Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
859 PM EDT Thu May 8 2025
Day 1
Valid 01Z Fri May 09 2025 - 12Z Fri May 09 2025
...THERE ARE A SLIGHT RISK OF EXCESSIVE RAINFALL OVER SOUTH TEXAS...
...Deep South Texas...
16Z Update: Little change with the overall expectations across Deep
South TX. Still looking at primary impacts from two rounds of
convection. The first is already occurring with a strong
thunderstorm slowly progressing eastward off South Padre as the
updraft and primary mesocyclone matured enough to drop a few inches
of rainfall in short succession along the South TX coast plain past
few hrs. 12z KBRO sounding came in with a deep moist profile with
PWATs settled at 2.15" putting it at the new 12z Daily Max for the
date. This is a testament to the environment available for any
convective regimes whether that be from more pulse variety
convection this morning and afternoon, and eventual MCS progression
as the potent shortwave ejects southeast out of MX generating a
more organized heavy rain prospect from the Big Bend, southeast.
12z HREF neighborhood probs for >3" are between 60-90% over the
area extending from CRP down to BRO to about 40 miles inland along
that stripe. Neighborhood >5" are highest near Brownsville to
McAllen (30-60%) lending credence to the higher flash flood threat
in the region, especially in those more urbanized corridors along
the Rio Grande Valley. The previous SLGT was relatively unchanged
considering the setup, in agreement with the local WFO's.
Kleebauer
...Portions Lower Ohio Valley, Tennessee Valley, Southeast, Mid
Atlantic, and Northeast...
Trimmed quite a bit of the Marginal Risk area across these areas,
given the weak mid-level lapse rates (~6-6.5 C/Km) and thus
diminishing CAPE trends following sunset. Slow-moving, favorable
area of large-scale forcing (DPVA/upper divergence) ahead of the
upper trough will be aided across the Mid Atlantic overnight due to
some left-exit region upper jet contribution via the 90-100kt upper
jet streak that pushes into the Southeast. This will lead to
slightly more favorable low-mid layer moisture transport, with
850-700 mb moisture flux anomalies closer to 1 standard deviation
above normal. Recent HRRR and other CAM runs show isolated pockets
of 3+ inches of rain overnight, especially across Upstate SC into
central NC, with 1-2+ inch totals over parts of VA/MD and the
northern Mid Atlantic. However despite the favorable forcing and
slow storm motions, expect the increasingly marginal instability
to limit the coverage and intensity of the stronger cells,
especially after 03-04Z per the 18Z HREF exceedance probabilities.
Thus the Marginal Risk will continue, with the expectation of
mainly isolated/localized short-term runoff issues, again aided by
the slow storm motions and especially where any west-east bands
set up and train.
Hurley
Day 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
859 PM EDT Thu May 8 2025
Day 2
Valid 12Z Fri May 09 2025 - 12Z Sat May 10 2025
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE NORTHEAST & SOUTHEAST...
20Z Update: The period remains pretty consistent in run to run QPF
variance with the general convective threat across the
Southeastern U.S. still anticipated with the main changes being the
positioning of local maxima. 12z HREF remained steadfast on its
probabilistic outputs with an elevated EAS probs for >1" located
across Southern MS/AL with lower probs elsewhere. The signal for
>2" is much lower (<10-20%) for anywhere within the area of
interest meaning guidance still has a wide variance in where the
strongest cells will materialize during the forecast cycle. The
antecedent conditions are most favorable across Southeast LA where
heavy rains in the prior periods have decreased areal FFG's to more
attainable exceedance values. The rest of the Southeast is pretty
much near normal for the 1/3/6 hr. FFG exceedance intervals leading
to a higher rate threshold necessary to exhibit flash flooding.
The best opportunity for a targeted upgrade will likely be within
that corridor from Southeast LA through Southern MS/AL just given
the location of highest theta_E's correlated with better upper jet
dynamics in the form of a jet coupling between the mid-latitude jet
trailing the cyclonic flow to the north and the southern jet
streak focused to the south.
Across the Northeast, total precip will remain firmly between 1-3"
thanks to maturing mid-latitude cyclone moving northward off the
Northeast coast with much of the Northern Mid Atlantic through New
England well positioned within the LER of a strengthening upper
speed max juxtaposed over the Central Mid Atlantic around the
trough base. The total precip and intra-hour rates ~1"/hr will be
the greatest factor for any flash flood prospects since the
majority of the precip will likely be within a stratiform scheme
during the peak of the event. Hi-res ensembles and overwhelming
global deterministic outputs are generally modest with the
anticipated rates over the most impacted areas. This is a stronger
case for river and small stream flooding which is the reasoning for
the widespread Flood Watch issuances and not so much the flash
flood variety. If there was an upgrade, it would likely be very
targeted within Southern New England where there is a better case
for elevated instability within the WCB process generally over
more urbanized zones. In any case, the threat is still within the
lower risk threshold and has merit with the D2 ML First Guess
Fields. The MRGL was relatively unchanged with a small expansion
into the Philadelphia metro to cover for recent QPF trends.
