Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
335 PM EDT Fri May 9 2025
Day 2
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 previous forecast remains on track for both the
Southeast and Northeastern U.S MRGL risk areas as the overall run
to run consistency in the synoptic pattern has yielded similar QPF
outputs within the recent 12z guidance. The heaviest rain across
the Southeast will translate eastward due to the mean trough
pattern over the Southern Plains shifting more towards the
Mississippi Valley longitude. Surface low across the Gulf will
occlude and wander north and northwest during the course of the
period with deep layer flow basically aligned south to north from
the FL Panhandle up through AL/GA. Surface and upper lows will
become vertically stacked by Saturday afternoon leading to slow
storm motions under the parent circulation with a steady push south
to north within the eastern flank of the circulations. Scattered
to numerous thunderstorms will accompany both areas of interest
leading to the risk area encompassing a sprawling zone with the
cyclonic arc back towards the Mississippi Valley on the northwest
side of the circulation.

Stalled front along the Southeast Atlantic coast extending through
Southern GA will act as an inflection point for convection with
mean storm motions likely aligned with the boundary as you work
from southwest to northeast (Tallahassee to Charleston, SC line).
This will lead to a secondary maxima developing over the course of
Saturday morning and afternoon, a consistent signature within the
latest CAMs. The primary axis of interest will reside over the
Western FL Peninsula up through the AL/GA line till about the I-20
corridor. Despite the overall setup indicating 1-3" of rainfall
with locally higher in that vicinity, the area in question is under
the influence of a D0-2 drought so the threat of flash flooding
with the current forecasted rates should help curb the threat
initially (See D3 discussion below). The MRGL was maintained
outside some expansions on the northern periphery of the risk area
to align with current QPF.

Northeast U.S. saw the western edge towards VT brought back further
east as the forecast surface low progression and subsequent dry air
advection pattern behind the vacating low will yield less of a risk
of persistent rainfall in that portion of New England. There was
enough consensus to trim part of the risk area to account for the
changes leading to NH and ME as the only areas where the risk was
maintained. QPF details remain steadfast from previous forecast
with locally moderate to heavy rainfall leading to an areal
average of 0.75-1.5" of rainfall with upwards of 2" plausible over
Central ME down towards the Southern ME coast.

Kleebauer


..Previous Discussions..

...Southeast...

A stationary boundary will remain draped across the Southeast,
with several waves of low pressure expected to form along the
boundary over the weekend as upper level short waves pivot around
the upper low over the region. Compared to Friday, the QPF
footprint will translate east into areas with relatively drier
antecedent conditions. Models are agreeable that widespread
rainfall totals of 1-2 inches are expected across southern and
central Alabama and Georgia and North Florida. Locally higher
amounts above 2 inches will also be possible, but exactly where may
vary within the region. The highest QPF will likely be along the
immediate coast in the Florida panhandle as convection moves
onshore, and training onshore convection will likely pose the
highest threat for flash flooding. Efficient rain rates (2+ inches
per hour) are expected due to a saturated atmosphere with ample
CAPE and shear to support thunderstorms. Flash flooding may be
fairly limited to urban and poor drainage areas given the higher
FFGs (3-5 inches). To account for this, a Marginal Risk area is in
place across much of the Southeast.

...New England...

As a coastal low tracks south of the coast, moderate precipitation
will focus over New England on the northern side of the system.
The low is expected to be fairly progressive, which might help to
limit rainfall totals, but a decent swatch of rainfall is forecast
from Vermont and New Hampshire through southern Maine. Soils in
Vermont and New Hampshire are expected to be somewhat sensitive
given heavy rains expected today. Rainfall will be mostly
stratiform across New England, limiting rain rates, but there will
likely be enough moisture and instability to produce localized
rates of 1+ inches over the course of a few hours, which may come
close to 3/6 hr FFGs. A Marginal Risk area remains in place across
Vermont, New Hampshire, and most of Maine, but northwestern Maine
was removed from the Marginal as model QPF has trended downwards.

Dolan


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