Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
428 PM EDT Fri Aug 13 2021
Valid 12Z Mon Aug 16 2021 - 12Z Fri Aug 20 2021
...Heavy rain associated with Tropical Storm Fred expected to
spread further inland into the Southeast early next week...
...Potential Tropical Cyclone Seven could approach Florida during
the latter half of next week...
...Pattern Overview...
An upper level trough will likely move through the Northwest
during the early part of next week before reaching the northern
Plains later next week, heralding the end of the ongoing heat wave
across the region. Meanwhile, Tropical Storm Fred is forecast to
make landfall on the eastern Gulf Coast early next week and spread
heavy rainfall inland across the Southeast U.S. into the southern
Appalachians. Toward the latter half of next week, Potential
Tropical Cyclone Seven is forecast to intensify into a tropical
storm and approach Florida.
...Guidance/Predictability Evaluation...
Models and ensemble means are in very good overall agreement on
the pattern change expected across the north-central U.S. through
the beginning of the week. However, deterministic models have
become more divergent toward the end of next week regarding
whether a low pressure wave will develop and intensify over the
northern Plains. A general model compromise with increasing
weights towards the ensemble means on Days 6 and 7 yielded a
solution fairly close to continuity. This is also true regarding
the remnants of Fred across the Southeast with the ECMWF still
favors more QPF pushing north into the southern Appalachians. In
contrast, the GFS/GEFS continues to advertise much less QPF across
the Southeast given a much weaker remnant circulation of Fred in
the forecasts. An intermediate solution was adapted as a
compromise given past performances of these two models on tropical
systems.
The WPC medium-range package was based on a blend of the 00Z
ECMWF/EC mean, 06Z GFS/GEFS, and some contributions from the 00Z
CMC/CMC mean with more weights from the ensemble means for Days 6
and 7.
...Weather Highlights/Hazards...
The ongoing heat wave over the Northwest is expected to be over by
Monday with temperatures returning to near normal levels. Some
portions of the Northern Rockies and adjacent portions of the
northern Plains could be 10 to 20 degrees below normal next
Tuesday into Wednesday. High are expected to be above normal
ahead of the cold front across the northern Plains on Monday,
followed by more pleasant readings by midweek.
Tropical moisture is expected to lift northward across interior
portions of the Southeast and southern Appalachians after Fred
makes landfall, potentially interacting with a stalled frontal
boundary and terrain enhancement. Exact locations of heaviest
rainfall depends on the eventual track of Fred, and this is
subject to additional changes in future forecasts.
Elsewhere, monsoon activity over the Desert Southwest should
increase and spread northward, again bringing a localized flash
flood risk. Moisture should stream northward ahead of the
incoming trough into the Northwest, with a potential for moderate
to heavy rainfall emerging over parts of the northern Rockies/high
Plains on Tuesday, with organized and enhanced rainfall then
focusing along the cold front as it advances across the Northern
Plains Tuesday into Wednesday. Toward the latter half of next
week, attention may once again be drawn to Florida where Potential
Tropical Cyclone Seven over the tropical Atlantic is forecast to
intensify into a tropical storm before approaching the Sunshine
State.
Kong/Hamrick
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
Hazards:
- Heavy rain across portions of the Central Great Basin, the
Tennessee Valley, the Mid-Atlantic,
the Southern Appalachians, the Southeast, the Ohio Valley, and the
Southwest, Mon-Wed, Aug 16-Aug 18.
- Heavy rain across portions of the Upper Mississippi Valley and
the Northern Plains, Wed-Thu, Aug 18-Aug 19.
- Heavy rain across portions of the Central Plains, the Southeast,
the Middle Mississippi Valley, the Upper Mississippi Valley, and
the Northern Plains, Thu-Fri, Aug 19-Aug 20.
- Heavy rain across portions of the Southeast, Mon, Aug 16.
- Flooding possible across portions of the Southeast.
- Flooding occurring or imminent across portions of the Southeast.
- Excessive heat across portions of the Central Great Basin,
California, the Northern Plains, and the Southwest, Mon, Aug 16.
- Excessive heat across portions of the Central Plains and the
Northern Plains, Mon-Tue, Aug 16-Aug 17.
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