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

Valid 00Z Sun Aug 13 2023 - 00Z Tue Aug 15 2023

...Dangerous heat persists across much of the South while another heat
wave is 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...

A broad upper-level ridge anchored over the South will keep dangerous heat
in place this weekend and into early next week. Widespread heat-related
advisories and warnings are in effect from Florida north to the Carolinas
and west to Texas/Oklahoma. The most excessive heat is expected to focus
across a large portion of Texas into the Lower Mississippi Valley. Daily
high temperatures are forecast to 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.
These temperatures when combined with high humidity ill allow heat indices
to rise as much 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. This heat streak will persist into
the coming week with little to no reprieve. Additionally, a new heatwave
will begin to develop Sunday across the Pacific Northwest thanks to a
strong ridge of high pressure that will settle 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 convection potential to the
Northeast/Midwest/Plains this weekend. The leading front tracking eastward
across the Great Lakes/Midwest into the Northeast through Saturday evening
focusing showers and thunderstorms from the Ohio Valley to the Northeast.
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.
Daily maximum temperatures are forecast to be near or just above seasonal
normal for the Central and Northern Plains region with upper 80s to lower
90s. Once the cold front passes much cooler air will filter in resulting
in maximum temperatures in the upper 60s to low 70s.

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.

Campbell/Putnam

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