743
FOUS30 KWBC 280841
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
441 AM EDT Fri Jun 28 2024

Day 1
Valid 12Z Fri Jun 28 2024 - 12Z Sat Jun 29 2024


...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE CENTRAL PLAINS TO THE MIDWEST TODAY AND TONIGHT...

The latest guidance continues to show a slight southward/slower
shift compared with earlier runs...and tended to follow suit...as
yet another shortwave makes its way eastward across the Plains.
This next setup has more potential due to the addition of a cold
front that will be progressing southeast out of the Upper Midwest.
With ample moisture in place ahead of the shortwave, a boundary to
help focus convection an low level mean flow roughly aligned with
the boundary...the potential for training cells/repeat convection
with some intense downpours sets up a risk of flooding especially
if there is overlap with the heavy rainfall footprint from the past
10 days to 2 weeks from northwest Missouri into portions of Iowa
as well as from overnight convection leading up to the start of the
Day 1 ERO. A Marginal Risk area extends northward into the western
portion of the Great Lakes region ahead of the northern stream
system scoots along the international border this afternoon and
evening.

Bann

Day 2
Valid 12Z Sat Jun 29 2024 - 12Z Sun Jun 30 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE EASTERN GREAT LAKES AND UPSTATE NEW YORK...

Coverage of showers and thunderstorms should be increasing over
portions of the Ohio Valley prior to the start of the Day 2 outlook
period at 29/12Z. Precipitable water values look to be approaching
2 inches over parts of the Ohio Valley will be in place by the time
height falls from a sharpening mid-level trough approaching from
Canada arrives. The broad-scale ascent afforded by the approaching
shortwave...the deep moisture and wet bulb zero values at or above
10 kft suggest some intense downpours are possible. With at least
some risk of cell training and perhaps some interaction with
terrain..felt an upgrade to Slight Risk was needed...although the
antecedent conditions may mitigate the flooding potential
somewhat. A second Slight risk was introduced across portions of
upstate New York which was more hydrologic sensitivity than the
Ohio Valley given recent rains. Concern for excessive rainfall
extends southward and westward from these Slight Risk areas
along/near a cold front making its way into the Tennessee
Valley/Central Plains and southern Rockies. Precipitable water
values in the 2 to 2.25 inch range should be pooling along the
front from the eastern seaboard as far west as Arkansas and
Oklahoma by 30/00Z. Model guidance shows a lot of spread in this
region and a Marginal Risk was maintained due to the potential for
any cells that form to be efficient rain makers.Bann

Bann

Day 3
Valid 12Z Sun Jun 30 2024 - 12Z Mon Jul 01 2024

...THERE IS A MARGINAL RISK OVER PORTIONS OF THE EASTERN U.S. /
SOUTHWEST U.S. and WESTERN HIGH PLAINS...

...Eastern U.S...
Showers and thunderstorms capable of producing heavy to possibly
excessive rainfall will persist along and ahead of a progressive
front associated with a northern stream trough. Current guidance
continues to focus the heaviest rainfall amounts and rates over
portions of the Mid-Atlantic seaboard at the time of maximum
instability extending into portions of the coastal Carolinas.
Overall the upper dynamics should keep cells moving but with
precipitable water values approaching 2 inches along/near the
front...flooding is possible in areas of poor drainage and urban
areas/

...Southwest U.S...
Moisture begins to get drawn northward from Mexico on the western
side of a sprawling mid-level high over the Southern Plains.
Isolated to widely scattered late afternoon and evening convection
has the potential to produce locally heavy rainfall as the sub-
cloud layer moistens up in response to precipitable water values
over 1.5 inches crossing the border into southern Arizona by early
evening.

...Plains of Western North Dakota and eastern Montana...
A mid-level trough will be moving out of the Northwestern US and
emerging from the northern Rockies by afternoon helping to focus
late day convection. Any convection that forms during the day
should encounter deeper moisture over North Dakota by the
evening...leading to a threat of excessive rainfall. Model
guidance does show increasing moisture transport into the region by
a strengthening low level jet during the evening with at least some
concern for convection to persist into the night given the model
shear profile.

Bann


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