Short Range Forecast Discussion
NWS Weather Prediction Center College Park MD
400 AM EDT Sun Aug 13 2023

Valid 12Z Sun Aug 13 2023 - 12Z Tue Aug 15 2023

...Flash flooding and severe thunderstorms possible for portions of the
Plains/Mississippi Valley Sunday, shifting to the Great Lakes/Ohio
Valley/Mid-Atlantic Monday...

...Dangerous heat persists across much of the South while another heat
wave begins across the Pacific Northwest Sunday...

...Monsoonal showers continue across the Four Corners region with some
locally heavy downpours possible...

An upper-level shortwave moving southeastward from Canada over a pair of
frontal boundaries over the Plains/Mississippi Valley will help to trigger
numerous showers and thunderstorms throughout the day Sunday. Ongoing
storms continuing through early Sunday morning are expected to be followed
by another round of storms Sunday afternoon into Sunday night along the
boundaries as well as any outflows. Moisture pooling between the
boundaries from portions of the Central/Southern Plains into the Middle
Mississippi Valley will help promote efficient rainfall rates with the
prospect of storms moving parallel to the fronts repeatedly over the same
region heigthening the risk for several inches of rainfall. A Moderate
Risk of Excessive Rainfall (level 3/4) has been introduced over
southwestern Missouri where the highest probabilities of heavier rainfall
totals and scattered to numerous instances of flash flooding are currently
forecast, with a broader Slight Risk (level 2/4) across the region. In
addition to heavy rainfall, the Storm Prediction Center has outlined
similar areas of southwestern Missouri with an Enhanced Risk of severe
weather (level 3/5), with a broader Slight Risk (level 2/5) in place as
well, as sufficient deep layer shear will be in place for some organized
severe storms capable of damaging winds, a few tornadoes, and large hail.

The frontal boundaries are expected to consolidate into a potent Summer
cold front as they push eastward overnight Sunday into the day Monday,
with storm chances shifting into the Great Lakes, Ohio Valley, and
Mid-Atlantic. Very moist, southerly flow will continue to help promote
storms with heavy rainrates, and a Slight Risk of Excessive Rainfall in
effect for the risk of some scattered instances of flash flooding.
Additional severe storms are possible as well, with a Slight Risk of
severe weather focused on the Upper Ohio Valley into the Mid-Atlantic,
mainly for the risk of some damaging winds. The cold front pushing
eastward will bring quite the contrast in high temperatures broadly from
the Plains to the East Coast. Much below average highs in the upper 60s to
low 70s are forecast for the Northern Plains into the Upper Midwest
Sunday. The cooler temperature will spread into the Midwest and portions
of the Central/Southern Plains Monday, with highs in the mid 70s to low
80s. Highs will be closer to and above summertime averages ahead of the
front, with upper 70s and low 80s for New England and mid-80s to low 90s
for the Mid-Atlantic.

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 and
the Carolinas, the upper 90s into portions of the Southeast, the low 100s
in Oklahoma and the Lower Mississippi Valley, and the mid 100s into
portions of Texas. These temperatures when combined with high humidity
will allow for heat indices 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. Some moderation of the
heat is expected for northern portions of the region by the middle of this
upcoming week as cooler air follows the southward moving cold front
cutting across the country, starting with the noted portions of the
Southern Plains Monday.

Unfortunately, another heatwave is forecast to begin 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 in Oregon, with
highs soaring into the 100s Sunday. This heat will continue to intensify
and expand across the region Monday, with highs ranging from upper 80s to
low 90s around Puget Sound, the upper 90s to low 100s east of the
Cascades, and the mid-100s to potentially over 110 degrees for the
interior west of the Cascades. Numerous near record-tying/breaking highs
are possible here as well. These temperatures combined with the duration
of heat, expected to continue through this week, will increasingly pose a
heightened health risk, especially for those without adequate air
conditioning.  

A lingering frontal boundary in addition to Monsoonal moisture will lead
to continued daily rounds of afternoon thunderstorms in the Four Corners
region with a few locally heavy downpours possible, and some isolated
concerns for flash flooding. Highs will generally be near summertime
averages across the southern tier of the West, with 70s along the
California coast, low to mid-90s for the Great Basin, upper 90s to low
100s for the interior California valleys, and 100s for the Desert
Southwest. Portions of the Central/Southern Rockies and adjacent High
Plains will be more influenced by the cold front passage, with much cooler
highs in the upper 70s to low 80s forecast.

Putnam

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