Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
148 AM EDT Sat Aug 07 2021
Valid 12Z Tue Aug 10 2021 - 12Z Sat Aug 14 2021
...Overview...
A typical August-like pattern is expected to evolve during the
medium range period with building heights aloft across much of the
western and southern CONUS. This will bring periods of excessive
heat to some as temperatures rise above normal. Frontal systems
should be confined to northern parts of the country as an upper
trough shifts east from the northern Plains Tuesday and settles
across the Great Lakes/Northeast by next weekend.
...Guidance/Predictability Evaluation...
Compared to recent days, the guidance is showing better agreement
regarding the details of individual shortwaves moving through
within the broad northern stream trough. Run to run continuity has
improved the last couple of model cycles with some wobble in the
exact trough axis, though fairly typical at these time ranges. A
general blend of the deterministic solutions provides a good
starting point for the forecast. The only other thing to note is
some timing differences in troughing as it moves into the Gulf of
Alaska or far western Canada. The GFS and CMC are faster while the
ECMWF holds the energy farther west. A blend of the GFS and ECMWF
along with modest amounts of the more agreeable ensemble means
seems to sort this out. This overall approach also provides good
continuity with the previous WPC forecast.
...Weather Highlights/Hazards...
Upper troughing and surface frontal systems should bring showers
and thunderstorms to portions of the Midwest/Upper Great Lakes
next week with rain chances moving into the Northeast as well by
late week and next weekend. Rain could be heavy at times in some
places, but detail issues results in plenty of uncertainty exactly
where and how impactful it may be. Across the Eastern U.S. into
the Deep South, daily scattered storm chances are likely within a
warm and moist environment. Monsoonal moisture is also expected to
result in diurnally driven rain and thunderstorms across parts of
the Southwest as well through much of the week.
Much of the country will be either near or above normal in terms
of temperatures through next week. Values 5-10 degrees, locally
higher, will shift eastward from the Central Plains/Midwest into
the Ohio Valley and the Northeast ahead of the main cold front.
Even though the temperature anomalies aren't all that impressive,
the combined heat and humidity could bring heat indices into the
low 100s for many. With little relief expected overnight,
excessive heat is possible most notably in and around Arkansas and
also parts of the Mid-Atlantic I-95 urban corridor. Out West,
temperatures will rise once again as strong upper ridging moves
in. By mid-week, expect daytime highs 10 to 15+ above normal,
especially for parts of the interior Northwest where daily high
temperature records could be challenged for a handful of locations.
Santorelli
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