Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
246 AM EDT Mon Sep 20 2021
Valid 12Z Thu Sep 23 2021 - 12Z Mon Sep 27 2021
***Heavy rainfall expected for portions of the Northeast U.S. to
close out the work week***
...Weather Pattern Overview...
The longwave trough that is currently crossing the central U.S. is
forecast to evolve into a closed upper low over the Ohio Valley
and Great Lakes region by early Thursday, and then lifting to the
northeast by the end of the week as the cold front exits the East
Coast. Another trough and surface cold front will be tracking
from the Pacific Northwest to the Upper Midwest by Friday night
and this may also evolve into an upper low, although not as strong
as the preceding system based on current model guidance. In its
wake will likely be a developing upper ridge centered over the
Intermountain West for next weekend, along with a weak upper low
trapped under the ridge near the California coast that may bring
some moisture northward across portions of the Desert Southwest.
...Overview and Guidance Evaluation/Preferences...
There has actually been an increase in model uncertainty with the
evolving upper low over the Great Lakes and Ohio Valley for the
end of the week compared to yesterday, and forecast confidence for
this system is below average for a day 3 and 4 forecast, while
near to above average for the remainder of the nation. The ECMWF
and CMC are considerably slower and more southwest with the core
of the upper low compared to the more progressive GFS and GEFS
mean. The UKMET and also the ICON model are generally between
these two sets of solutions, and after reviewing trends in the
guidance and previous WPC continuity, the forecast was hedged more
in the faster direction through Friday. However, if the slower
CMC/ECMWF solutions come to pass, the passage of the cold front
could be delayed and heavier rainfall will likely persist longer
across the Northeast.
Elsewhere across the continental U.S., another synoptic scale
trough is likely to develop over the Great Lakes region in time
for the weekend with perhaps a closed low forming over Michigan
and southern Ontario, with the GFS slightly faster and holds onto
the feature longer over the Northeast U.S. by next Monday, whereas
the other guidance lifts the trough out faster. There are also
some longitudinal differences with the upper low near the
California Coast, with the CMC/ECMWF farther offshore and the
GFS/GEFS mean near the coast.
The WPC fronts/pressures forecast was primarily based on the
12Z/18Z GFS, UKMET, and 18Z GEFS mean for Thursday and Friday,
particularly for the East Coast storm system with slightly more
weighting on the 12Z UKMET given its middle ground solution with
the closed low over the northern Ohio Valley. The ECMWF and its
ensemble mean were introduced into the forecast blend for the
second half of the forecast period after the departure of the East
Coast system.
...Weather and Potential Hazards...
Widespread moderate to locally heavy rain is expected to be
ongoing Thursday across northern portions of the Mid-Atlantic and
much of the Northeast U.S. as the cold front intercepts a deep
plume of moisture ahead of it. The potential exists for
widespread 1-2 inch rainfall totals, with isolated amounts up to 3
inches possible through Friday evening, particularly across the
elevated terrain of upstate New York and New England. There may
be some instances of flooding if higher rainfall rates repeatedly
affect the same areas and for those regions with saturated soils.
There may be some strong thunderstorms as well, but the overall
severe potential appears limited for now.
In terms of temperatures, it will feel fall-like behind the cold
front across the Ohio Valley and the Appalachians on Thursday, and
the cold front should make it to the East Coast by Friday along
with improving humidity levels. It should also feel more pleasant
across the Deep South and Southeast after the front passes through
the region. The opposite will hold true across much of the
western U.S. as a warming trend is likely to commence by next
weekend in response to a building upper level ridge, with highs
expected to be on the order of 10 to 20 degrees above normal
across the western High Plains next Sunday and then expanding
eastward to the Midwest by Monday. Meanwhile, temperatures should
be close to seasonal averages for the eastern third of the nation
for this same time period.
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