Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
340 AM EDT Mon Mar 31 2025

Day 1
Valid 12Z Mon Mar 31 2025 - 12Z Tue Apr 01 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
CENTRAL GULF COAST...

..Central Gulf Coast..

A fairly amplified longwave pattern over CONUS with a deep low
over Ontario will allow for a trailing cold front to slowly
propagate east-southeastward through the Deep South with a defined
convective footprint along and ahead of the frontal boundary.
Recent trends have shifted the previous QPF max closer to
southeastern LA within the axis of the stronger LLJ providing the
best regional shear and low-level convergence progs near the
southwestern edge of the front. 00z HREF neighborhood probs for >2"
are running between 60-90% within the confines of the southeastern
parishes, including the urban corridor between Baton Rouge to New
Orleans. Considering the urban factors and the previous heavy rain
impacts over the past 36 hrs, especially within Baton Rouge, the
prospects for flash flooding are relatively elevated. Local PWATs
will trend between 1.7-2.0" when assessing the HREF mean, a
suitable environment for increased rates approaching up to 2"/hr.
This falls within the lower threshold for flash flood potential
according to the recent FFG's in place.

The threat remains focused within the zone of southeastern LA and
southern MS where the best frontal convergence and instability is
co-located within the bounds of the front. Heavier QPE footprint
lingering over towards MOB from the past 12 hrs will allow for a
heightened concern for flash flooding as the convective
cluster/line progresses eastward. Thankfully, models have backed
off a bit on the heavier precip being located over the areas that
were heavily impacted this afternoon/evening. There's still an
opportunity for a few heavier cells to impact that portion of the
I-10 corridor from Biloxi to Mobile, so wanted to maintain some
continuity from the previous forecast SLGT in the area to cover for
the sensitivity factors. The main change was the broad SLGT
coverage was shifted westward into the southeastern LA parishes and
southern MS to encompass the highest probs for 2-4" of rainfall,
most of which is forecast to fall within a 3-6 hr time frame
between 12-18z. A broad MRGL exists over the Deep South into the
southern Appalachians for isolated threats of heavier precip within
the confines of the terrain of eastern TN, north GA, and the
western edge of Upstate SC.

..Mid Atlantic..

The deep longwave pattern across the eastern half of the CONUS will
translate into a bout of heavier rain across the Mid Atlantic
later this afternoon and evening thanks to strengthening right-
entrance region jet dynamics that will develop over the course of
the period. Recent guidance has been keen on an increasingly potent
upper jet motioning across the eastern Great Lakes with forecast
speeds approaching 140-150kts as it traverses north of the
northeastern CONUS during the middle of the period. Considering the
deep layer ascent provided by the jet being coupled with sufficient
low to mid-level moisture advection occurring along and ahead of
the front/mean trough progression, the pattern will yield a
blossoming of convection beginning over the northern Mid Atlantic,
eventually building southward towards a weak frontal wave
approaching from the southwest as it exits out of the Tennessee
Valley. A line of heavy rain is likely over parts of northern VA up
through the Delmarva into eastern PA and NJ with the northern
extent reaching the lower Hudson into NYC and eastern LI as the low
rides northeastward. The max potential for this convective
signature is capped due to the progressive nature of the precip
field with most of the rain falling in a 3 hr window as it advances
eastward. Neighborhood probs are elevated for >1" totals (60-90%)
within a corridor from northern VA up through NYC, however the
probabilities drop off considerably when moving closer to 2"
(10-30%), a testament to the progressive nature of the pattern. The
areal footprint of urbanization is one of the main reasons why the
threat of flash flooding exists in this scenario leading to a MRGL
risk maintained over the above corridor. There was an expansion
further southwest into northern VA and the DC metro to account for
recent trends within the probability fields and the bias for
convection to develop further southwest within a frontal dynamic.

Kleebauer


Day 2
Valid 12Z Tue Apr 01 2025 - 12Z Wed Apr 02 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE CENTRAL PLAINS & MIDWEST...

