Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
233 AM EDT Tue Sep 07 2021
Valid 12Z Fri Sep 10 2021 - 12Z Tue Sep 14 2021
...Overview...
The medium range period will feature a trend toward quasi-zonal
flow as an upper high over the Four Corners region weakens into
the southern Plains and Southeast. Troughing will reload over the
Northeast as a couple systems traverse the U.S./Canadian border.
The pattern remains rather dry with generally near to above normal
temperatures.
...Guidance Evaluation and Preferences...
Models and ensembles showed decent longwave agreement with much
more spread in the shorter wavelength features. Perhaps the ECMWF
has been more consistent than the GFS or Canadian/UKMET, but even
it has wavered in how it handles the Pacific flow across the
CONUS. Thus, continued to favor a blended solution of the recent
deterministic guidance to start, with increasing weight to the
ensemble means (GEFS and ECMWF EPS) by the end of the period as
uncertainty continues to grow. Trend has been toward a deeper
trough in the northern Rockies early next week, but the east-west
(fast-slow) spread continues, with the GFS quickest and Canadian
slowest. This left the ECMWF mostly in between nearer to the
ensemble means.
Per the latest NHC outlooks, a tropical system is likely to form
in the East Pacific over the next day or so but is forecast to
track offshore/west of Baja California. In the Gulf of Mexico,
tropical development remains uncertain but by the medium range it
could be on its way out into the Atlantic. Please see the NHC
outlooks for more information.
...Weather/Hazard Highlights...
Rainfall will focus along and ahead of a cold front (perhaps fused
with some additional tropical moisture) into the Deep South and
Florida to start the period. Daily showers/storms are likely for
much of the FL peninsula to the south of the boundary as it washes
out. Some moisture will lift northward on the backside of the
upper high into the northern Rockies, with generally light
amounts. With shortwaves and frontal systems traversing across the
north-central to northeastern U.S., periods of rain showers can be
expected there. By around next Monday, tropical moisture could
spread some organized rain toward western parts of the Gulf Coast
(including south Texas), but uncertainty remains high.
Temperatures will be near to above normal over most areas,
especially over the Front Range on Friday with highs 10-20
degrees above normal. This area of warmth will move into the
southern/central Plains over the weekend and toward the Middle
Mississippi and Ohio Valleys next week. Temperatures in the Great
Lakes/Northeast will be near to a bit below normal via northwest
flow behind the cold front Friday, moderating a bit over the
weekend as the surface high settles over the region. The next cold
front may bring in another shot of slightly cooler than normal
temperatures across the northern tier next Mon-Tue.
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