Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
248 AM EDT Sun Aug 15 2021
Valid 12Z Wed Aug 18 2021 - 12Z Sun Aug 22 2021
...Heavy Rain Threat with Fred to focus over the
Southeast/South-Central Appalachians early next week...
...Tropical Storm Grace may threaten southern Florida and move
into the Gulf of Mexico later next week...
...Monsoonal moisture from the Southwest/Great Basin leads into
emerging Heavy Rain Threat over the north-central U.S. mid-later
next week...
...Pattern Overview...
An upper level shortwave trough will work through the northern
Rockies and northern Plains early in the forecast period then lift
well north into Canada as upper level ridging off the Southeast
U.S. asserts itself westward over the southern U.S. by next
weekend. This will favor synoptic storm tracks across the Plains
and Great Lakes. Attention focuses also on two tropical systems -
Fred as it moves inland across the Southeast and South-Central
Appalachians and Tropical Storm Grace as it nears southern Florida
late in the week and then could move into the Gulf of Mexico by
next weekend. Both systems will pose a heavy rainfall threat.
...Guidance/Predictability Evaluation...
The latest model guidance suite was reasonably clustered for the
Northwest to northern Plains shortwave trough, though there
remains some uncertainty in how much if any energy splits into a
closed low over the Intermountain West. For now, the WPC blend
favored a more progressive/flatter system that leaned on the
ensemble means by day 5 onward. By the end of the period, signals
are that the upper pattern takes on a more zonal direction with
several impulses quickly moving through the flow - overall
uncertainty/spread is higher than normal for day 6 and day 7.
Other areas of uncertainties are associated with the tropics -
particularly for Grace as it emerges into the Gulf of Mexico late
in the week and next weekend. The strength/position of the upper
level ridging off the Southeast U.S. may shunt the system further
west/southwest but the ECENS/GEFS members show a wide spread, with
the GFS being the southern solution scenario while the ECENS,
including its deterministic run, show a weaker ridge that allows
the system to curve northward toward the central Gulf Coast. For
now, following the latest NHC advisory, a compromise of the two
scenarios was used with the caveat that model spread and
uncertainty is higher than normal for the system.
...Weather Highlights/Hazards...
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. There remains some uncertainty
in the heaviest rainfall axis however across the southern
Appalachians, Deep South, and TN/OH Valleys are the favored
locations for several inches of rainfall that may lead to flash
flooding. For Tropical Storm Grace, heavy rainfall hazards may
impact the southern Florida peninsula and then the system may
emerge into the Gulf of Mexico by next weekend. It's too early to
tell where the system goes beyond that. Follow the latest NHC
advisories for the most updated forecast information.
Elsewhere, monsoonal moisture activity over the Desert
Southwest/Great Basin will again bring a localized flash flood
risk. Moisture should stream northward and northeastward ahead of
the incoming Northwest to north-central U.S. upper trough, with a
potential for moderate to heavy rainfall/convection emerging from
the northern Rockies/high Plains to the north-central Plains and
mid-upper MS Valley mid-later next week, with organized and
enhanced rainfall focusing along a developing frontal system.
Taylor
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