Short Range Forecast Discussion
NWS Weather Prediction Center College Park MD
400 AM EDT Sat Aug 12 2023

Valid 12Z Sat Aug 12 2023 - 12Z Mon Aug 14 2023

...Dangerous heat will persist across much of the South through this
weekend, with another heat wave expected to develop across the Pacific
Northwest Sunday...

...Severe thunderstorms and flash flooding possible Saturday for parts of
the Ohio Valley and Northeast, shifting to the Plains and Mississippi
Valley Sunday...

...Heavy rainfall expected for the High Plains late Saturday with
Monsoonal showers expected across the Four Corners region...

Dangerous heat will continue for much of the southern U.S. through the
weekend as a west to east oriented upper-level ridge of high pressure
remains in place. Widespread heat-related advisories and warnings are in
effect from Florida west through the Gulf Coast and into the Lower
Mississippi Valley and Southern Plains, with the most excessive heat
expected over portions of Texas and the Lower Mississippi Valley. Forecast
highs this weekend will range from the mid-90s in Florida, the upper 90s
into portions of the Southeast, the low 100s in Oklahoma and the Lower
Mississippi Valley, and the mid 100s to 110 into portions of Texas. When
combined with high humidity values, heat indices will reach as high as 110
to 120F for many locations. Numerous record-tying/breaking highs are
forecast both days, with similarly record-level warm low temperatures in
the upper 70s to low 80s providing little relief from the heat overnight.
The heat wave looks to continue beyond the current forecast period into
next week. Unfortunately, a new heat wave is also forecast to begin
building across the Pacific Northwest by Sunday as a strong ridge of high
pressure settles in overhead. The more excessive temperatures will
initially be focused along the interior valleys west of the Cascades, with
highs soaring into the upper 90s to mid-100s. This heat wave will also
continue to intensify and expand across the region next week. 

Energetic flow with embedded shortwaves north of the ridge and the passage
of multiple meandering frontal systems will bring shower and storm chances
to the Northeast/Midwest/Plains this weekend. An initial frontal system
will push eastward across the Great Lakes/Midwest into the Northeast
through Saturday evening, focusing storm development over portions of the
Northeast into the Ohio Valley. The Storm Prediction Center has issued an
Enhanced Risk of Severe Weather (level 3/5) as strong flow at low levels
and aloft will provide enough shear for storms to produce damaging winds,
along with some hail and the threat of a few tornadoes. Some locally heavy
downpours are also possible, with a Slight Risk of Excessive Rainfall
(level 2/4) over portions of southern New England where the greatest
threat of some scattered instances of flash flooding will exist. A
shortwave dropping south over the Northern Plains from Canada will help to
better organize a southward moving frontal system late Saturday into early
Sunday with storm chances shifting southeastward into the Central/Southern
Plains and the Middle Mississippi Valley through the day Sunday. High
surface moisture pooling between this frontal system and another front to
the south will help storms to produce some locally very heavy downpours.
There is a Slight Risk of Excessive Rainfall in effect for the risk of
flash flooding, particularly if storms grow upscale into a more organized
convective system Sunday night, enhancing rainfall totals and coverage. In
addition, strong buoyancy and sufficient deep layer shear will also lead
to the threat of some severe weather, with a Slight Risk in place for the
threat of damaging winds, large hail, and a few tornadoes. Forecast highs
will range from the upper 70s to low 80s in New England to the mid-80s to
low 90s for the Mid-Atlantic and Ohio Valley west into portions of the
Midwest/Mississippi Valley.

Areas of showers and thunderstorms are also expected on Saturday over
portions of the central and southern High Plains as a cold front sags
southward across the region and moist, upslope flow settles in place. An
additional Slight Risk of Excessive Rainfall is in effect here as well as
upscale growth of storms later in the evening may lead to some more
widespread, higher rainfall totals and a few instances of flash flooding.
Highs will generally be at or just above average for much of the
Central/Northern Plains Saturday, with upper 80s to low 90s forecast. Much
cooler temperatures will follow the passage of the frontal systems Sunday
over the Northern Plains with highs dropping into the upper 60s to low
70s. Over the West, monsoonal moisture in the Four Corners region will
lead to daily rounds of afternoon thunderstorms with a few locally heavy
downpours possible, and some isolated concerns for flash flooding. This
will also help to keep temperatures a bit below average, with highs only
into the upper 70s for some locations. Areas of the Central/Southern
Rockies and Great Basin will be a bit closer to average, seeing highs in
the mid-80s to low 90s, with upper-90s to mid-100s for the Desert
Southwest. A lingering upper-level low will keep temperatures unseasonably
cool over portions of southern California, as highs are expected to be in
the upper 60s to mid-70s for the coast. Highs will warm up through the
weekend further north, with 70s along the coast and 90s for the interior
valleys. Some locally heavy showers will be possible over the Sierra.

Putnam

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