Kleebauer
..Previous Discussions..
...Gulf Coast & Southeast...
A large upper low positioned over the Lower MS Valley will linger
over the region through Friday night with heavy rainfall possible
from the central Gulf Coast to the Southeast coast. Precipitable
values of 1.5-1.75" are forecast, implying nearly saturated
soundings. Effective bulk shear could be sufficient to help
sustain areas of organized thunderstorms. There is wide variance in
the QPF output from the various pieces of guidance, and most of
the guidance isn't terribly wet, so have kept the excessive
rainfall risk level at Marginal.
...Northeast...
Out ahead of the upper low, guidance is showing strengthening
onshore flow which would transport copious amounts of moisture into
the Northeast. Moisture will wrap around the northern flank of the
500 hPa low into a potential comma head pattern across northern NY
and central New England. Farther east, the warm sector will
feature highly saturated and deep warm cloud layers that could
contain some weak elevated instability. There remains some spread
in where the axis of heaviest rainfall sets up, which is why a
Slight Risk remains un-added. However, given trends in guidance
are all pointing towards a cut-off low over the Northeast and soils
throughout the region are highly sensitive, there may be the need
for a Slight Risk upgrade in future forecast updates once
confidence increases in where the heaviest rainfall takes shape.
Roth/Mullinax
Day 2 threat area:
www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
859 PM EDT Thu May 8 2025
Day 3
Valid 12Z Sat May 10 2025 - 12Z Sun May 11 2025
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SOUTHEAST & NEW ENGLAND...
20Z Update: The main change this period was over the Northeastern
U.S. MRGL risk were the southern edge was pulled a bit northward
given the trends in QPF distribution. The ECMWF was one of the
furthest south in terms of the heavier QPF placement over the past
cycle of runs, however the ML disagreed with the assessment and had
the heavier precip further north as the the low will move steadily
northeast before occlusion. 12z ECMWF is now in line with the rest
of the deterministic and subsequent ensembles and ML output
leading to confidence in pulling the southern portion of the risk
area further north. As for the general pattern, little has changed
otherwise with the core of heaviest precip focused across ME where
precip is very much welcomed in all locales given current drought.
The MRGL risk was maintained due to the forecasted 1-2" rainfall
forecast with some terrain focused areas and urbanized zones the
most likely to see any hydro impacts.
Across the Southeast, beginning to see a greater consensus on
heavier precip entering near the Big Bend over Apalachicola
National Forecast, an area that is notoriously hard to flood
outside significant convective training and/or tropical influence.
Current moisture advection regime off the Gulf is situated for a
prolonged training threat within the eastern flank of the maturing
surface low positioned directly over the Central Gulf Coast. Deep
layer mean flow is progged to be due southerly across the entire
Western FL Panhandle which could ultimately lead to a targeted SLGT
risk issuance if the current setup stands. There's still a bit of
discrepancy on specifics with where the most prolific moisture
transport will end up, but ensemble and ML depictions are close to
that aforementioned area, but either a west or east displacement
would cause differences in potential impacts as some larger
population centers would get involved if the setup takes aim a bit
more upstream. The MRGL is in place currently, but would not be
surprised if a targeted upgrade is necessitated in future
forecasts, especially once in range of the CAMs.
Kleebauer
..Previous Discussion..
Southeast...
A cold low drops down to the Gulf coast with time, spurring a
development of a low vaguely near the Loop Current. Despite good
agreement aloft, there is a definite difference in the guidance in
lower levels, with the 00z NAM a bit north of the 00z GFS at 850
hPa. This sort of pattern usually leads to a semi-convective low
near the Loop Current which then moves northeast towards the
Florida Panhandle or Big Bend, though the low's approach to the
region looks more evident on Sunday or so/beyond the day 3 period.
Some of the guidance has a decent QPF signal near the FL Big Bend,
generally agreeing on 2-3" areal average, but it there is a bit of
dispersion. It appears the 850 hPa boundary is still down in the
Gulf much of the day. However, there could be enough moisture,
instability, and effective bulk shear for issues in northern FL.
