Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
339 AM EDT Thu May 29 2025
Day 1
Valid 12Z Thu May 29 2025 - 12Z Fri May 30 2025
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE SOUTHEAST, OHIO VALLEY, SOUTHERN PLAINS, AND MID MISSISSIPPI
VALLEY...
...Mid Mississippi and Ohio Valleys...
A shortwave currently analyzed across the Central Plains will
continue to progress eastward overnight under the base of a broad
ULL situated over the Midwest. Increased mid and upper forcing will
aid in scattered convection through the course of the period in the
area spanning from Eastern KS through the Central Ohio Valley.
Some of the stronger convective cores will be capable of locally
enhanced rainfall rates between 1-2"/hr at peak intensity leading
to isolated flash flood prospects in-of the above zone. The
positive news in the setup is the moisture anomalies across the
region of interest remain near to just slightly above normal with
NAEFS anomalies signaling less than +1 deviation for the area.
This will limit the upper threshold of flood concerns for the
setup, but some of the heavier cores could still cause some flash
flood concerns within more urbanized settings and over some of the
terrain focused areas of Southern MO, as well as Eastern KY/TN. The
previous MRGL risk was generally maintained with only some minor
adjustments on the northern periphery of the risk area.
...Southeast U.S...
A shortwave trough over the Southern Plains will roll eastward
overnight and through the period with focused ascent within the
confines of the Southeastern U.S. Weak jet coupling from a vacating
jet over the Tennessee Valley and approaching jet from the west
will create a period of stronger forcing in-of the Central Gulf
Coast early this morning with convective activity rooted in a
narrow corridor of elevated theta_E's stemming from southerly flow
near the coast. Expectation is for scattered heavy convection to
initiate across Southern LA and migrate eastward with the stronger
forcing advancing across the Southeast. Scattered thunderstorms
will be found across off the Southeast by late-morning and beyond,
but the progressive nature of the threat should limit the chances
for widespread flash flooding. QPF mean between 1-2" is forecast
along the Central Gulf Coast with some deterministic maxima between
4-5" showing up within the 00z CAMs. This put modest (30-50%) HREF
neighborhood probs for >3" within that zone between New Orleans to
Mobile, but even still that signature is not going to be enough to
move the needle outside a MRGL threat. A cold front will sweep
through the region from northwest to southeast leading to a sharp
shift in the environmental moisture availability and limiting
instability region-wide. This will ultimately end the threat for
any convection and put an end to any risk in place. For now, the
MRGL risk remains across the Southeastern U.S. with emphasis on the
Central Gulf Coast for best threat in the period.
Further east over Southeastern SC, another round of strong
thunderstorms will likely occur late-afternoon with the best flash
flood risk likely within more urban zones like Charleston and small
areas like Hilton Head. This actually the zone with the highest >3"
probs in the country, but again, the threat is very localized for
flash flood concerns considering the sandier soils prompting higher
FFG's. In this case, the threat will continue to be more in-line
with a MRGL risk leading to general maintenance of the previous
forecast.
...Southern Plains...
Increased ascent within the confines of a cold front pressing south
through the Southern Plains will lead to a corridor of heavy
rainfall developing upstream across the Caprock, moving southeast
within the mean flow and expected upwind trajectory of any cold
pool generation. Regional instability parameters point to a
sufficiently buoyant environment along and ahead of the advancing
cold front leading to a suitable MCS maintenance regime that will
begin over Eastern NM and eventually work its way southeast through
the rest of the Texas Caprock, Permian Basin, and Concho Valley by
nightfall. HREF EAS probs for >1" are modest (20-40%) for the
probability scheme, but a stronger neighborhood prob for >2"
(50-80%) exists over that area referenced above. Progressive nature
of the eventual complex should curb some of the threat, but
scattered instances of flash flooding will be plausible where the
MCS migrates overhead. The highest threat will likely be within
more urban settings and over the Concho Valley where 3-hr FFG
exceedance probs are highest (30-40%) due to antecedent soil
moisture.
Kleebauer
Day 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
339 AM 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...
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
339 AM 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...
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
Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
322 AM EDT Thu May 29 2025
A series of shortwaves will continue to dig into an amplified
upper trough working slowly off the East Coast and potentially
developing a separated closed low. A wavy trailing front will
settle nearby from the western Atlantic southwestard to the Gulf.
While uncertain lows could act to bring some activity into coastal
areas up the Seaboard Sunday and into next week, particular runoff
issues may develop with time for south Florida urban areas given
deepest lower latitude moisture availabiliy and support to monitor.
Guidance continues to overall favor upper trough amplification with
multiple systems into the West along with lead ejection of a
activty focusing southern stream closed system from offshore Baja
through the Southwest into the Rockies. This transition will act
to provide relief from weekend heat over portions of the West with
the building of a much more unsettled and increasingly wet flow
pattern farther inland and downstream over the Intermountain West
and especially the northern Rockies through midweek. Despite the
calendar flipping into June, this pattern will support some threat
for enhanced mountain snow in the highest elevations of the
northern Rockies. Faborable upper support and frontal translation
along with downstream cyclo/frontogenesis over the Plains into next
week should also act to enhance northern Rockies to northern
Plains precipitation and emerging convective pattern to develop
over the north-central U.S. during next week.
Schichtel
Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
322 AM EDT Thu May 29 2025
A series of shortwaves will continue to dig into an amplified
upper trough working slowly off the East Coast and potentially
developing a separated closed low. A wavy trailing front will
settle nearby from the western Atlantic southwestard to the Gulf.
While uncertain lows could act to bring some activity into coastal
areas up the Seaboard Sunday and into next week, particular runoff
issues may develop with time for south Florida urban areas given
deepest lower latitude moisture availabiliy and support to monitor.
Guidance continues to overall favor upper trough amplification with
multiple systems into the West along with lead ejection of a
activty focusing southern stream closed system from offshore Baja
through the Southwest into the Rockies. This transition will act
to provide relief from weekend heat over portions of the West with
the building of a much more unsettled and increasingly wet flow
pattern farther inland and downstream over the Intermountain West
and especially the northern Rockies through midweek. Despite the
calendar flipping into June, this pattern will support some threat
for enhanced mountain snow in the highest elevations of the
northern Rockies. Faborable upper support and frontal translation
along with downstream cyclo/frontogenesis over the Plains into next
week should also act to enhance northern Rockies to northern
Plains precipitation and emerging convective pattern to develop
over the north-central U.S. during next week.
Schichtel
