729
FOUS30 KWBC 072012
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
412 PM EDT Sat Jun 7 2025

Day 1
Valid 16Z Sat Jun 07 2025 - 12Z Sun Jun 08 2025

...THERE IS A RISK OF EXCESSIVE RAINFALL ACROSS MUCH OF THE EASTERN
UNITED STATES...WITH SLIGHT RISKS IN SOUTHERN NEW ENGLAND... THE
OHIO RIVER VALLEY...AND MID-SOUTH...

An active day of heavy rainfall and flash flooding is expected
across a large portion of the eastern United States, with the
Marginal Risk area now encompassing the majority of the land area
in the CONUS east of 95W longitude. This is due to a broad region
of near to above normal tropospheric moisture (via PW anomalies)
and sufficient instability for convective rain bands and
thunderstorms.

Generally, the broad zone of heavy rainfall risk can be divided
into three main areas of concern, all indicated by a regional-scale
Slight Risk maximum now:

1. In the Northeast, where low-topped convection has organized into
narrow bands of efficient heavy rainfall. MRMS analysis indicates
almost no ice present in these convective bands, with 30 dBZ echo
tops generally shy of the -10C level. In other words, warm rain
processes are dominant and any organized convective rain band could
produce localized heavy rainfall. The area of greatest concern is
probably from active area of flash flooding in the Hudson Valley
near Albany, NY eastward through Massachusetts and northern
Connecticut to the Boston metro area. There should naturally be
increasing low-level convergence with the existing front draped
across the region, and increasing easterly flow near the coast
around the north side of the offshore low. Aloft, a coupled jet
structure should lead to enough divergence to focus convection.

2. The Ohio River Valley region, particularly the middle and upper
Ohio Valley from late this afternoon well into the overnight hours.
This will be due to scattered convective rain bands and perhaps
some thunderstorms developing in advance of an eastward propagating
mid-upper level wave, and developing surface low by this afternoon,
currently centered over IA- MO. This should allow for some
organized convective bands in the warm air advection regime. Model
forecast soundings show increasing PWs to near or above the 90th
percentile for early June and tall, skinny CAPE profiles that are a
good match for efficient, low-topped convective flash flood events.
Of particular concern will be the potential for flash flooding
overnight in the central Appalachian region from eastern Kentucky
and southeast Ohio into West Virginia and southwest Pennsylvania.
Models show southwest low-level inflow increasing into the
mountains in a very moist environment, with the right entrance
region of an upper level jet streak providing divergence aloft. All
of this should support an organized heavy rain threat and the
structure of the instability profile will keep convection low-
topped and dominated by warm rain processes. Locally significant
flash flooding will be possible given the terrain in the area and
relatively wet antecedent conditions.

3. Across the mid-South where a forward propagating MCS has already
generated some flash flooding in central Arkansas, but appears to
begin accelerating. Renewed development and/or stalling and
training will be possible along the southern periphery of the cold
pool later today, and hi-res models do show localized corridors of
heavy rainfall that would be sufficient to cause flash flooding.
The Slight Risk was shifted south to account for observational
trends indicating the current position of the cold pool and
convective line, and the projected motion over the next few
hours.

Lamers


Day 2
Valid 12Z Sun Jun 08 2025 - 12Z Mon Jun 09 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL IN THE CENTRAL
APPALACHIANS AND MID ATLANTIC... AS WELL AS THE SOUTHERN PLAINS...

...Central Appalachians into the Mid-Atlantic...
The heavy rain and associated flash flood threat is likely to be
ongoing across portions of the region at the beginning of the
outlook period (12 UTC Sunday). Model guidance today has generally
begun to focus the QPF signal across the region such that there is
much better consistency between the global ensembles and hi-res
ensembles (both RRFS experimental ensemble and HREF operational).
Therefore, confidence was sufficiently high to go with a relatively
big departure from the existing more focused Marginal Risk area,
and go with a broad Slight Risk. The biggest change was to areas of
eastern Virginia and central Maryland, which previously had ERO
probabilities of less than 5 percent, but hi-res guidance is now
consistently showing some localized maxima in excess of 3 inches
into the coastal plain.

