Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
406 AM EDT Wed Jul 28 2021

Day 1
Valid 12Z Wed Jul 28 2021 - 12Z Thu Jul 29 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE UPPER GREAT LAKES INTO THE UPPER MIDWEST...

...Upper Midwest/Great Lakes...
The next shortwave trough makes its way across the area during the
period with sufficient instability and PW values reaching 1.75 to
2.0 inches. Soils have been primed through previous days rainfall.
Soil moisture is already sitting in upwards of the 98th percentile
and with previous days QPF, the day 1 periods will see much lower
FFG than what is currently seen, which is already low. Flash
flooding concerns are on the higher end confidence wise. The
Slight Risk area has been adjusted for portions of WI into MI to
better align with latest QPF forecast and model guidance. The
Marginal Risk area already in play for this region was expanded
slightly to account for differences among the models and latest
QPF forecasts.

...Great Basin/Intermountain West...
Monsoonal moisture continues to surge north across the region with
weak mid-level impulses helping to focus convection farther north.
This becomes no more evident than by the precipitable water values
remaining above 1.25 inches in some locations under broad
southerly low level flow. With plenty of diurnal heating,
anticipate MUCAPE to climb above 1000 J/kg allowing for
differential heating across the terrain. Therefore, afternoon
convection will largely be tied to the mountains and eventually
shift off into the valleys/deserts through the late
afternoon/evening. Hourly rain rates could reach between 0.5-1
inches, though isolated higher amounts are possible, especially
with slow moving convection. Given the complex terrain, wet
antecedent conditions across the Southwest/Central-Southern
Rockies, and burn scars, localized flash flooding could occur and
the Marginal Risk area has been maintained.

...Southeast...
A boundary should stall near the FL/GA line extending west to the
central Gulf Coast. Convergence of tropical air along this
boundary should allow heavy enough rainfall to locally exceed the
currently high flash flood guidance and combined with previous
days QPF, the Marginal Risk area was kept in play.

Chiari




Day 2
Valid 12Z Wed Jul 28 2021 - 12Z Thu Jul 29 2021

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS
OF ARIZONA...

...Southwest into the Northern Rockies...
Monsoonal moisture will continue to cause flooding concerns for
portions of the Southwest northward into the Northern Rockies. Low
FFG and periods of rain leading up to this period all lead to
concerns for flooding. Both the Slight Risk area and the Marginal
Risk area were adjusted to account for latest model guidance and
updated QPF forecast. PW values continue to exceed 1.00 inches
with anomalies reaching 2.5-3 std deviations above climo for these
areas.

...Northeast south through the Ohio Valley...
The upper trough will continue to bring periods of heavy rains to
this region with latest guidance showing signals of 1 to 3 inches.
Much of this area sits near 150-400% of normal for precipitation
in the past few weeks and with antecedent conditions priming soils
even more, flooding could be of concern for this period. PW values
sit near 1.5-1.75 inches for much of this area during this time.

...Eastern Florida coastline...
Tropical moisture will continue to pump into the region creating
periods of heavy rainfall. PW values remain near 2.25 inches
during this timeframe with latest model guidance showing signals
of 2 to 4 inches. Urban areas will be the most susceptible to
flooding during this time period.

Chiari

Day 3
Valid 12Z Thu Jul 29 2021 - 12Z Fri Jul 30 2021

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE SOUTHWEST INTO THE NORTHERN ROCKIES AND PARTS OF THE
CENTRAL PLAINS INTO MID MISSISSIPPI VALLEY...

...SOUTHWEST INTO NORTHERN ROCKIES...
Monsoonal moisture will continue to impact the Southwest CONUS
northward into portions of the Northern Rockies. PW values
continue to sit near 1.75 inches for much of this area and with
antecedent conditions making soils even more susceptible to
flooding, confidence in additional flooding events continues into
the day 3 time period. A Marginal Risk was put into effect

...CENTRAL PLAINS INTO MID MS VALLEY...
Troughing across the region coupled with modest instability will
increase chances of heavy rounds of precipitation with convection
and training storms. PW values sit near 2.0 inches during this
time period with MU CAPE values sitting near 2000 J/kg. FFG is on
the higher end for some of these areas and steering flow is on the
faster end, however the possibility of training is the main
concern with some of these storms expected to drop heavier
amounts. Guidance is wavering a bit on where to place the swath of
precipitation and as such, confidence in placement for the
Marginal Risk for this area is on the lower end. Adjustments will
likely be made as newer guidance comes in and hopefully a better
handle on where the band of precipitation will set up.

Chiari




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