Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
300 PM EDT Wed Sep 29 2021
Valid 12Z Sat Oct 02 2021 - 12Z Wed Oct 06 2021
...Overview...
The slow-moving front extending across the Great Plains should
begin to move eastward as the medium range period begins on
Saturday, spreading scattered showers/thunderstorms and possibly
embedded heavy rain eastward across the Ohio Valley on Sunday and
Monday, and into the East Coast Tuesday and Wednesday. Over the
Pacific Northwest, the next moisture plume is forecast to slowly
edge into the region by early next week. Meanwhile, some
monsoonal moisture appears to make a return across the
Four-Corners region by early next week.
...Guidance Evaluation/Preferences...
Model guidance are coming into better agreement with respect to
shifting the slow-moving frontal boundary across the Plains
progressively eastward beginning on Saturday. Both of the ECMWF
and GFS have reached relatively good agreement on this eastward
shift, with a more typical slightly faster GFS verses the slightly
slower ECMWF. The slower ECMWF favors a weaker low pressure wave
moving across the central Plains behind a main low pressure center
forecast to drift southeastward across the Great Lakes by both
models. By next Tuesday, the 00Z ECMWF develops this system over
New England ahead of an amplifying upper low over the Great
Lakes--a scenario that is not yet supported by its ensemble mean.
Therefore, a consensus of the 00Z ECMWF/00Z EC mean with the 06Z
GFS/GEFS was adopted to handle this system, with more weights
given to the EC mean for Days 5-7 relative to its deterministic
component. This blend should provide a proper handle to the next
system of interest moving into the Pacific Northwest. The 00Z CMC
has not been included in the blend due to significant deviation in
the general synoptic pattern across the U.S. through much of the
medium-range period.
...Weather and Potential Hazards...
Scattered showers and thunderstorms can be expected to spread
gradually eastward across the Ohio Valley on Sunday and Monday
ahead of a frontal system, with some embedded heavy rain possible.
Steady rain is now forecast for New England Sunday and Monday
north of a stationary front. The rain could linger into Tuesday
as a low pressure center drifts across the Great Lakes along the
stationary front toward the northern Mid-Atlantic region. The
scattered showers and storms should then affect much of the
eastern U.S. Monday and Tuesday and possibly into Wednesday as the
front slowly moves across and could reach the East Coast on
Wednesday. Meanwhile, some monsoonal moisture appears to make a
return across the Four-Corners region by early next week as an
upper low off the southern California coast is forecast to move
onshore. Over the Pacific Northwest, the next moisture plume
ahead of a positively-tilted trough is forecast to slowly edge
into the region by early next week.
Warmer than average temperatures are forecast for the Midwest and
Ohio Valley especially in terms of lows, but even highs should be
around 10 degrees above normal on Saturday, though are likely to
moderate to near normal by Monday. The Northern Rockies/Northern
Plains can expect highs around 10 to 15 degrees above average for
the first half of next week underneath the upper high forming.
Meanwhile, temperatures a few degrees below normal are forecast
for the Four Corners states and into southern/central parts of the
Plains through the period.
Kong/Tate
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