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
402 AM EDT Fri Jun 20 2025
Day 1
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...
...Northern Plains into Upper Midwest...
Model guidance indicates a well defined mid level shortwave will
eject east out of the Rockies and into the Northern Plains this
afternoon into tonight, with a surface low moving from southern MT
into the Dakotas. High res guidance show convection developing near
the MT/ND border this afternoon to the north of a warm front. An
increasing low level jet by evening likely supports upscale growth
of this activity as it moves eastward across ND. This convection
will likely be quick moving, however 1"+ per hour rainfall appears
probable and a localized flash flood risk is possible. A Marginal
risk was expanded west into these areas.
A Slight risk was maintained across portions of northeast MN,
and was expanded some with this update to include more of northern
MN and far northern WI. An impressive low level jet and moisture
transport axis will quickly evolve this evening into the overnight
hours. Also looking at a pool of extreme instability on the order
of 4000-5000 j/kg, and PWs increasing towards 2". While the
progressive MCS tracks east across ND, we should see some
downstream development over eastern ND into northern MN on the nose
of this intense low level jet and along the instability gradient.
Some west to east training of this activity is possible ahead of
the upstream MCS that will also eventually push across the area.
The degree of training remains a bit unclear, but do note that both
the 00z HRRR and RRFS show a narrow swath of 3-5" of rainfall.
Given the instability and moisture in place, totals of this
magnitude do seem plausible. Also some uncertainty on the axis of
heaviest rainfall, which should end up pretty narrow. The better
instability will be south, however a strong mid level cap will be
advecting northward likely putting a limit on how far south
organized convection will get. The current Slight risk area
encompasses the highest HREF probabilities, and while a bit broader
than what will probably happen, accounts for some latitudinal
uncertainty. Overall think isolated to scattered flash flooding
could evolve from this setup, especially if there is overlap with
the heavy rainfall axis from a couple days ago over northeast MN.
...Iowa...
Convection is currently moving southeast across portions of MN and
IA, and some of this will likely be still ongoing at 12z this
morning. The 12z HREF and recent HRRR/RRFS runs show some brief
backbuilding potential this morning on the nose of the low level
moisture transport axis across eastern IA. Localized amounts over
3" appear possible, and an isolated flash flood threat could exist
this morning.
...Northern Rockies...
A strong mid level low moving into the Pacific Northwest will
focus impressive synoptic ascent over MT today into tonight. This
will result in increasing shower coverage, with PWs increasing
towards the climatological 90th percentile as well. Instability is
likely the main limiting factor for excessive rainfall, and the
probability of exceeding 1" per hour rain is pretty low because of
this. However do think we will get some weak instability, which
combined with the impressive dynamics, should still allow for some
embedded heavier convective cores with over 0.5" per hour rain.
Most areas will probably see closer to 1" of rain, however
localized amounts over 2" are supported by the 00z high res models
which could result in an isolated flash flood risk.
Chenard
Day 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
402 AM EDT Fri Jun 20 2025
Day 2
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...
Convection will likely be ongoing at 12z Sat across portions of
northern MI into the UP. The overall convective complex should be
propagating eastward pretty quickly by this time, although it may
still have a west to east orientation to it allowing for some
training. The 00z HREF indicates 1" per hour exceedance
neighborhood probabilities upwards of 60%, and 2" exceedance around
20%. HREF blended mean QPF is in the 2-3" range and these amounts
seem plausible wherever the convective training axis is Saturday
morning. Amounts higher than this will probable be hard to come by
given the quick system movement, however even totals of this
magnitude could result in a localized flash flood threat. Some
latitudinal differences in the convective axis are noted, with both
northern MI and the UP of MI potentially impacted.
...New England...
The aforementioned convection over MI will try to move into
portions of NY during the day Saturday. The maintenance of the
activity is unclear as it will be moving into a more stable
airmass. Generally think convection will weaken as it moves into
NY...however with daytime heating some increase in instability over
central NY could support at least some additional convective
development during the day.
