Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1157 AM EDT Wed Jun 18 2025

Day 1
Valid 16Z Wed Jun 18 2025 - 12Z Thu Jun 19 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
GREAT LAKES AND THE MID-SOUTH STATES...

...Great Lakes through the Mid-Mississippi Valley...
Strung out shortwave embedded within amplifying mid-level flow
will swing northeastward today as the primary trough axis shifts
from the Central Plains to the Ohio Valley. The collocated height
falls/PVA accompanying this energy will interact with the modest
RRQ of a departing upper jet streak to help deepen a surface low
moving into the Great Lakes. This low will strengthen along the
front, leading to scattered/widespread showers and thunderstorms,
but with two focused regions of excessive rainfall -- northern
Illinois across Lower Michigan and from near Memphis into Kentucky.

The greatest risk for heavy rain capable of producing flash flooding
appears to be centered across the lower peninsula (L.P.) of
Michigan. Heavy rain is favored along a warm front ahead of the
low track, and along a deformation axis in the same region with
embedded convection (some occurring already this morning/noontime).
850mb flow ahead of the low will surge to 20-30kts, drawing PWs as
high as 1.75 inches into MI, coincident with MUCAPE nearing 1000
J/kg. This will support rainfall rates that have a moderate chance
(40-60%) of exceeding 1"/hr at times. The heaviest rain rates are
expected along the leading warm front thanks to stronger
instability this evening, and during that time the combination of
20-30 kts of bulk shear and 0-6km mean winds aligned to the front,
despite progressive speeds near 30 kts, will support rainfall that
will likely (70% chance) reach 3", and may reach as much as 5" in a
few isolated locations. This could result in instances of flash
flooding (15-30% chance of 3-hr FFG exceedance) despite generally
dry antecedent conditions, and the SLGT risk was maintained here.

Farther to the south, the cold front will be the primary mechanism
for ascent/convective development as it tracks east from the Central
Plains to the Tennessee Valley. This front will impinge upon intense
thermodynamics characterized by MUCAPE above 3000 J/kg and PWs
approaching 2 inches (>90th percentile). Thunderstorms will likely
develop along the length of this front during peak instability,
and where bulk shear exceeds 25 kts, generally from IN through AR,
clusters of more organized convection are expected. While cells
will generally move rapidly along the front, Corfidi vectors
progged to collapse to just 5-10 kts suggests some
backbuilding/short term training, which could enhance rainfall to
more than 3 inches in some areas (max ensemble rainfall is 5-7").
There is still some latitudinal spread in the axis of highest
rainfall potential between the HREF and REFS members, leading to
lower confidence in placement of heaviest rain. However, the EAS
probabilities from both ensemble clusters, while varying greatly in
magnitude, are focused generally in the same area, and atop a
region of more sensitive soils and lower FFG due to 7-day rainfall
more than 300% of normal. The SLGT risk was maintained in this area
due to the potential for this heavy rain falling atop vulnerable
soil conditions.


...Mid-Atlantic into New England...
Southwest return flow around a Bermuda-type ridge off the
Southeast coast will continue today, pumping elevated moisture
northward into the Mid-Atlantic States. PWs are forecast to reach
around 2 inches (12Z IAD sounding was 2.15" / 99th percentile /
near-record for this time of year per the ESAT). This PW will be
spread through the entire column as reflected by long-skinny CAPE
profiles among the regional soundings, indicating efficient
rainfall processes within thunderstorms that develop along a weak
surface trough skirting across the area. Both the HREF and REFS
probabilities indicate a moderate potential (30-50%) for 1"/hr
rates, which, despite generally progressive motion, could cause
axes of 2-3" of rain where multiple rounds occur (especially over
the Mid-Atlantic DC-PHL-TTN). Recent HREF/RRFS members show
potential for >4" rainfall in this urban corridor. If any of this
heavy rain falls atop these urban areas or more sensitive soils
from recent rain (where FFG is as low as 1-1.5"/3hrs), instances
of flash flooding could result. Maintained the Marginal risk here
due to the expected coverage (more isolated) though local impacts
may be significant depending on the exact placement.

Fracasso/Weiss


Day 2
Valid 12Z Thu Jun 19 2025 - 12Z Fri Jun 20 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM THE MID-
SOUTH THROUGH INTERIOR NEW ENGLAND...

Mid-level trough axis initially positioned across the Mississippi
Valley will become more progressive on Thursday as it weakens in
response to an expanding ridge over the Southern Plains. As this
trough weakens and move quickly towards the Atlantic coast, a
surface cold front beneath it will additionally track eastward,
likely exiting the Mid-Atlantic and New England coasts by Friday
morning, while lingering into the Southeast. This front will
provide a focus for convective development Thursday, especially
during peak heating as downstream SW 850mb flow of 25-35 kts surges
PWs to 1.75+ inches, reflective of an environment that will support
extremely efficient rainfall within thunderstorms. This indicates
that despite the rapid frontal progression, and accompanying
forward motion of thunderstorms (0-6km mean winds 20-25 kts),
intense rainfall rates of 1-2"/hr could still produce isolated
impacts, especially where any repeating storms can occur, or if
heavier rain falls atop urban areas or more sensitive soils.

At this time no SLGT risks have been introduced as storm motions
may be more perpendicular to the front, and thus, mostly
progressive. However, there is at least a subtle signal for some
heavier rainfall across the Central Appalachians. This area is
extremely sensitive due to recent heavy rainfall (7-day rainfall
150-300 percent of normal) which has primed soils and reduced FFG
to 1-2"/3hrs. If this signal increases, a targeted SLGT risk may be
needed across these most vulnerable areas.


Weiss


Day 3
Valid 12Z Fri Jun 20 2025 - 12Z Sat Jun 21 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR THE NORTHERN
GREAT LAKES AND PORTIONS OF THE NORTHERN ROCKIES...

...Northern Great Lakes...
Low pressure emerging from the Northern High Plains will extend a
warm front downstream and into the Upper Midwest/Great Lakes. As
this warm front lifts steadily northeast, especially the latter
half of D3 /Friday night/, it will serve as a focus for blossoming
convection. The downstream amplifying 850mb flow lifting out of the
Gulf will reach as high as 40-50 kts into the Upper Midwest, and
with the presence of the low to the west, these winds will back
steadily to the SE to surge higher PWs, approaching 1.75 inches,
into the warm front and into MN/WI. The combination of moisture
convergence and isentropic ascent invof this front will help expand
clusters of thunderstorms Friday night, leading to an axis of heavy
rain that has a modest potential (10% or less from the ECENS/SREF)
of exceeding 3 inches. Despite high FFG across this region,
isolated flash flooding could result from this rainfall.


...Northern Rockies...
Extremely anomalous mid-level low will dig into the Pacific
Northwest Friday, pushing 500mb heights to below the 00Z minimum of
the climatological database from NAEFS. Where intense height
falls/PVA combine with the increasing diffluence along the LFQ of a
Pacific jet streak, surface pressure falls will result, leading to
a deepening low pressure moving from ID into MT. The increasingly
backed low-level flow ahead of this system will cause 850mb winds
to surge out of the E at 15-25 kts, lifting favorably to upslope
into the terrain while additionally transporting a ribbon of PWs
above 1 inch into the High Plains and banking into the eastern side
of the Rockies. This will cause expanding precipitation, with
embedded rainfall rates above 0.5"/hr where a narrow corridor of
instability exists over northern MT. Locally heavy rainfall of
1-2", with locally higher amounts, will result, leading to the
potential for isolated flash flooding atop FFG that is only
1-1.5"/3hrs.

Weiss

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