Short Range Forecast Discussion
NWS Weather Prediction Center College Park MD
356 PM EDT Thu Jul 29 2021

Valid 00Z Fri Jul 30 2021 - 00Z Sun Aug 01 2021

...Severe thunderstorms likely from the Middle Mississippi Valley to the
Mid-Atlantic through Friday morning...

...Excessive rainfall with isolated flash flooding possible over parts of
the Northeast through Friday morning...

...Potentially heavy rainfall fueled by monsoonal moisture could produce
scattered flash flooding in the Southwest and Central Great Basin for the
first half of the weekend...

...Widespread Heat Advisories and Excessive Heat Watches/Warnings are in
effect for much of the central US and Pacific Northwest...

A low pressure wave positioned over the Lower Great Lakes with a cold
front trailing southwest through the Ohio Valley to the Central High
Plains and a warm front pushing ahead over the Mid-Atlantic and Northeast
will be the focus for much of the activity affecting the eastern half of
the CONUS during the short-range period. An unstable, moist airmass within
the warm sector is expected to interact with upper-level shortwave energy
and forcing at the surface via the cold front to generate scattered
thunderstorms as well as multicell clusters. Due to the potential for
these storms to become severe and produce damaging gusts and possibly a
tornado or two, the Storm Prediction Center has issued a Slight Risk for
severe thunderstorms for the Ohio Valley, Central Appalachians, and
Mid-Atlantic through Friday morning. Because southeastern Pennsylvania,
southern New Jersey, northern Delaware, and northeastern Maryland have the
greatest likelihood of experiencing these conditions, an Enhanced Risk of
severe thunderstorms has been issued for these areas. Taking into
consideration the potential for localized flash flooding in the Northeast
due to antecedent saturated soils and expected rainfall totals of 1 to 3
inches with the passage of this system, the Weather Prediction Center has
issued a Slight Risk of Excessive Rainfall for the region through tomorrow
morning.

Overnight tonight the western tail end of the cold front is expected to
become stationary over the Central High Plains and combine with a low
pressure wave that slowly drifts from southern to northern Colorado. As
the wave becomes embedded within the front it is expected to lift further
north into Nebraska with a warm front extending out to the east across the
state. This warm front will serve as the focus for convective development
on Friday as it interacts with a moist and unstable boundary layer. As
afternoon vertical wind profiles become more favorable a cluster of severe
thunderstorms will likely develop. Seeing as these storms could generate
large hail and gusty winds, the Storm Prediction Center has issued a
Slight Risk of severe thunderstorms for parts of eastern Nebraska from
Friday morning until Saturday morning. Localized flash flooding will be
possible Friday through Saturday along a northwest-southeast axis from
southern South Dakota to Tennessee due to the potential for these storms
to produce rainfall totals of 1 to 2+ inches in 24 hours. Southeastern
Missouri will be at greatest risk for flash flooding on Saturday,
therefore, the Weather Prediction Center has issued a Slight Risk of
Excessive Rainfall for the area beginning Saturday morning.

Shifting focus to the Southwest, Great Basin and Central/Southern Rockies,
showers and thunderstorms are expected to continue throughout the period
as the pressure pattern aloft aids the transport of monsoonal moisture
into the aforementioned regions. Anomalously high precipitable water
values are expected to combine with upper-level energy as well as surface
instability due to diurnal heating to produce heavy rainfall on top of
already moist soils. Given the potential for high rain rates, antecedent
wet conditions, and scattered burn scars throughout the affected areas,
the Weather Prediction Center has issued a Slight Risk of Excessive
rainfall for parts of the Southwest through Friday morning due to the
possible occurrence of localized flash flooding. On Friday, the flash
flooding concern is expected to spread north into the the Central Great
Basin and Central Rockies, prompting the issuance of Slight Risk areas in
those regions for Friday along with the Southwest. By Saturday the flash
flooding risk is expected to shift even further north into the Northern
Great Basin while simultaneously abating in the Southwest. As such, for
Saturday a Slight Risk for Excessive Rainfall has been issued for the
southern Oregon/Idaho border and parts of the Central Rockies.

Widespread Heat Advisories are currently in effect from the Central
Plains/Ohio Valley to the Lower Mississippi Valley/Southeast as hot
afternoon temperatures and high humidity are expected to mix to generate
heat index values between 100 and 110 degrees, with isolated higher values
possible. Excessive heat warnings are also in effect for parts of Kansas,
Missouri, Oklahoma, and Mississippi. Throughout northern California and
the Pacific Northwest Excessive Heat Watches and Warnings as well as Heat
Advisories are also in effect for areas where temperatures on Friday are
forecast to be 10+ degrees above average, with highs reaching the upper
90s and low 100s. The main concern with these hot temperatures and/or high
heat index values is the onset of heat related illness if the proper
precautions aren't taken. Residents within the previously mentioned
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 to safeguard their health.

Elsewhere, tropical moisture being pumped into the Gulf Coast states and
Florida will help fuel the development of daily showers and thunderstorms
through Saturday. Anomalously high precipitable water values over Florida
could allow rain rates within these storms to reach 1.5 - 2 inches per
hour, which may lead to isolated flooding, primarily in urban areas,
through Friday morning. Lastly, Air Quality Alerts are currently in effect
for portions of Idaho, Montana, Wyoming, Colorado, Minnesota, and Iowa due
to smoke from ongoing wildfires out west and in Canada being advected into
their areas by the prevailing northerly-northwesterly winds.

Zavadoff

Graphics available at www.wpc.ncep.noaa.gov/basicwx/basicwx_ndfd.php