By Saturday night steeper lapse rates advect into the
Northeast, resulting in what should be a pretty rapid increase in
MUCAPE over western NY into northern New England. A well defined
mid level shortwave and increasing low level moisture transport
will probably kick of organized convective development over
southern Canada by Saturday evening. This activity should track
into portions of NY and northern New England overnight into early
Sunday. It seems likely that this activity will have a better
chance of persisting into NY and New England given what should be
a very strong axis of low level moisture transport over top steep
mid level lapse rates. In fact we could very well have an
impressive MCS diving into NY and northern New England later
Saturday night. The specifics remain a bit unclear, but at least a
localized flash flood risk seems probable even with a forward
propagating system. Certainly a chance this setup keeps some
backbuilding convection across the area, and possible we will
eventually need a Slight risk. However, given the uncertainty
regarding the details, think a Marginal risk will suffice for now.
...Northwest Montana...
A Marginal risk was maintained across portions of northwest MT
where rainfall will continue into Saturday. By this time this
region will be within the comma head of the low, and looking at
more stratiform rain with lower rates. Thus probably not really a
flash flood risk by this point, but with storm total rainfall
potentially into the 3-4" range, some possible flood impacts still
justify a Marginal risk. Snow levels will drop towards 4000 feet
by this time as well, so flood impacts will stay below that level.
Chenard
Day 2 threat area:
www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
402 AM EDT Fri Jun 20 2025
Day 3
Valid 12Z Sun Jun 22 2025 - 12Z Mon Jun 23 2025
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF EASTERN NEW MEXICO AND SOUTHWEST TEXAS...
Sunday is likely the beginning of what will be a multi day period
of excessive rainfall potential over NM. The mid/upper level
pattern features a near record to record ridge over the eastern
U.S. and a well defined longwave trough over the west. Southerly
flow in between these features will supply ample moisture to NM,
with a connection all the way to the Caribbean Sea. Thus no
surprise that forecast PWs are well over the 90th percentile, and
possibly approaching near record levels for late June. The better
forcing from the western trough should hold of until Monday and
Tuesday (see the day 4 and 5 EROs), but by Sunday we should at
least have enough moisture and instability around for isolated to
scattered convective development. This appears to be a solid
Marginal risk with localized flash flooding expected...and can not
rule out eventually needing a Slight risk. Sometimes these first
day of the event situations can overperform as the clean start can
allow for more destabilization and greater convective coverage
than expected. However with Monday and Tuesday currently expected
to be the bigger days, will keep this day 3 as a Marginal for now
and continue to monitor trends.
Chenard
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 Fri Jun 20 2025
The upper trough/ridge pattern setting up across the lower 48 will
draw in ample subtropical moisture to the southern Rockies/High
Plains. The right entrance region of the upper jet will provide
support to develop and maintain rain and thunderstorms, while
moisture levels should be high. Specifically, precipitable water
values are likely to be over the 95th percentile if not 99th, while
instability will be sufficient for widespread thunderstorms. New
Mexico is likely to see the heaviest rain, and have delineated
Slight Risks in the Day 4/Monday and Day 5/Tuesday EROs for
potential flash flooding. By Day 5/Tuesday this is considered a
higher-end Slight Risk given slightly better moisture parameters
and wetter antecedent conditions by then. Areas like terrain and
burn scars (particularly the Sacramento Mountains) would be
particularly sensitive. Some moist inflow is likely to continue
Wednesday and beyond, continuing rain chances, though the upper jet
should get gradually weaker for less support for heavy amounts.
Moisture is forecast to wrap around on the northern side of the
upper ridge and spread rain chances eastward into the north-central
Plains and Midwest in conjunction with shortwaves. A meandering
frontal boundary will focus this moisture on the cusp of the
instability gradient and lead to thunderstorms with potentially
heavy rain rates that could train west to east. A Slight Risk
remains in place for the Day 4/Monday ERO for portions of Nebraska
to Iowa, and for Day 5/Tuesday show a similar idea but just to the
north into southeast South Dakota/southern Minnesota/western
Wisconsin as the front and thus QPF shifts north. Another rather
wet day is currently forecast for the Upper Midwest Wednesday with
some eastward shift toward the Upper Great Lakes region possible by
Thursday. Some rain and storms could spread into the Mid-Atlantic
and Northeast mid to later week as well.
