Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
250 PM EDT Sat Sep 18 2021
Valid 12Z Tue Sep 21 2021 - 12Z Sat Sep 25 2021
...Heavy Rainfall Threat from the Great Lakes/Ohio Valley through
the Mid-Atlantic/Northeast...
...Guidance Evaluation/Preferences and Weather/Threats
Highlights...
Medium range model forecast spread has decreased Tue into Wed, but
uncertainty grows through later next week. The WPC medium range
product suite was primarily derived from a blend of the 00 UTC
ECMWF with the 06 UTC GFS/GEFS and the 13 UTC National Blend of
Models. The last few ECMWF ensemble runs were not available at WPC
due to computer issues. There has been a significant recent trend
in most guidance toward more robust flow amplification through
medium range time scales and this WPC blend adjusts product
continuity in that direction. The 12 UTC GFS has bucked this
trend, offering limited outlier uncertainty, but the rest of newer
12 UTC guidance remains in line to bolster forecast confidence.
In this scenario, potent system energy set to dig through the
Midwest may prove to be a gradually progressive heavy rainfall
producer and local runoff threat from the Great Lakes/OH Valley to
the Mid-Atlantic Northeast Tue-Thu, perhaps lingering into Fri
over the New Eng depending on uncertain upper system ejection by
approaching upstream northern stream energies. The threat is
expected to be fueled by pooling moisture/instability as guidance
now develops a supporting closed upper low and slows progression.
The system will be accompanied by a strong and focusing cold front
that will progress across the central and eastern states that
would also provide some focus for more modest rainfall down across
the South/Southeast. The lead large surface high and upper ridge
that will be across the eastern U.S. early in the week will
eventually give way to the approaching trough and front, which
will herald a pattern change compared to the recent extension of
summer-like heat and humidity. Steady and amplified upstream upper
trough progression through western Canada now noted in guidance
Wed/Thu would also be primed as a kicker to dig into the
north-central U.S./Midwest later next week along with another
associated frontal system, but with much less precipitation
potential with depleted moisture availability in the wake of the
main lead system. Best potential may over vicinity of the Great
Lakes under the meat of the trough. Meanwhile, weaker eastern
Pacific upper trough energy set to reach the Northwest midweek is
trending to now dig down the West Coast and eastern Pacific to
offer minimal additional precipitation potential for the region.
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