Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
249 AM EDT Sun Aug 01 2021
Valid 12Z Wed Aug 04 2021 - 12Z Sun Aug 08 2021
...Heavy rainfall threat for parts of the South and the Southeast
Wednesday...
...Overview...
Troughing will persist through the week over the East, in between
flattening ridging out of the northern Rockies and over the
Atlantic. This favors a quasi-stationary boundary astride the East
Coast with embedded areas of low pressure that will promote bouts
of rainfall across especially the Carolinas but also up the coast
to New England. Multi-day rainfall could exceed several inches
over parts of the coastal Carolinas. A system over the Pacific
will slowly work its way into Washington/Oregon later in the week.
...Guidance/Predictability Evaluation...
Latest models and ensembles remain in good agreement on the large
scale pattern through the period. Favored a blend of the
deterministic guidance (GFS/ECMWF/Canadian/UKMET) days 3-5,
trending toward the 18Z GEFS/12Z ECENS means by day 7 as the
models diverge on timing of the systems over the central CONUS.
Trends have been for a bit stronger lead-in system into northern
CA/southern OR by early Thu (touching off more showers for
northern ID) and a tad quicker with the upper low into British
Columbia. With support from the deterministic models in each area,
blended/middle ground solution was prudent.
...Weather Highlights/Hazards...
Areas of the North/South Carolina coast in the vicinity of the
frontal boundary will see the highest chance for multi-inch
rainfall totals during the first few days of the period. With
ample moisture off the Atlantic, daily rounds of showers and
storms will be likely with a slow trend in decreasing
coverage/intensity by next weekend as the front washes out. Less
rainfall is expected farther west along the boundary along the
Gulf Coast back into Texas, but some could be enhanced over New
Mexico into southeastern Arizona as mid-level energy meanders over
the area. Over the Northwest, lead-in and Pacific system will
bring in some light rain to the northern Rockies and coastal
Washington, respectively, as the upper trough and surface front
into WA/OR move ashore.
Temperatures will be within a few degrees of normal over much of
the West but trending cooler in the Pacific Northwest as the front
moves ashore. Areas along the Canadian border from eastern Montana
to the western Great Lakes will likely see temperatures 5 to 10
degrees above normal under upper ridging. 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 on some days over Texas/New Mexico and over the
Carolinas. Low temperatures will be closer to normal in the
Southeast owing to cloud cover but perhaps a few degrees below
normal in the mid-Mississippi Valley where skies will be clearer.
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