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 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt