Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
225 AM EDT Sat Jul 31 2021
Valid 12Z Tue Aug 03 2021 - 12Z Sat Aug 07 2021
...Heavy rainfall threat for parts of the South and the
Southeast...
...Overview...
Troughing will persist next week over the East, in between
flattening ridging out of western/central Canada and over the
Atlantic. This favors bouts of rainfall along a
slowly-moving/stalled front across the Carolinas back through the
Southeast into the southern Plains. Multi-day rainfall could
exceed several inches over parts of the Carolinas. The upper high
over the Southwest will meander in place, shifting the moisture
axis primarily to the Rockies and Front Range.
...Guidance/Predictability Evaluation...
Latest models and ensembles remain in good agreement on the large
scale pattern through the period. Recent deterministic runs
continue to waver on how quickly (or slowly) to bring height falls
into the Pac NW later in the week with ECMWF/Canadian/ECENS mean a
little bit slower than the GFS and many GEFS members. The lack of
a clear upstream kicker over Alaska would favor the slower
solutions, but the upper low could unwind in pieces which would
send in some lower heights into WA/OR. Favored a blend of the
deterministic guidance days 3-5, trending toward the 12Z ECMWF and
the 18Z GEFS/12Z ECENS means by days 6 and 7.
...Weather Highlights/Hazards...
Focus for impactful weather will lie over the North/South Carolina
coast in the vicinity of the frontal boundary. With ample moisture
off the Atlantic, daily rounds of showers and storms will be
likely through the period. Guidance indicates several inches of
rainfall over a fairly large area which seems reasonable given the
setup. Less rainfall is expected farther west along the boundary
along the Gulf Coast back into Texas. Over the Northwest,
approaching system will eventually bring in some light rain to
coastal Washington by Friday into the northern Rockies (ID/MT) as
the front moves ashore.
Temperatures will be within a few degrees of normal over much of
the West with areas along the Canadian border likely to see
temperatures 5 to 10 degrees above normal from eastern Montana to
the western Great Lakes. Much of the East/Southeast will see near
to below normal max temperatures along and behind the cold front,
perhaps as much as 10 degrees below normal for early August on
some days over Texas/New Mexico and over the Carolinas. Low
temperatures will be closer to normal in the Southeast.
Fracasso
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, winter weather outlook probabilities
and heat indices 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/wwd/pwpf_d47/pwpf_medr.php?day=4
https://www.wpc.ncep.noaa.gov/heat_index.shtml