Farther south, there is some uncertainty with how much convection
may occur under the upper ridge across the Gulf Coast and
southeastern U.S. as surface based instability likely battles with
subsidence aloft. In general, scattered thunderstorms may increase
in coverage as the week progresses as the upper high gradually
weakens. The southern Appalachians may be a particular focus for
convection with orographic lift.
The strong upper high/ridge stretching across the south-central to
eastern U.S. will allow for well above average temperatures that
could approach or exceed daily records at a few dozen locations
peaking Monday and Tuesday. 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. HeatRisk values will be Major to Extreme for much of
the Midwest and Ohio Valley 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. Temperatures are expected to moderate by a
few degrees but remain above average into the latter part of next
week. Meanwhile, temperatures (particularly highs) are forecast to
be below average by 5-15 degrees in interior portions of the West
early next week, but should gradually warm closer to or a bit above
average as the week progresses.
Tate
Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
258 AM EDT Fri Jun 20 2025
The upper trough/ridge pattern setting up across the lower 48 will
draw in ample subtropical moisture to the southern Rockies/High
Plains. The right entrance region of the upper jet will provide
support to develop and maintain rain and thunderstorms, while
moisture levels should be high. Specifically, precipitable water
values are likely to be over the 95th percentile if not 99th, while
instability will be sufficient for widespread thunderstorms. New
Mexico is likely to see the heaviest rain, and have delineated
Slight Risks in the Day 4/Monday and Day 5/Tuesday EROs for
potential flash flooding. By Day 5/Tuesday this is considered a
higher-end Slight Risk given slightly better moisture parameters
and wetter antecedent conditions by then. Areas like terrain and
burn scars (particularly the Sacramento Mountains) would be
particularly sensitive. Some moist inflow is likely to continue
Wednesday and beyond, continuing rain chances, though the upper jet
should get gradually weaker for less support for heavy amounts.
Moisture is forecast to wrap around on the northern side of the
upper ridge and spread rain chances eastward into the north-central
Plains and Midwest in conjunction with shortwaves. A meandering
frontal boundary will focus this moisture on the cusp of the
instability gradient and lead to thunderstorms with potentially
heavy rain rates that could train west to east. A Slight Risk
remains in place for the Day 4/Monday ERO for portions of Nebraska
to Iowa, and for Day 5/Tuesday show a similar idea but just to the
north into southeast South Dakota/southern Minnesota/western
Wisconsin as the front and thus QPF shifts north. Another rather
wet day is currently forecast for the Upper Midwest Wednesday with
some eastward shift toward the Upper Great Lakes region possible by
Thursday. Some rain and storms could spread into the Mid-Atlantic
and Northeast mid to later week as well.
Farther south, there is some uncertainty with how much convection
may occur under the upper ridge across the Gulf Coast and
southeastern U.S. as surface based instability likely battles with
subsidence aloft. In general, scattered thunderstorms may increase
in coverage as the week progresses as the upper high gradually
weakens. The southern Appalachians may be a particular focus for
convection with orographic lift.
The strong upper high/ridge stretching across the south-central to
eastern U.S. will allow for well above average temperatures that
could approach or exceed daily records at a few dozen locations
peaking Monday and Tuesday. 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. HeatRisk values will be Major to Extreme for much of
the Midwest and Ohio Valley 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. Temperatures are expected to moderate by a
few degrees but remain above average into the latter part of next
week. Meanwhile, temperatures (particularly highs) are forecast to
be below average by 5-15 degrees in interior portions of the West
early next week, but should gradually warm closer to or a bit above
average as the week progresses.
Tate
