Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
238 AM EDT Mon Oct 25 2021
Valid 12Z Thu Oct 28 2021 - 12Z Mon Nov 01 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...
A deep cyclone lifting from the Mississippi Valley to the
Northeast this weekend will keep much of the medium range quite
active. The guidance with this system continues to show good
agreement on the large scale evolution/track, increasing
confidence a bit, but do offer some uncertainties in the details,
which may take until the short range to resolve. A general model
blend seemed a good starting point for this system, maintaining
good continuity with the previous shift. This system may threaten
October low pressure records across parts of the mid-Mississippi
to Ohio Valley.
Elsewhere, the last couple of model runs show the next trough
moving towards the West Coast by the middle of this week may split
sending a northern stream shortwave across the northern Rockies
and northern Plains into next weekend. The guidance has trended
more north and stronger with this system suggesting the Northeast
U.S. low may eventually get wrapped up into this system late
period. The last few model runs hint at a possible southern stream
upper low near California next Sat-Mon, but the ensembles offer
plenty of uncertainty on whether this feature even exists at all.
This cycle of the WPC medium range progs used a blend of the
deterministic models days 3-4 transitioning towards and almost
equal weighting of the GFS/ECMWF with the GEFS/ECENS means by day
7. This helps to mitigate the late period differences.
...Weather/Hazards Highlights...
The biggest weather threat during the medium range period will be
the deep low pressure system working across the eastern third of
the U.S. later this week into next weekend bringing widespread
moderate to locally heavy rains. Closed upper low development and
a slowed frontal system progression will allow rainfall to expand
across the Ohio Valley/Appalachians/Southeast later this week then
the Mid-Atlantic/Northeast into next weekend as aided by a
sustained Atlantic moisture feed. Repeat/duration activity and
favored orographic lift should focus a possible flood/runoff
threat for parts of the central Appalachians.
Out West, shortwave energy progressing towards the West Coast 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 on Thursday into
Friday. Conditions should finally dry out by next weekend though
as upper ridging builds over the region.
For temperatures, the Midwest/South to the East Coast should
remain near or more likely below normal through much of the period
underneath cloudy and unstable skies. Humidity levels though
should allow overnight mins to be more above normal. Behind this,
upper ridging shifts across the central U.S. bringing a period of
above normal (+5-10F) days before the next shortwave moderates
temperatures back towards or even below normal. Out West,
temperatures should generally stay within a few degrees on either
side of normal.
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