The Weather Prediction Center

College Park, MD

Extended Forecast Discussion

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
234 AM EDT Wed Jul 28 2021

Valid 12Z Sat Jul 31 2021 - 12Z Wed Aug 04 2021

...Widespread Monsoonal Moisture and Rainfall Pattern over the
...Heavy Convection/Rainfall Threat for the
South/Southeast/Southern Mid-Atlantic...

...Pattern Overview with Weather Highlights/Hazards...

A closed upper high slid over the south-central U.S. will
gradually weaken this weekend as ridging builds over the Rockies,
Intermountain West and western Canada into next week. Ridge
amplitude will support an unseasonably deep and persistent eastern
Canada/U.S. upper trough.

Impulses and moisture lifting around the ridge and on the eastern
periphery of eastern Pacific upper troughing will present
widespread monsoonal rainfall with local runoff issues. Best
chances are from the Southwest/CA to the central Great
Basin/Rockies where there will also be increasing coverage of
daytime highs 5-15F below normal. On the other hand, the Pacific
Northwest may see up to 10-15F above normal highs Saturday, before
a gradual moderating trend as heights aloft decline. Record warm
minimum temperature records will be possible west of the Rockies
over the Northwest Saturday and elsewhere with monsoonal

Meanwhile, shortwaves rounding the upper ridge will periodically
dig downstream to reinforce an eastern U.S. upper trough.
Favorable support aloft may help to produce strong convection and
locally heavy downpours as moisture/instability pool near wavy
fronts slowing on the southern trough periphery. Rainfall chances
shift through the south-central Plains, mid/lower MS and TN
Valleys and southern Mid-Atlantic/Southeast. The pattern favors
below normal temperatures from the Midwest/Northeast to the
Mid-Atlantic where spotty/modest record lows are possible. The
Deep South will be steamy with record warm lows possible along and
south of a main and wavy front.

...Guidance/Predictability Evaluation...

The medium range pattern offers above average predictability. The
WPC medium range product suite was derived from best clustered
guidance of the 18 UTC GFS/GEFS and 12 UTC ECMWF/ECMWF ensemble
mean along with the 01 UTC National Blend of Models. Transitioned
WPC blend weights increasingly from the models to the ensemble
means over time consistent with gradually growing forecast spread.
This maintains good WPC product continuity that seems in line with
newer 00 UTC guidance.


Additional 3-7 Day Hazard information can be found on the WPC
medium range hazards outlook chart at:

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

Last Updated: 234 AM EDT WED JUL 28 2021