Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
258 PM EDT Mon Aug 17 2026
Day 3
Valid 12Z Wed Aug 19 2026 - 12Z Thu Aug 20 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE MID-MISSISSIPPI AND OHIO VALLEYS...

...Mid-Mississippi and Ohio Valley...
Mid-level trough will gradually sharpen across the eastern third of
the CONUS on Wednesday in response to a shortwave diving from the
Central Plains into the Mid-Mississippi Valley. This impulse will
help to amplify the longwave pattern in the east, leading to
increasingly impressive forcing for ascent through height falls and
overlapping RRQ diffluence as an upper jet streaks into New
England. The amplifying trough will help dig a cold front south
towards the Tennessee Valley by the end of D3, and the accompanying
ascent along this front may help spawn a wave of low pressure as
well to enhance lift.

While there is still spread among the models as to how widespread
activity will be on Wednesday (both in space and time due to
multiple possible MCS/convective clusters), any storms will be
influenced by impressive thermodynamics to support rainfall rates
of 1-2"/hr. Bulk shear forecast to be 35-45 kts will help organize
this convection, and with both Corfidi vectors and mean 0-6km winds
aligned to the front, expect there will be at least short-term
training to lengthen the duration of these intense rates. Where
this occurs, 1-3+" of rain is possible, some of this falling atop
primed soils from prior rainfall. The adjusted SLGT risk was
modified to match the greatest probabilities from the GEFS/ECENS,
with some extension in the MRGL risk to capture potentially lower
rainfall, but atop very vulnerable soils into the Ohio Valley from
recent heavy rain.

...Southwest into the Central High Plains...
Upper ridge centered over the Four Corners will re-center and
amplify as an inverted trough pushes west on Wednesday. This will
promote a more favorable setup for monsoonal convection as a surge
of thermodynamics (PWs above 1.25" overlapping MUCAPE of 1000-2000
J/kg) lifting northward behind the inverted trough. At the same
time, weak shortwaves embedded within the flow and rotating around
the ridge will produce areas of locally enhanced ascent, which into
the improving thermodynamics suggests more widespread convective
activity is likely on Wednesday. While a more substantial risk for
greater coverage and intensity exists just beyond D3, a large MRGL
risk for excessive rainfall appears supported from the Deserts of
Arizona northeast into the Central High Plains of CO/KS. With
rainfall rates of 0.5-1"/hr likely at times, and mean storm motions
of just 5-10 kts, (higher in CO but with stronger bulk shear for
organization) this will support at least isolated instances of
flash flooding, especially over vulnerable terrain, urban areas, or
recently saturated soils.

Weiss

Day 3 threat area: www.wpc.ncep.noaa.gov/qpf/99epoints.txt