Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
254 AM EDT Wed Sep 01 2021
Valid 12Z Sat Sep 04 2021 - 12Z Wed Sep 08 2021
...Guidance Evaluation and Preferences...
Models and ensembles continue to show fairly good agreement on the
large scale upper flow pattern through much of the medium range
period for much of the nation, but still struggle with smaller
scale individual systems, especially by early next week.
Accordingly, the WPC medium range product suite was primarily
derived from a composite blend of best clustered guidance from the
18 UTC GFS/GEFS mean, 12 UTC ECMWF/ECMWF ensemble mean and the 01
UTC National Blend of Models. This maintains reasonably good WPC
continuity for a pattern with near average forecast
predictability. This forecast baseline remains in line with newer
00 UTC guidance composites despite lingering embedded system
timing variances.
...Pattern Overview with Weather/Hazard Highlights...
A series of energetic northern stream upper troughs will force
several wavy frontal systems into the Northwest and out though the
north-central U.S. and increasingly southward into next week
across the central and eastern U.S. with main upper trough
reinforcement/amplification. Conditions seem best for some locally
enhanced lead system convection from the Great Lakes to the
Mid-South and central Plains Saturday, with more modest potential
downstream more into the southern and eastern U.S. Sunday into
Monday. The next trough to move into the north-central U.S. should
bring another chance for organized rainfall across parts of the
Midwest and Upper Great Lakes by next Monday into Tuesday, with
scattered trailing frontal activity then again settling down over
the southern and eastern states into next midweek in response to
upper trough amplification.
The breakthrough approach of northeast Pacific systems into
western Canada and the Northwest U.S. may foster some moderate
rain potential into northwest WA this weekend. Farther south,
lingering deepened moisture may fuel some local downpours as
enhanced by terrain over the southern Rockies into Saturday around
the western periphery of a potent south-central U.S. upper ridge.
The hot ridge shifts slowly westward over the Southwest/West next
week forcing a modest monsoonal channel back into southern
NM/AZ/CA to the east of an upper troughing off CA.
This ridge transition may also offer some room for
moisture/convection to lift across the western Gulf of Mexico in
about a week. NHC has collaborated bringing a tropical wave into
the Bay of Campeche to monitor for any development.
Schichtel
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