Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
852 PM EDT Thu May 29 2025
Day 1
Valid 01Z Fri May 30 2025 - 12Z Fri May 30 2025
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
CENTRAL TEXAS...
A Slight risk was maintained across central TX. Convection is
expanding in coverage this evening along a southward shifting cold
front. This activity should continue to expand in coverage and
grow upscale into an MCS or convective cluster or two tonight.
Generally looking like a forward propagating convective system,
which should limit the extent/magnitude of flash flooding. However
a few cell mergers this evening as convection organizes will
likely lead to some flash flood risk. Even once activity becomes
more progressive high rainfall rates should be enough for isolated
to scattered FFG exceedance, especially where cell mergers briefly
extend rainfall duration. Recent HRRR runs have indicated some
upstream development overnight near the Rio Grande, which does seem
plausible given forecast increase in low level moisture transport
and instability around 4000 j/kg. If this does occur, and then
merges with the approaching MCS (as the HRRR indicates) then a more
substantial flash flood risk could evolve later tonight near the
Rio Grande in south central TX.
A Marginal risk was maintained across portions of the eastern
Carolinas into far southeastern VA. Convection will continue to
pose a localized flash flood risk this evening, although the
activity over NC and VA is expected to weaken fairly rapidly this
evening, diminishing the risk.
The approach of a strong shortwave will potentially bring a
localized flash flood risk to portions of KY/TN and southern IL/IN
later tonight as well. Limited instability will keep rainfall
rates in check...however the very dynamic nature of the approaching
shortwave should still allow for some heavier convective elements
within the broader rain shield. Rainfall could locally approach or
exceed 2", potentially enough for some minor flood concerns,
especially over any more sensitive urban or low lying areas.
Chenard
Day 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
852 PM EDT Thu May 29 2025
Day 2
Valid 12Z Fri May 30 2025 - 12Z Sat May 31 2025
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE OHIO VALLEY AND MID ATLANTIC...
...20Z Update...
Continued increases in the forecast rainfall from the 12Z guidance
suite, sufficient instability for storms, ample upper level
forcing from a strong shortwave that rapidly turns negatively
tilted, and soil sensitivity due to recent heavy rainfall have all
worked to increase the concern for flash flooding across portions
of the Ohio Valley and Mid-Atlantic Friday and Friday night.
Generally, the convection will track from west to east...so the
portion of the Slight in Kentucky really focuses on Friday morning,
whereas the portion in northern New Jersey is primarily focused on
after midnight Friday night.
Soil sensitivity to flash flooding is high as soil moisture levels
remain well above the normal for this time of year due to rain as
recently as yesterday, and not including any shower and isolated
thunderstorm activity probable around the DMV region the afternoon
and this evening. Further, possible repeating rounds of heavy rain
on Friday interacting with the terrain and full rivers should cause
onset of flash flooding to occur sooner...closer to the rainfall
start time...in these areas, making any potential flooding more
hazardous in West Virginia. Further east, urban concerns will
increase the potential impacts from flash flooding. The storms will
also occur in the afternoon and evening, during the diurnal warmest
time of day, which will add more instability for the storms to feed
on. Given all of the above, have introduced a higher end Slight
from eastern Kentucky to the Baltimore metro. The 12Z HREF suite
shows an over 50% chance of exceeding 6-hour FFGs across the DMV.
The Slight Risk area was expanded northeast across the Philadelphia
Metro and much of northern New Jersey with this update. While the
storms will impact this region Friday evening and into the
overnight, here too recent heavy rainfall will make flash flooding
more common compared to if the soils were dry. By Friday evening
a coastal low will be rapidly forming, and so instability will be
waning as the precipitation shield evolves into more of a comma
shape, with the heaviest rains along the Delaware River on the
cold/more stable side of the low. Nevertheless, remnant instability
and long duration of rainfall will still cause widely scattered
instances of flash flooding. Since the low will have a comma-shape
Friday night, the dry slot will end the rainfall sooner in some
areas. While there is some uncertainty where it will be, it looks
to focus along the Jersey Shore. Thus the Slight was kept from the
coast a bit expecting the rain to end there sooner than areas along
the Delaware River.
The surrounding Marginal Risk was also expanded into NYC and southern
New England for Friday night. Here too the dry slot could limit
rainfall amounts, especially the further east you go. Upslope into
the terrain of Connecticut and recent rains around NYC could still
cause locally heavy rain before the dry slot moves in.
Wegman
...Previous Discussion...
