Short Range Forecast Discussion
NWS Weather Prediction Center College Park MD
355 PM EDT Wed Jul 28 2021
Valid 00Z Thu Jul 29 2021 - 00Z Sat Jul 31 2021
...Severe thunderstorms likely from the Upper Midwest to the Mid-Atlantic
through Friday morning...
...Excessive rainfall with isolated flash flooding possible over parts of
the Upper Midwest through Thursday morning...
...Potentially heavy rainfall fueled by monsoonal moisture could produce
scattered flash flooding in the Southwest through Friday...
...Widespread Heat Advisories and Excessive Heat Warnings are in effect
throughout the central US...
A long, slow moving frontal boundary extending from the Central High
Plains through the Upper Midwest to the Mid-Atlantic will be the focus for
much of the activity over the CONUS during the short-range period. A low
pressure wave set up over the Upper Midwest is expected to interact with
upper-level shortwave energy and surface instability to produce both heavy
rain and thunderstorms over the region. Taking into consideration the
antecedent saturated soils and the potential for rainfall accumulations up
to and exceeding 2" in 24 hours, which could lead to localized flash
flooding, the Weather Prediction Center has issued a Slight Risk of
Excessive Rainfall for portions of Wisconsin and Michigan through Thursday
morning. As afternoon vertical wind profiles become more favorable, severe
thunderstorms and supercells will likely develop throughout the Upper
Midwest that hold the potential to produce damaging winds, large hail, and
a few tornadoes. Much of Wisconsin and the surrounding areas have the
greatest likelihood of experiencing these conditions, therefore, the Storm
Prediction Center has issued a Moderate Risk of severe thunderstorms for
portions of Wisconsin through Thursday morning.
The low pressure wave and its associated frontal boundary are forecast to
move from the Great Lakes area overnight and into the Ohio
Valley/Mid-Atlantic by Thursday morning. Though rainfall totals are
expected to be a bit lower than the previous day's, saturated soils
throughout the regions due to 150-400% above normal precipitation will
still make isolated flash flooding a concern. Additionally, a moist,
unstable airmass extending from the Ohio Valley to the Mid-Atlantic could
interact with the advancing cold front to produce linear thunderstorm
clusters and supercells. Due to the potential for this storms to produce
damaging gusts and possibly a tornado or two, the Storm Prediction Center
has issued a Slight Risk of severe thunderstorms for the Ohio Valley,
Central Appalachians, and Mid-Atlantic from Thursday through Friday
Shifting focus to the Southwest and Central Great Basin, showers and
thunderstorms are expected to continue throughout the period as high
pressure aloft aids the transport of monsoonal moisture into the region.
Anomalously high precipitable water values are expected to combine with
surface instability due to diurnal heating to produce hourly rain rates
between 0.5 and 1 inches, with isolated higher amounts possible. Given
these high rain rates, antecedent wet conditions, and scattered burn scars
throughout the regions, the Weather Prediction Center has issued a Slight
Risk of Excessive Rainfall for portions of Arizona, Nevada, Utah, and
Colorado from Thursday through Friday due to the potential for localized
flash flooding to occur.
Widespread Heat Advisories are currently in effect from the Northern
Plains/Upper Midwest to the Lower Mississippi Valley/Southeast as hot
afternoon temperatures and high humidity are expected to produce heat
index values between 100 and 110 degrees, with isolated higher values
possible. The main concern with heat index values this high is the onset
of heat related illness if the proper precautions aren't taken. Residents
of these regions are urged to drink plenty of fluids, stay in air
conditioned rooms, stay out of the sun, and reschedule strenuous
activities to the early morning or evening. Excessive Heat Warnings are
also in effect for portions of Minnesota, Kansas, Missouri, Oklahoma, and
Mississippi. In Northern California and the Pacific Northwest, Excessive
Heat Watches and Warnings as well as Heat Advisories are in effect for
areas where temperatures on Friday are expected to be above average, with
highs reaching the high 90s and low 100s.
The Storm Prediction Center has issued an Elevated Risk of Fire Weather
for portions of the Northern Plains through Thursday morning due to
prevalent dry fuels throughout the region, low relative humidity values,
and winds between 15 and 20 mph. Air Quality Alerts are in effect for
portions of Washington, Idaho, Montana, Wyoming, and Colorado due to
wildfire smoke from ongoing wildfires.
Graphics available at www.wpc.ncep.noaa.gov/basicwx/basicwx_ndfd.php