FOUS30 KWBC 271907
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
307 PM EDT Sat Jun 27 2026

Day 1
Valid 16Z Sat Jun 27 2026 - 12Z Sun Jun 28 2026

...THERE IS A MODERATE OF EXCESSIVE RAINFALL OVER PORTIONS OF THE
OHIO VALLEY...

1600Z Update...

Based on a consensus of the 12Z HREF/REFS guidance along with
recent HRRR/RRFS solutions, and the current radar/satellite trends,
it has been decided to upgrade portions of the OH Valley to a
Moderate Risk of excessive rainfall. Multiple rounds of very heavy
rainfall with locally extreme rainfall rates of 2 to 3 inches/hour
have been occurring already this morning locally across areas of
far southern IN down through central KY with rather widespread
flash flooding ongoing. The latest satellite imagery shows a parade
of MCVs transiting the Mid MS and OH Valley region with the deeper
layer, but very moist west-southwest flow across the region. Very
high PWs running 2 to 2.5+ standard deviations above normal and
tall, skinny CAPE profiles support high-end rainfall potential and
especially with weak and locally opposing Corfidi vectors across
sizable areas of the OH Valley. There is some variance with respect
to the details of the convective band evolutions going through this
afternoon and evening as basically each vort center will be
potentially able to focus its own axis of very heavy rainfall.
There is evidence to support a nocturnal low-level jet enhancement
tonight with respect to the current MCV over southern MO as this
feature advances downstream into western and central KY. Several
hires CAM members support a threat for a high-end training band of
convection over parts of central/southern KY and possibly northern
TN. Additional rainfall amounts near and adjacent to the Moderate
Risk area could locally approach 3 to 6+ inches.

Elsewhere, an expansion of the Marginal Risk area was accommodated
across the northern Plains in connection to the deeper layer trough
over the Intermountain West that will be ejecting gradually off to
the east. Expecting a rather well-organized outbreak of
thunderstorms across ares of the northern High Plains with
downstream advance of activity across the Dakotas. Some localized
flash flooding concerns will be possible.

Finally, a Marginal Risk has been introduced across areas of the
southern High Plains and mainly for western and southwest TX as a
channel of deeper layer moisture coupled with the ejection of a
weak MCV/shortwave out of southeast NM helps to initiate and focus
convection with the diurnal heating cycle. Given local terrain-
focused sensitivities, a localized threat of flash flooding will be
possible here later today and this evening.

Orrison

Previous discussion...

...Mid-Mississippi Valley through Southern Appalachians...

Another expansion of a high-end Slight Risk area was made along a
west to east axis based on convection occurring prior to the start
of the outlook period at 27/12Z...with the expectation that the
risk persists through much of the Day 1 period in proximity to an
east-west oriented stationary front.  Low level westerly flow
remains parallel to this boundary, raising concerns for
repeating/training activity. The 27/00Z HRRR and the 27/00 RRFS QPF
placement was similar to each other...but the RRFS appeared to be
showings its bias with higher QPF and higher neighborhood
probabilities of exceedance. The concern is greatest upon overlap
of any additional rainfall with the footprint of recent heavy
rainfall events.

...Mid-Atlantic...

00Z guidance offered a split decision of whether or not a frontal
zone lingers over the central Mid- Atlantic through tonight rather
than shifting shifting south to the Virginia/North Carolina border.
Assuming it stays over the Mid-Atlantic...enhanced moisture and
instability will remain over the DC area metros and the DelMarVa
with a risk of isolated heavy to excessive rainfall otherwise the
focus shifts south with time. Despite that uncertainty...any
storms that form in such an airmass can easily produce localized
downpours and excessive rainfall. This still looks to be a
localized flash flood threat, but a focused Slight Risk may be
needed eventually.

...Northern Plains...

The upper low tracks over the interior Northwest on today with a
shortwave troughing ejecting across northern Wyoming and eastern
Montana by this evening. Low level forcing east of the approaching
wave triggers repeating thunderstorms over eastern Montana through
North Dakota from this afternoon into the evening. Increasing Low
Level Jet should enhance rainfall within the Slight Risk
maintained/expanded eastward in North Dakota. Activity should be
fairly progressive, but the route of the upper trough may
pivot/allow repeating activity.

Bann/Jackson


Day 2
Valid 12Z Sun Jun 28 2026 - 12Z Mon Jun 29 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS EASTERN
NORTH CAROLINA INTO FAR SOUTHEAST VIRGINIA...

19z Update: A Slight risk was introduced across eastern NC into far
southeast VA. The MCV currently over southern IL will be moving
across NC tomorrow. By afternoon a convective focus ahead of this
feature should occur across the aforementioned Slight risk area.
Guidance indicates a mainly progressive convective mode, however
coverage should be enough to result in some cell mergers and brief
training enhancing rainfall rates/duration. Both the 12z HREF and
REFS indicate a 15-40% chance of FFG exceedance, and rainfall
today/tonight could lower FFG a tad more before then. Thus an
isolated to scattered flash flood threat appears probable tomorrow
afternoon across this region.

The Marginal risk was made broader over the Mid-Atalntic as well.
Isolated to scattered convection over WV into VA likely won't be
as organized as the activity over NC, but could be slower moving
and within a favorable environment for heavy rainfall rates. Over
eastern TN into northern GA a training convective axis could be
ongoing at 12z Sunday, although should be weakening.

