Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
226 AM EDT Wed Sep 08 2021
Valid 12Z Sat Sep 11 2021 - 12Z Wed Sep 15 2021
...Overview...
Quasi-zonal flow over the CONUS next week amid a mostly dry
pattern should spell a relatively quiet period, but with
uncertainty in system details. The western Gulf/south Texas has
the best chance for appreciable rainfall depending on how much
moisture a possible tropical wave brings to the area. Temperatures
will be near to below normal along the U.S./Canadian border and
near the Gulf, but warmer than normal in between.
...Guidance Evaluation and Preferences...
Models and ensembles showed decent longwave agreement with
continued spread in the shorter wavelength features. Continued to
favor a blended solution of the recent deterministic guidance to
start, minus the UKMET which was quite slow into the Northwest
this weekend, with increasing weight to the ensemble means (18Z
GEFS and 12Z ECENS) Mon-Wed as uncertainty grows.
Unclear/inconsistent trends or shifts in the guidance (not unusual
in lower amplitude flow) have made it difficult to favor one
solution over another.
In the Gulf of Mexico, the GFS remained the strongest with any
tropical entity (though likely just a weak surface reflection) and
thus rainfall. Favored a more modest depiction and subsequent QPF.
...Weather/Hazard Highlights...
Daily showers/storms are likely for much of the FL peninsula to
the south of the boundary as it washes out early next week.
Frontal boundary over the northern tier will likely only support
light rainfall for the High Plains before a modest injection of
moisture from the south may promote more scattered showers across
the Great Lakes Monday night/Tuesday. This should slide into the
Northeast next Tuesday into Wednesday per current timing. Will
continue to adjust rainfall forecasts for the western Gulf,
particularly over south Texas/the lower Rio Grande Valley pending
any organization of favorable ingredients.
Temperatures will be above normal under the ridge and south of the
surface boundary this weekend from the southern Plains eastward.
High temperatures may be 5 to 15 degrees above normal where
several high temperature records will be challenged. This area of
warmth will temper a bit as it moves toward the Middle Mississippi
and Ohio Valleys next week and then to the Mid-Atlantic.
Temperatures along the Canadian border will linger near to below
normal through the period. Areas around the Gulf will see typical
mid-September temperatures in the upper 80s to around 90F.
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