751
FXUS02 KWBC 120708
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
308 AM EDT Wed Aug 12 2026

Valid 12Z Sat Aug 15 2026 - 12Z Wed Aug 19 2026


...Hazardous Heat in the South-Central and Southeast U.S...

...Heavy Rain Threats for the Southwest, Rockies, Midwest, and
Mid-Atlantic...


...Overview...

A broad upper ridge will continue to build through mid-week across
the southern tier, bringing a prolonged period of hazardous heat
across south-central U.S. and Southeast. Furthermore, an active
regime continues as shortwave energy progresses through the
northern stream. This will lead to cyclogenesis over the northern
Plains. Multiple convective episodes are possible from parts of the
Plains/Midwest into the Great Lakes, Mid-Atlantic and Northeast,
leading to flooding concerns. Heavy rain will also be possible
with daily monsoonal thunderstorms from the Southwest into the
Great Basin and southern Rockies.


...Guidance/Predictability Assessment...

The latest guidance continues to show reasonable agreement on the
larger-scale pattern, but meaningful spread develops with the
evolution of individual shortwaves and associated surface features.
Ensemble means provide a more stable signal than the
deterministic, particularly towards late period. The forecast
therefore favors a multi-model approach, with emphasis on the
ECMWF/ECmeans and GFS/GEFS for the synoptic pattern. Confidence is
higher in the broad placement of ridging across the southern tier
and the presence of periodic shortwave energy within the zonal flow
across the northern and central states, while confidence is lower
on the timing and magnitude of individual surface waves and their
associated precipitation maxima.


...Weather/Hazards Highlights...


The persistent upper ridge across the southern tier will continue
to support widespread well above normal temperatures across the
South- Central U.S. to the Southeast. Portions of the southern
Plains and Lower Mississippi Valley will reach the lower 100s,
while widespread 90s extend eastward across the Southeast. High dew
points/humidity will keep heat indices elevated, resulting in
dangerous heat conditions, and record warm overnight temperatures
will provide little relief. Widespread Major to Extreme HeatRisk
conditions are expected across these regions. Several consecutive
days of high heat stress can lead to heat-related illnesses.

Meanwhile, monsoonal moisture and flow embedded vorts will channel
over the Southwest, Great Basin, and southern Rockies between an
eastern Pacific upper trough and the western periphery of the
southern U.S. upper ridge this period. This will lead to daily
rounds of scattered to numerous thunderstorms, particularly over
portions of Southwest into central Plains, where the Day
4/Saturday and Day 5/Sunday Excessive Rainfall Outlook (ERO)
depicts a broad Marginal Risk area across portions of the Southwest
into central Plains, while a Slight Risk focuses in on northeast
Colorado on Day 4. Some storms will produce excessive rainfall
over short periods while moving rather slowly. Repeated convection
may lead to flash flooding in vulnerable terrain, including burn
scars, slot canyons, and urbanized areas. Over into the Pacific
Northwest, the approaching trough will bring some isolated showers
by early next week.

A series of shortwaves moving through the northern-stream flow will
interact with a moist unstable airmass along a frontal boundary
from the northern Plains into the Midwest, which will lead to
showers and thunderstorm development through the weekend. Abundant
low- level moisture, instability, and forcing will support
clustered convection. Additionally, slow- moving storms could
result in locally excessive rainfall and flash flooding,
particularly across portions of the Upper Midwest where there is a
Slight Risk for Day 4/Saturday ERO, while a broad Marginal Risk
highlights parts of the Midwest and Great Lakes. On Sunday, the
focus for organized convection and heavy rainfall shifts towards
portions of the Great Lakes, Ohio Valley/Mid- Atlantic, and
Northeast as the frontal system advances eastward, which is
highlighted in Day 5/Sunday ERO. By Monday, the surface low moves
into the Northeast bringing additional showers and thunderstorms
across portions of the Northeast, Mid- Atlantic, and Southeast
along the boundary and surface trough.


Oudit/Dolan



Additional 3-7 Day Hazard information can be found on the WPC
medium range hazards outlook chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php

WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation forecast, excessive rainfall outlook,
winter weather outlook probabilities, heat indices and Key Messages
are at:

https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif
https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif
https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html
https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml
https://www.wpc.ncep.noaa.gov/#page=ero
https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4
https://www.wpc.ncep.noaa.gov/heat_index.shtml
https://www.wpc.ncep.noaa.gov/#page=ovw




$$