A strong mid-level trough ejecting out of the western CONUS will
aid in initiating a deep surface cyclone across the High Plains of
the Central U.S. by late-Tuesday through Wednesday morning. The
process is manifested through a well-defined axis of difluence
downstream of the mean trough/closed-low reflection over the
Rockies, kicking eastward into the plains. Current deterministic
output indicates sub-990mb pressures with continued intensification
through the end of the period generating significant low-level
moisture advection within the lead side of the circulation when
assessing the forecast 850-700mb RH field. A textbook warm conveyor
belt signature will mature with a connection based from the Gulf
driving warm, moist air poleward and enhancing a blossoming QPF
shield by the time we reach Tuesday evening. The primary focus for
heavy rainfall will ensue after sunset as the initiation of a
powerful LLJ across the Central Plains on the order of 65-75kts
will create an axis of growing low to mid-level shear juxtaposed
within a budding warm sector downstream of the primary circulation.
As the surface cyclone matures, a more defined cold front will
develop along the tail end of the low as it moves into the Plains
creating a focus for convective development as it maneuvers towards
the warm sector. Most guidance now indicate a rapid development of
convection across portions of northern KS into southeast NE
advancing east-northeast within the bounds of the cold front and
warm sector of the cyclone.

The normally conservative NBM probs for >1" have grown to 50-60%
within a area focused over northeastern KS, southeast NE, into
western IA with some 30-50% probs extending southwest within the
southern edge of the convective flare up along the cold front. Most
deterministic output signals 1-2" with locally higher totals
embedded within those above zones. ML output has been fairly
consistent with that area being the primary focus for the heaviest
QPF core which makes sense synoptically considering the expected
rapid cyclone intensification. Warm front over the Mississippi
Valley will have to be monitored for some enhanced convective
concerns as it motions to the north with some guidance dropping
appreciable precip in an area that has some relatively lower FFG
signals. The previous MRGL was maintained with some expansion south
and east to address the extension of the forecast heavier
convection across the maturing warm sector and within the axis of
strongest isentropic ascent over western IL.

Kleebauer


Day 3
Valid 12Z Wed Apr 02 2025 - 12Z Thu Apr 03 2025

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE LOWER MISSISSIPPI AND OHIO VALLEYS...

The D3 period will represent the initial stages of a prolonged
heavy rain event that will reside over the Lower Mississippi and
Ohio Valleys with a multitude of frontal waves riding along a
persistent quasi-stationary front that will undulate at times when
interacting with the aforementioned surface waves. The general
longwave pattern will become increasingly favorable for a "highway"
of moisture and mid-level impulses to eject northeastward out of
the southern Rockies/Plains and up through the Mississippi and Ohio
Valleys beginning Wednesday afternoon, onward. The consensus among
all ensembles and much of the deterministic fields are the first
wave of the pattern to maneuver over the mid-Mississippi Valley
around the base of a broad trough located over the Plains/Midwest
creating ample surface-based ascent to couple with the powerful
upper jet component situated over the Central CONUS in response to
the evolving pattern the day prior.

At the surface, a cold front will run into a stout western ridge
extension from a surface high located over the Western Atlantic
into the Canadian Maritimes. The pattern essentially comes to a
stalemate thanks to an expected +2 to +3 standard deviation height
anomaly centered over the Western Atlantic. This ridge will counter
the deepening trough over the Central U.S. creating a funneling
affect for deep moisture advection through the Mississippi and Ohio
Valley's, along with ample mid-level impulses that will be preside
over the same areas. The gridlocked setup will create a formidable
moderate to heavy rain event across the above areas with
depictions from guidance indicating a widespread 2-4" with some
areas exceeding 5" increasingly likely within the corridor impacted
by the multitude of convective elements spawning southwest to
northeast in proxy to the quasi- stationary front. A Moderate Risk
remains over an area situated from northeast AR up through the mid-
Mississippi Valley of western TN/KY into southern IN. A broad
Slight Risk encompasses the MDT with a footprint extending as far
north as Lower Michigan.

This is an increasingly significant setup approaching with
potential for high impacts and life-threatening flash flooding
spanning the course of several days. Be sure to prepare if you live
in a flood zone anywhere from Arkansas, northeast through the
Ohio/Tennessee Valley's. Anyone surrounding will want to monitor
this setup closely as small changes could have heightened impacts
given the forecasted setup.

Kleebauer


Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: www.wpc.ncep.noaa.gov/qpf/99epoints.txt