Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
240 PM EDT Sun Oct 24 2021
Valid 12Z Wed Oct 27 2021 - 12Z Sun Oct 31 2021
...Major storm to focus a heavy rainfall/runoff threat from the
Midwest/Southeast Thursday to the Mid-Atlantic/Northeast into next
weekend...
...Overview and Guidance/Predictability Assessment...
As one deep cyclone exits the Northeast to start the period on
Wednesday, the next one will be spinning up over the southern
Plains, keeping the weather pattern quite active across the
eastern half of the nation into next weekend. Guidance continues
to trend less progressive and more amplified with this system as
it tracks from the Southern Plains into the Mid-Atlantic/Northeast
by next weekend. The latest models actually now show much better
agreement on the evolution/track of this system overall,
bolstering forecast confidence, with a possible southern stream
closed low and deep associated surface low. Lingering
uncertainties remain in the details however, especially late
period. The ECMWF/ECMWF ensembles continues to be slightly
south/less progressive than the rest of the global guidance, but
the trend has been to the south so the ECMWF is not unreasonable.
Well upstream, a favorable shortwave and upper jet pattern back
into the Pacific Northwest mid-late week will foster another
multi-day threat for heavy rain/mountain snows for the Northwest.
Guidance shows reasonable agreement with this setup, though timing
and strength differences arise once the system moves inland across
the northern Rockies and into the northern Plains next weekend.
Accordingly overall, a blend of the ECMWF/ECMWF ensemble mean
along with WPC continuity and the National Blend of Models was
primarily used to produce the WPC product suite.
...Weather/Hazards Highlights...
The bulk of the rainfall on the backside of the low exiting the
Northeast should push offshore Wednesday, but some lingering
wrapback activity will remain possible across New England.
The biggest weather threat during the medium range period will be
with a deep low pressure system working across the central to
eastern U.S. later this week that is slated to slowly lift into
the Northeast into next weekend. Ample Gulf of Mexico moisture
ahead of this system will present a protracted rainfall/convective
focus with widespread moderate to locally heavy rains possible.
The Storm Prediction Center also is highlighting the potential for
severe weather across the central Gulf Coast. Closed upper low
development and a slowed frontal system progression will allow
rainfall to expand into the Ohio Valley/Appalachians then
Mid-Atlantic/Northeast into next weekend as aided by Atlantic
moisture feed. Repeat/duration activity and favored terrain lift
will focus the runoff threat.
Out West, shortwave energies progressing entering the Northwest
mid to late this week will direct a plume of moisture to support
another round of heavy rain/mountain snows to mainly parts of the
Pacific Northwest, but also farther inland across the northern
Rockies. Conditions should finally dry out by next weekend as
upper ridging works into the region.
Schichtel/Santorelli
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