Greatest concern within the Slight Risk area is in the Allegheny
Mountains, Potomac Highlands and Laurel Highlands of northeast
West Virginia, the Maryland Panhandle, far southwest Pennsylvania,
as well as the Shenandoah Valley and Blue Ridge Mountain region of
far northwest Virginia. These areas have been very wet lately, with
30-day rainfall double to triple the normal amount, and a
significant portion of the same area received between 1 and 4
inches of rain in the past day or so.

Rainfall may be enhanced in these areas by a subtle southeasterly
low-level jet that should increase between 06 UTC and 18 UTC Sunday
due to a developing low level cyclone ahead of an eastward
propagating mid-upper level wave. If the low becomes situated near
the OH-WV border by the beginning of the outlook period, which has
been a trend over the past couple model cycles (along with a deeper
low overall), the LLJ would be likely to be enhanced. This may
create orographic enhancement to the rainfall and could help some
convective features and bands persist for a greater duration where
the orographic enhancement is most pronounced. Any convection is
likely to be low-topped, with model forecast soundings showing a
high melting level (above 12,000 feet) and a low equilibrium level
(between -10C and -20C), with PWs above the 90th percentile for
early June. This would be likely to lead to dominance of
efficient, warm rain processes with little to no ice present in the
cloud. This could support rapidly developing flash flooding where
convective features can persist.

Meanwhile, further east into the coastal plain and I-95 urban
corridor from Richmond to Baltimore, models have also converged on
an increased heavy rainfall threat. In these areas, greater
instability will be present -- enough to support organized clusters
of thunderstorms that could produce very heavy rain rates at times.
The heavy rain should be less persistent as compared to the
mountainous areas, but that would be offset by higher instantaneous
rain rates. Therefore, these areas were also included in the Slight
Risk and could see some flash flooding impacts.

...Southern Plains to the Southeast...
Fewer changes were made to the existing Day 2 outlook from the
Southern Plains into the Gulf Coast region. A Slight Risk remains
centered on North Texas and southern Oklahoma, and many models now
show a rainfall maxima in excess of 3 inches somewhere in the
area. Although a forward propagating MCS may develop with time,
there is enough signal in the model guidance for a corridor of
enhanced QPF that some training of intense convection seems
possible. This would be most likely either via (1) individual cells
and clusters developing near the front ahead of the advancing
convective line and subsequent cell mergers, or (2) slowing or
stalling of the convective line on the southwest periphery of the
developing cold pool. If either of these scenarios develops, flash
flooding would be possible via heavy rainfall sustained for a
couple hours at any one location.

Although the Gulf Coast is not in a Slight Risk at this time, it
will need to be monitored, as some (but not all) recent hi-res
model runs show significant localized rainfall extremes in excess
of 6 inches. An examination of individual model runs indicates this
would be most likely where convective outflows interact with the
coastal convergence zone.

Lamers


Day 3
Valid 12Z Mon Jun 09 2025 - 12Z Tue Jun 10 2025

...THERE IS A MARGINAL OF EXCESSIVE RAINFALL ACROSS A LARGE PORTION
OF THE SOUTHERN AND EASTERN UNITED STATES...

Very few changes were made to the existing Marginal Risk, which
encompassed a broad area in the southern and eastern United States
along and ahead of a slowly advancing cold front. The air mass
ahead of the front will be characterized by above normal PWs and
widespread moderate to strong instability. Therefore, a day with
abundant areas of active convection containing relatively high
rain rates is expected. Confidence is not very high at this time in
the detail, particularly where concentrated areas of heavy rainfall
and/or localized extreme events may occur. However, based on
today`s model guidance, this seems most likely in the following
locations, where a Slight Risk upgrade may eventually be needed:

1. Eastern Great Lakes and Central Appalachians. Instability is
projected to be more limited in these areas, but recent rainfall
may lead to wet conditions in advance, and there is better model
consensus on areas of over 1 inch QPF.

2. The Gulf Coast. A combination of strong instability and high PWs
would support organized convection and high rain rates, but there
is less model agreement on placement.

3. West Texas into Northeast New Mexico. PWs will be increasing and
quite anomalous (over the 90th percentile) along with fairly strong
instability. These areas will be removed from stronger mid-upper
level flow, likely leading to overall slower storm motions.

Lamers

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