A potent upper trough will slowly pivot eastward while taking on a
more neutral tilt as it swings through the Great Lakes and Ohio
Valley on Friday. At the surface, low pressure will develop within
the middle of an evolving jet couplet in-of the eastern portion of
the Ohio Valley with deterministic output placing the low nearby
Eastern KY. The entire dynamic between sfc-500mb will play an
integral role in the potential for heavy rainfall in areas of KY/WV
the first half of the period, shifting eastward through the
Shenandoah of VA by the afternoon and evening Friday before
providing quite the convective posture over the DMV and points
northeast. 00z suite has come in much more aggressive in the
presentation of heavy rainfall across the above areas with a dual
maxima signature present within the ensemble means. The first max
is over Eastern KY into Southern WV thanks to intense large scale
ascent being driven by a powerful shortwave rotating into the
region along the base of the mean trough as it approaches the
region. At the surface, a narrow tongue of surface based
instability will be co-located within the mid and upper level
ascent max being delivered by the trough, a product of a warm front
lifting north ahead of the aforementioned surface low off to the
west. A line of thunderstorm activity will likely transpire within
proxy of the warm front leading to enhanced rainfall rates likely
to spur totals between 1.5-3" with potentially higher over a span
of 3-6 hrs. before a cold front sweeps through the area in wake of
the lows progression to the east-northeast.
Current FFG exceedance probs for both 3 and 6hr increments are
pretty robust (>50%) within the corridor referenced above, along
with some hourly rate exceedance probs between 50-80% between the
12-18z time frame on Friday. The signature typically correlates to
a better than average threat for flash flooding, especially when
you account for the antecedent soil moisture and the complex
topography situated over Eastern KY and Southern WV. This allowed
for an addition of a SLGT risk across these two areas.
Further east, soil moisture anomalies will remain elevated thanks
to a barrage of rainfall the past 24 hrs. and very wet conditions
overall the past 2 weeks. As the surface low matures on its trek to
the east and the trough begins a more negative tilt to the west,
expectation is the for the low to begin intensifying steadily with
a sharper deformation axis beginning to take shape across the
Central Mid Atlantic. Heavy rains on the western flank of the low
will likely occur across the Northern and Central Shenandoah, an
area that has been hit hard over the course of May and is already
experiencing relatively higher stream flows. Current HREF depiction
for 6hr exceedance probs over Northwestern VA are between 50-80%
over the Blue Ridge and adjacent valley areas from Harrisonburg to
just north of Winchester. A smaller max is co-located with the more
urban centers of Northern VA towards the DC metro, but that is at
the end of the hi-res temporal window. The assessment of the final
6 hr period from the CAMs and the global deterministic and ensemble
outputs in the final 12 hr time frame indicate a mass of heavy
rainfall being depicted within the MD Piedmont to the Chesapeake.
NAM Nest is incredibly bullish in its interpretation, likely due to
the strong low to mid-level lapse rate signature across the area
looking at forecast soundings, as well as persistent 250-750 J/kg
MUCAPE still present prior to the cold front arrival. QPF output is
around 1-2" currently for the DC/Balt Metro and surrounds for the
period, a signal strong enough to warrant a targeted upgrade in the
forecast risk. As a result and with coordination from the local
Sterling, VA WFO, a SLGT risk was issued for the period across the
above area.
Heavy rainfall will occur further north as the low matures, but the
instability gradient is fairly tight on the northwest periphery of
the low as we move closer to Saturday morning. It's plausible
places like Philadelphia and areas of Southern NJ and Southeast PA
could see a targeted upgrade as well for the D2, but the signal is
more borderline for those locales. The MRGL risk was maintained for
those areas up to Northern NJ, but will be something to monitor as
we move closer.
Kleebauer
Day 2 threat area:
www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
852 PM EDT Thu May 29 2025
Day 3
Valid 12Z Sat May 31 2025 - 12Z Sun Jun 01 2025
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF NORTHERN NEW ENGLAND...
...20Z Update...
Very few changes were needed to the inherited Marginal Risk, with
maybe a small southward adjustment across Massachusetts, where
rainfall occurring just before the start of the period 12Z Saturday
will be ongoing into the start of the D3 period. The surface low
will be well-formed by 12Z Saturday, with surface pressures
dropping into the 980 mb range. However, instability will be
lacking as New England is on the "cold" side of the low...so almost
all of the rainfall will be stratiform. Terrain interactions will
locally increase rainfall rates, so the Marginal, while low-end and
low-confidence, remains in place.
Wegman
...Previous Discussion...
Powerful surface low across the Mid Atlantic on D2 will motion to
the northeast, eventually dropping down into the 980s as it
occludes and drifts north over New England. Residual heavy rains
will occur across parts of Northern New England with the heaviest
rainfall likely over Northern NH and ME, especially within the
northeastern flank of the low where WCB process will advect deeper
moisture poleward with weak buoyancy likely remaining over the
region during the first 12 hrs of the D3 period. Totals of 1-2"
with locally higher are forecast over the above zones leading to
localized flash flood potential given the complexity of the terrain
impacted and relatively good dynamics at play.
The MRGL risk inherited was only adjusted on the western half of
the risk as trends have pushed the heavier rain a little further
east compared to previous forecasts. The area over Central and
Western ME is the most susceptible in this pattern with potentially
greater impact into Southern New England if the low slows down and
brings a stronger convective output towards Eastern MA and RI the
front end of D3. Will be an area to assess closely in the coming
updates, but maintained the nil there for now.
Kleebauer
Day 3 threat area:
www.wpc.ncep.noaa.gov/qpf/99epoints.txt