For the moment, the Marginal Risk area remains in place due to the
dispersion seen in the guidance.
Northeast...
The guidance has a signal for moderate to heavy rainfall across
portions of New England over an area of relatively low flash flood
guidance values. This is near and ahead of a cold low racing
through the area. Precipitable water values rise to 1-1.25", which
given the cool 1000-500 hPa thickness values, should lead to
saturation. Given the strong 500 hPa height falls during the
afternoon hours, some instability is bound to be available. Added a
Marginal Risk area per the above. Per coordination with
CAR/the Caribou ME forecast office. left northern ME out as the
seven day rainfall across the state shows a fairly strong gradient,
with northern ME generally left out of the recent rainfall.
Roth
Day 3 threat area:
www.wpc.ncep.noaa.gov/qpf/99epoints.txt
Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
300 PM EDT Thu May 8 2025
A wavy frontal system and a closed upper low meandering over the
Southeast will support a multi-day heavy rainfall and flooding
threat through next midweek as fueled by the pooling of highly
anomalous moisture. Given the increased signal for heavy rains by
Sunday (and some heavy rainfall potential starting on Saturday),
a Slight Risk for northern Florida and southern Georgia remains
depicted on the WPC Day 4 Excessive Rainfall Outlook (ERO). By
Monday, this potential expands southward into the Florida peninsula
and northward across much of the Southeast as storms may train
along a slow moving cold front lifting through the region. As such,
a slight risk is included for much of the Southeast on the Day 5
ERO as well. Protracted enhanced rainfall should gradually lift
northward with time increasing across the Mid-Atlantic and up the
Appalachians next Tuesday-Thursday before finally losing influence.
Out West, rain and higher elevations snows will become more
widespread through the weekend and into next week with main
amplified system cooling and slow progression inland through early
and mid next week. Modest snows are possible in the higher terrain.
As the system progresses east, moderate to heavy rainfall may
develop on the north and west side of a surface low over the north-
central Plains.
Upper troughing over the South will keep post-frontal temperatures
below normal through early next week, especially daytime maxes
given widespread and protracted rain/cloud potential around the
upper low. Upper ridging from the Southwest to the Plains will
support much above normal temperatures across much of the
intermountain West and northern U.S., with daytime highs 20-30
degrees above normal possible in the Northern Plains where record
values are in reach. Parts of the Southwest should exceed 100
degrees with at least some localized heat threat given earlier in
the season timing Sunday and Monday. Above normal heat will
moderate some as it progresses across the rest of the Plains and
into the Midwest and East next week. Upper troughing and frontal
surge over much of the West will drop and maintain temperatures
below normal Tuesday-next Thursday in unsettled flow.
Santorelli/Schichtel
Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
300 PM EDT Thu May 8 2025
A wavy frontal system and a closed upper low meandering over the
Southeast will support a multi-day heavy rainfall and flooding
threat through next midweek as fueled by the pooling of highly
anomalous moisture. Given the increased signal for heavy rains by
Sunday (and some heavy rainfall potential starting on Saturday),
a Slight Risk for northern Florida and southern Georgia remains
depicted on the WPC Day 4 Excessive Rainfall Outlook (ERO). By
Monday, this potential expands southward into the Florida peninsula
and northward across much of the Southeast as storms may train
along a slow moving cold front lifting through the region. As such,
a slight risk is included for much of the Southeast on the Day 5
ERO as well. Protracted enhanced rainfall should gradually lift
northward with time increasing across the Mid-Atlantic and up the
Appalachians next Tuesday-Thursday before finally losing influence.
Out West, rain and higher elevations snows will become more
widespread through the weekend and into next week with main
amplified system cooling and slow progression inland through early
and mid next week. Modest snows are possible in the higher terrain.
As the system progresses east, moderate to heavy rainfall may
develop on the north and west side of a surface low over the north-
central Plains.
Upper troughing over the South will keep post-frontal temperatures
below normal through early next week, especially daytime maxes
given widespread and protracted rain/cloud potential around the
upper low. Upper ridging from the Southwest to the Plains will
support much above normal temperatures across much of the
intermountain West and northern U.S., with daytime highs 20-30
degrees above normal possible in the Northern Plains where record
values are in reach. Parts of the Southwest should exceed 100
degrees with at least some localized heat threat given earlier in
the season timing Sunday and Monday. Above normal heat will
moderate some as it progresses across the rest of the Plains and
into the Midwest and East next week. Upper troughing and frontal
surge over much of the West will drop and maintain temperatures
below normal Tuesday-next Thursday in unsettled flow.
Santorelli/Schichtel