Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
245 AM EDT Mon Oct 04 2021
Valid 12Z Thu Oct 07 2021 - 12Z Mon Oct 11 2021
...Southeast/Southern Appalachians/Mid-Atlantic Heavy Rain
Threat...
...Overview...
Guidance overall agrees that the upper flow amplifies this week as
an upper low closes off over the east-central U.S. and an upper
ridge builds over the north-central states and southern Canada. A
leading front and upper support will spread an enhanced rainfall
threat across the eastern third of the country. Meanwhile, an
eastern Pacific upper trough will amplify and become more
elongated/split and work inland through this period, pushing a
wavy cold front over the West and across the Rockies into the
north-central U.S. along with moderate precipitation chances.
...Guidance Evaluation/Preferences...
Guidance continues to offer fairly good agreement on the large
scale, bolstering forecast confidence, albeit with continued
variability in individual features and details. There is improving
forecast clustering with the difficult upper low meadering over
the east-central U.S. into later this week and out West, amplified
troughing along the coast splits with northern stream energy
driving a cold front over the Northwest/West then central U.S. as
southern stream energy energy ejects over the Southwest and over
the southern Rockies to the Southern Plains late period.
The WPC medium range product suite was mainly derived from a
composite blend of the 18 UTC GFS/GEFS mean, the 12 UTC
ECMWF/ECMWF ensemble mean/Canadian and the 01 UTC National Blend
of Models. This maintains max WPC continuity. Latest and
compatible 00 UTC models and ensembles overall remain in line with
the aforementioned composite blend.
...Weather and Potential Hazards...
Widespread areas of rain and thunderstorms will accompany a
frontal system as it slows across the Southeast/Mid-Atlantic this
week, with enhanced focus as a closed upper low rotates over the
region. This should keep rainfall over parts of the Southeast/East
for several days, with a gradual focus from the Southeast into the
Mid-Atlantic. Guidance continues to signal that some locations
could see some heavier local totals, whether from an episode of
heavy rain and/or a multi-day accumulation due to persistence of
the pattern. One area to monitor for a threat of runoff issues in
particular will be the Southern Appalachians where terrain
enhancement will also be a factor. Lighter precip should also
eventually work its way into the Great Lakes and the Northeast by
the weekend.
A wavy cold front digging slowly but robustly across the West will
spread areas of rain gradually southeastward mid-later week and
the weekend, eventually working across the Rockies into the
north-central states. Most activity should be in the
light-moderate range, along with snow at higher elevations. The
West should trend cooler with time as the upper trough approaches
and the cold front makes its way through the region. Daytime highs
by later next week could be 5-15 degrees below normal for some
locations. Downstream upper ridging traversing the north-central
U.S. will support warmer than average temperatures across the
Northern Plains and vicinity next week with some plus 10-20F
anomalies that equate to highs in the upper 70s and 80s.
Persistence of clouds and areas of rain with the emerging closed
upper low/trough and lead frontal system over the Southeast/East
should keep daytime temperatures near normal, but also maintain
decent coverage of plus 10F and greater anomalies for morning
lows.
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