A Marginal risk was also added over west TX, where dryline
convection should pose a localized flash flood risk.

Chenard

...Previous Discussion...

...Montana...
Upper low center pivots over Montana Sunday with the surface low
stalling along the international border . This maintains comma
head/deformation zone rain shifting into northwest MT where terrain
enhances rates at lower elevations and snow generally at or above
6500 feet. This is more of an areal flood concern given a lack of
instability, but localized rates may warrant some rapid flooding
concerns where precip bands are most stagnant. 00Z consensus
rainfall remained in the 1 to 3 inch range around Glacier National
Park onto the adjacent high plains in northwest MT. A marginal risk
was maintained for this area.

...Upper Midwest...
The warm sector of this anomalous low over MT will shift eastward
allowing an airmass with robust Gulf moisture advection up the
length of the Great Plains through the Upper Midwest. The
development of surface low pressure over South Dakota will aid the
moisture transport at the low levels and provide a lifting
mechanism. Focusing on this...there was an eastward expansion and a
bit of a southward expansion of the previously issued Marginal
risk area.

...Coastal Carolinas/Mid-Atlantic...
Added a small, targeted Marginal along the eastern seaboard where
precipitation lingers from the Day 1 period. Blending disparate QPF
in such cases tends to yield low amounts...but the area had the
best overlap with highest precipitable water values and an area
where ensemble members with differing cores tended to cluster. With
such high moisture...localized downpours and heavy to excessive
rainfall is a concern from any storm that can develop regardless of
the meager focusing mechanism.

Bann/Jackson


Day 3
Valid 12Z Mon Jun 29 2026 - 12Z Tue Jun 30 2026

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM MONTANA INTO
THE NORTHERN PLAINS AND UPPER MISSISSIPPI VALLEY, AS WELL AS ACROSS
WEST TEXAS...

...Montana into the Northern Plains and Upper MS Valley...
Main change was an expansion of the Marginal risk across this
region. Convection should be ongoing Monday morning across
portions of the Dakotas within a highly unstable airmass with very
strong low level moisture transport. This activity should then
push into central/northern MN through the day. Main uncertainty
deals with the latitude of the convective axis and mode/movement.
Will note that the environment is quite impressive...with extreme
CAPE, very strong moisture transport, high PWs and weak Corfidi
vectors. Thus the potential is certainly there for an axis of
backbuilding convection and a locally significant flash flood risk.
However, it's also a possibility that the strong mean flow and
somewhat progressive forcing help move storms along and limit the
coverage/magnitude of the flash flood risk. So we will just need to
continue to monitor trends over the coming days.

While mostly stratiform rain by this time, event total rainfall of
2-4" is expected across portions of central and western MT. Even
though rainfall rates by this time should be low enough to preclude
much of a flash flood risk, this is an impressive amount of rain
for late June over this area, and thus some areal/stream flood
impacts appear probable. Thus we will include them within the
Marginal risk.

...Southwest Texas...
Increasing instability should lead to late day convective
development along and near the dry line in west Texas. Isolated
flash flooding is again possible

Chenard


Day 4 and Day 5

Valid 12Z Tue Jun 30 2026 - 12Z Thu Jul 02 2026

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS NEW
ENGLAND, THE UPPER MIDWEST, SOUTHERN HIGH PLAINS, & CENTRAL GULF
COAST...

...Northern New York State into Northern New England...
Shortwave energy riding up over the top of the Ohio Valley upper
high across southeast Canada into northern New England will help
strengthen the low level westerly flow into the frontal boundary
forecast to stretch northwest to southeast from eastern Ontario
into the region. There is potential for organized convection late
on the 30th/early on the 1st near this boundary. There is a strong
enough model signal for organized convection moving northwest to
southeast resulting in heavy rains and isolated runoff issues,
enough for a Marginal Risk, but not cohesive enough amongst the
guidance for a stronger risk level.

...Southern High Plains...
PW values expected to rise to 1 to 1.5+ standard deviations above
the mean in the broad southwest mid to upper level flow across the
Southern High Plains on the west side of the Lower MS closed upper
high. Embedded shortwaves in this southwest mid to upper level flow
will support increasingly scattered convection in this high PW
axis, with locally heavy rains and isolated runoff issues possible
causing problems in burn scars and dry washes/arroyos.

...Upper MS Valley/Upper Lakes...
Convection expected to become increasingly active Tuesday into
early Thursday along and ahead of the surface front moving slowly
across the Upper MS Valley. Favorable right entrance region jet
dynamics ahead of this front in an axis of above average PW values
will support potential for heavy rains with organized convection
along and ahead of this cold front with isolated runoff issues
possible. There was a southward adjustment in the guidance away
from the International Border into portions of the Great
Lakes/Northern Plains/Upper Midwest, which caused a similar shift
in the existing Marginal Risk area. Additional southerly shifts
cannot be ruled out, given the flow pattern expected.

...Central Gulf Coast...
Some of the guidance has a signal for heavy rainfall across the
area, caused by an easterly wave coming off the tail end of a
frontal boundary. Considering the moisture and instability
expected, at least urban areas could receive flash flooding so
added a Marginal Risk for this possibility.


Roth


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



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