Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
150 AM EDT Thu Aug 12 2021
Valid 12Z Sun Aug 15 2021 - 12Z Thu Aug 19 2021
...Tropical Storm Fred Heavy Rain Threat from Florida this weekend
to the Southeast/Appalachians/Mid-Atlantic early-mid next week...
...Pattern Overview...
Models and ensembles show amplified troughing moving into the
Northwest on Sunday shifting slowly eastward early next week.
Meanwhile, an upper level trough will move through the Northeast
Sunday-Monday, with heights beginning to rise again across much of
the East by later in the medium range period. Tropical Storm Fred
will likely affect Florida this weekend bringing heavy rainfall,
and then also into parts of the Southeast, southern to central
Appalachians and Mid-Atlantic early-mid next week as tropical
moisture feeds inland and interacts with a stalled leading frontal
zone draped to the north of Fred and terrain.
...Guidance/Predictability Evaluation...
The NHC tracks Tropical Storm Fred over the far eastern Gulf of
Mexico as the medium range period begins on Sunday before lifting
inland into the Southeast and up the Appalachians early next week
as a gradually weakening but very wet system both along/east of
the track and in advance with deep moisture feed into a draped
front. Fred offers a maritime and inland heavy rainfall/runoff
threat. There remains some considerable spread regarding Fred's
track through the eastern Gulf and into the Southeast. Please
refer to NHC guidance regarding the details and official forecast
track.
Elsewhere, there's fairly good agreement to suffice a
deterministic blend for days 3-4 including the latest runs of the
ECMWF, GFS, and CMC. The second half of the period there remains
some uncertainty around the amplified Western U.S. troughing. The
GFS is more amplified and brings a closed low into the northern
Rockies, while the ECMWF and CMC are a little weaker and both lift
the closed low feature well into central Canada. Given the late
period timing, opted to lean more on the ECENS/GEFS ensemble means
which were more agreeable. Axis wise, there's decent agreement,
but it's the detail and dynamics differences which make the QPF
forecast across the northern Rockies/northern High Plains very
uncertain. A blend towards the ensemble means also supports the
growing forecast spread surrounding Fred after it makes landfall
likely near the Florida Panhandle.
...Weather Highlights/Hazards...
The ongoing heat wave over the Northwest during the short range
should finally break by the medium range period. Sunday
temperatures should still be above normal, but not nearly as
extreme as previous days. By Monday, troughing looks to quickly
move into the region so temperatures should actually rebound below
normal across the Northwest. Some portions of the Northern Rockies
could be 10-15+ degrees below normal next Tuesday into Wednesday.
Above to much above normal temperatures will shift eastward Sunday
and Monday into parts of the Northern Plains as well.
Heavy rainfall and gusty winds across Florida associated with
Tropical Storm Fred will be ongoing on Sunday, especially along
the western Florida Gulf coast. Tropical moisture will likely
spread northward even ahead of any landfall by Fred into the
Southeast, Appalachians, and Mid-Atlantic, interacting with a
leading stalled frontal boundary and favorable terrain. Exact
locations of heaviest rainfall depends on the eventual track of
Fred and may still take another few days to resolve.
Elsewhere, as an upper ridge axis in the Western U.S. shifts east
with advent of upper troughing over the Northwest, 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 moderate to heavy rain threat emerging
over parts of the northern Rockies/high Plains on Tuesday, with
organized and enhanced rainfall then focusing along the cold front
as it shifts into the Northern Plains Tuesday into Wednesday.
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