Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
230 AM EDT Fri Sep 17 2021
Valid 12Z Mon Sep 20 2021 - 12Z Fri Sep 24 2021
...Weather Pattern Overview...
The strong storm system that is currently approaching the Pacific
Northwest is expected to be over the central and northern Plains
by Monday morning, accompanied by a rather strong cold front that
should steadily progress across the central and then eastern U.S.
The large surface high and upper ridge across the eastern U.S.
will eventually give way to the approaching trough and cold front,
which will herald a pattern change for much of the eastern half of
the nation. In addition, a second storm system is forecast to
reach the northwestern U.S. by Wednesday and into Thursday as the
storm track across the northeast Pacific remains active next week.
...Overview and Guidance Evaluation/Preferences...
The deterministic guidance is in above average agreement for the
beginning of next week, with the 18/00Z GFS beginning to show a
faster solution by Tuesday night with the trough/cold front
crossing the Great Lakes and Midwest region. This is even more
apparent with the second trough crossing the northern Rockies and
northern Plains by Thursday, and also ahead of its own ensemble
mean. By next Friday, model spread becomes substantial across
much of the northern tier states and southern Canada, with much of
this owing to the eventual evolution of a possible closed upper
low over southern Manitoba as the second Pacific trough moves
inland.
The WPC front/pressures forecast was primarily based on a nearly
equal blend of the 12Z deterministic guidance through early
Wednesday, and then primarily the 12Z GFS/ECMWF and their
respective ensemble means through Thursday and Friday. The 12Z
run of the GFS was substituted for the 18Z run since the latter
was considerably faster with the cold front across the eastern
U.S. compared to the model consensus, whereas the former is more
in line with the CMC/UKMET/ECMWF.
...Weather and Potential Hazards...
Widespread showers and scattered thunderstorms are expected to
develop along and ahead of the cold front from the Midwest to the
Northeast and Mid-Atlantic states, with some locations getting on
the order of 1 to 2 inches of rainfall with locally higher totals
possible. Some strong to severe storms will also be possible
where favorable parameters of wind shear and instability exist,
although location specifics are uncertain. Winter weather will
also be likely across the higher terrain of northern Wyoming and
western Montana with accumulating snow likely on Monday, followed
by a drier weather pattern by Tuesday and beyond. There may also
be enhanced fire weather concerns for portions of the West,
depending on the magnitude of the pressure gradient and resulting
winds in the wake of the cold front.
It will still feel like summer across much of the southern Plains
and extending northward to the Great Lakes and the Ohio Valley to
begin the week, and more like late October across much of the
Rockies and Intermountain West with highs running 10 to 20 degrees
below average. These cooler temperatures are expected to reach
the Midwest by midweek and finally to the East Coast by Thursday
and Friday, along with much more pleasant humidity levels.
Hamrick
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