Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
253 AM EDT Thu Aug 05 2021
Valid 12Z Sun Aug 08 2021 - 12Z Thu Aug 12 2021
...Overview...
Troughing in the West will move into the central CONUS around the
middle of next week as heights build over much of the area in
another push of summertime heat. Rainfall will largely focus along
the Canadian border (Northern Rockies Sunday, then the Upper
Midwest/Great Lakes into the Ohio and Tennessee Valleys) in an
overall drier pattern than in recent weeks. Uncertain flow
evolution out of the Pacific may squeeze some ridging into the
Pacific Northwest late in the period.
...Guidance/Predictability Evaluation...
Through the 12Z/18Z cycle, the deterministic models largely paired
well with the ensembles, but this represented a change out of the
Pacific toward more ridging into the Pacific Northwest around
midweek next week rather than persistent, albeit weak, troughing.
Though this may be a relatively small shift eastward, this also
nudged the downstream trough eastward to the Great Lakes rather
than the Plains or Upper Midwest. The GFS/ECMWF were stronger with
this troughing than earlier runs and the ensemble means, but since
this change was manifested in just the recent 12Z cycle, opted to
rely mostly on the ensemble means in case an adjustment back
emerges. This was a good midpoint between continuity and the more
aggressive GFS/ECMWF. Otherwise, still expect heights build over
much of the CONUS midweek next week and beyond in line with the
CPC day 6-10 forecast.
...Weather Highlights/Hazards...
Rainfall will expand across the Corn Belt and Great Lakes Sunday
as a shortwave lifts toward Wisconsin. Some rainfall could be
locally heavy as this system has trended more defined in recent
runs. As the system lifts into southern Ontario/Quebec, rainfall
coverage and intensity may wane a bit before perhaps picking up
again next Tue-Thu along the tail-end of the lead boundary and in
advance of another Canadian front, in addition to afternoon
convection over the Tennessee and Ohio Valleys.
Temperatures will be about 5-15 degrees cooler than normal in the
Pacific Northwest and northern Rockies behind the lead cold front
this weekend into Monday with a moderation thereafter. Ahead of
this cold front, temperatures will be 5 to 12 degrees above normal
over the Plains Sun-Mon and expanding to the East Coast Mon-Thu. A
ridge over the Southeast Monday will maintain temperatures into
the 90s with heat indices into the low 100s as dew points rise
into the 60s and low 70s. Temperatures into the 80s and 90s will
reach into the Northeast/ Mid-Atlantic ahead of the Great Lakes
trough which is about 5-10 degrees above normal.
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