Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
201 AM EDT Mon Aug 02 2021
Valid 12Z Thu Aug 05 2021 - 12Z Mon Aug 09 2021
...Lingering heavy rainfall threat for the coastal Carolinas
Thursday...
...Overview...
Upper pattern will undergo a reshuffling of the anomaly centers
from the Pacific across CONUS into the western Atlantic later this
week into next week. Initial trough/ridge/trough pattern over the
PacNW/Rockies/Great Lakes and Midwest will shift toward broad
troughing from the northern Rockies to the Upper Midwest with
weaker ridging into the East. This translates to a generally drier
pattern with warming temperatures out of the Plains into the East.
The Northwest will see cooler and unsettled weather overall.
...Guidance/Predictability Evaluation...
Through the 12Z/18Z cycle, models and ensembles showed good
clustering to start, but then drifted apart one-by-one from Friday
onward. The UKMET was first to depart from otherwise good
agreement in the West (stronger than most other guidance) and East
(slower than most). The GFS became farther south in the PacNW with
the incoming upper low late Fri into Sat vs the ECMWF/Canadian
which were farther north in southern BC (which has had good
continuity). Favored the ECMWF/Canadian as the pattern unfolded
into quasi-zonal flow but increased ensemble weighting from both
the GEFS and ECMWF ensemble mean to account for increased
uncertainty in an inherently more unpredictable pattern.
...Weather Highlights/Hazards...
Stalled frontal boundary over the Carolinas and along the East
Coast will slowly dissipate this coming weekend, after likely
being a focus for several days of widespread rain and locally
heavy amounts that could cause flooding issues. Upper trough
responsible for its orientation will weaken in concert, allowing
the focused rainfall to abate. With increased flow across the
U.S./Canadian border, rainfall will expand out of the
PacNW/northern Rockies through the Upper Midwest and Great Lakes
as a system traverses the northern tier. Rainfall will focus near
and north of an area of low pressure as well as south of a
northern stream boundary. Rainfall in the Southwest will be more
isolated this weekend but could increase a bit next week as the
upper high shifts away and weak troughing settles into California
and Nevada.
Temperatures will trend cooler as the Pacific front moves ashore
the Pacific Northwest Friday--about 5-15 degrees cooler than
normal. Ahead of the front, temperatures will be 5 to 10 degrees
above normal over the Plains Thu/Fri then into the Corn Belt/Great
Lakes this weekend and to the East Coast by next Monday. Much of
the East/Southeast will see near to below normal max temperatures
along and behind the stationary front--perhaps as much as 10
degrees below normal on some days over Texas/New Mexico and over
the Carolinas Thu-Fri before a warm-up this weekend back toward
more typical values.
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