
into the southern Rockies, leading to some increase in rainfall
potential over New Mexico. Introduced a Marginal risk in the
Excessive Rainfall Outlook as there was a bit of an uptick in
heavier amounts over some sensitive areas/terrain. By Monday, the
continued feed of moisture northward will interact with the frontal
boundary and could lead to a wider expanse of heavier rainfall all
the way northward/northeastward through the Plains to the Corn
Belt and Upper Midwest. Maintained a Slight Risk over Iowa to
account for the highest chance of heavier rain but there is typical
model spread in exact location. Thereafter, expect rounds of
rainfall over much of the same area (New Mexico to Nebraska to the
Upper Midwest) which will continue the flash flooding threat.
Farther west, troughing into the Northwest will be strong enough to
bring in colder air this weekend and some snow to the highest
elevations around Glacier National Park.
Farther east, the strong upper high will favor dry conditions
except for along the Gulf Coast and northern New England though
some increase in isolated showers/storms is possible in the East as
the high starts to fade later in the week. Temperatures will be
well above normal and approach/exceed daily records at a few dozen
locations in total between Sunday and at least Wednesday. This
translates into highs well into the 90s to around 100F with heat
indices to near 110F as dew points will be in the 60s to
low/mid-70s. Overnight lows will only drop into the low/mid 70s for
many areas but may stay around 80F in the urban centers like
Washington, D.C., Baltimore, Philadelphia, and New York City. Many
locations may exceed daily or perhaps even monthly record high
minimum temperatures. Some relief is expected toward the end of
next week.
Heat Risk values will be Major to Extreme for much of the Midwest
to Eastern states -- levels 3 and 4 on a scale from 1 to 4 (4 being
Extreme). This indicates an intensity and duration of heat that is
extremely dangerous to anyone without adequate cooling or
hydration. Successive overnight readings in the mid 70s to around
80F will bring little relief from the heat and can pose a
significant danger. Extreme heat is the number 1 weather-related
killer.
Highs will be cooler and below average by as much as 10-20 degrees
across portions of the Pacific Northwest, Great Basin, and
northern Rockies/High Plains this weekend with a deep- upper level
trough overhead. Temperatures will moderate closer to average
through early/mid next week.
Fracasso/Putnam
Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
256 PM EDT Thu Jun 19 2025
For Sun/Mon, subtropical moisture will start to push northward
into the southern Rockies, leading to some increase in rainfall
potential over New Mexico. Introduced a Marginal risk in the
Excessive Rainfall Outlook as there was a bit of an uptick in
heavier amounts over some sensitive areas/terrain. By Monday, the
continued feed of moisture northward will interact with the frontal
boundary and could lead to a wider expanse of heavier rainfall all
the way northward/northeastward through the Plains to the Corn
Belt and Upper Midwest. Maintained a Slight Risk over Iowa to
account for the highest chance of heavier rain but there is typical
model spread in exact location. Thereafter, expect rounds of
rainfall over much of the same area (New Mexico to Nebraska to the
Upper Midwest) which will continue the flash flooding threat.
Farther west, troughing into the Northwest will be strong enough to
bring in colder air this weekend and some snow to the highest
elevations around Glacier National Park.
Farther east, the strong upper high will favor dry conditions
except for along the Gulf Coast and northern New England though
some increase in isolated showers/storms is possible in the East as
the high starts to fade later in the week. Temperatures will be
well above normal and approach/exceed daily records at a few dozen
locations in total between Sunday and at least Wednesday. This
translates into highs well into the 90s to around 100F with heat
indices to near 110F as dew points will be in the 60s to
low/mid-70s. Overnight lows will only drop into the low/mid 70s for
many areas but may stay around 80F in the urban centers like
Washington, D.C., Baltimore, Philadelphia, and New York City. Many
locations may exceed daily or perhaps even monthly record high
minimum temperatures. Some relief is expected toward the end of
next week.
Heat Risk values will be Major to Extreme for much of the Midwest
to Eastern states -- levels 3 and 4 on a scale from 1 to 4 (4 being
Extreme). This indicates an intensity and duration of heat that is
extremely dangerous to anyone without adequate cooling or
hydration. Successive overnight readings in the mid 70s to around
80F will bring little relief from the heat and can pose a
significant danger. Extreme heat is the number 1 weather-related
killer.
Highs will be cooler and below average by as much as 10-20 degrees
across portions of the Pacific Northwest, Great Basin, and
northern Rockies/High Plains this weekend with a deep- upper level
trough overhead. Temperatures will moderate closer to average
through early/mid next week.
Fracasso/Putnam
