Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
254 AM EDT Thu Sep 30 2021
Valid 12Z Sun Oct 03 2021 - 12Z Thu Oct 07 2021
...Overview...
The medium range period will feature the potential for an upper
low to close off in the east-central U.S. to the south of an upper
ridge/high, while at the surface a cold front moves slowly
eastward and spreads showers to eastern third of the country.
Farther west, troughing should amplify through the period along
the West Coast and push a cold front through the Northwest and
California.
...Guidance Evaluation/Preferences...
Model guidance is reasonably agreeable with the overall pattern
described above. Though with slightly different evolutions, models
show multiple shortwaves joining over the Midwest around Monday to
form a closed low as the week progresses, as upper ridging
building over portions of the central U.S. could spread into
south-central and southeastern Canada around midweek and separate
the low from the northern stream flow. The 12/18Z GFS and 12Z
ECMWF show this type of solution, and both center an upper low
over the Mid-South on Wednesday/Thursday. However, the placement
of the closed low has varied from run to run in each model, along
with the upper ridge/high location and track. The newer 00Z GFS
has an upper low closing off and diving even farther southwest
toward the central Gulf Coast by midweek, with an upper high over
the Midwest. The 00Z ECMWF shows a bit of a southern trend as
well, with the center of the low over the Deep South. The
differences in the placement reduce confidence for the exact
positioning of the features, but at least the general idea for the
pattern has been persistent.
Saw fairly good model agreement regarding the western U.S., as an
initial relatively weak southern stream low tracks from the
eastern Pacific into southern California around Tuesday and then
lifts northeastward and potentially gets absorbed midweek. This
feature is in addition to/ahead of the positively tilted
amplifying trough along the West Coast, which also shows good
consistency among model guidance.
Overall, the WPC forecast was based on a multi-model deterministic
blend of the 12/18Z GFS runs and the 12Z ECMWF/UKMET/CMC for the
first half of the period, before reducing CMC weighting due to
some inconsistencies with the pattern in the East and a weaker
initial western shortwave, and adding components of the GEFS and
EC ensemble means by the latter part of the forecast period. The
main change to the previous forecast was a faster entry of the
trough and cold front into the Northwest per model trends.
...Weather and Potential Hazards...
As the frontal system tracks across the central and eastern U.S.,
showers and thunderstorms are likely in its vicinity, shifting
across the Mississippi Valley toward the Ohio and Tennessee
Valleys early in the week, and spreading to the Eastern Seaboard
as the workweek begins. Showers should then linger in the East
underneath and on the eastern side of the upper low through at
least Thursday. Some embedded heavy rain is possible, and while it
is still uncertain where higher totals will occur, terrain
enhancement of the rainfall in the Southern Appalachians could
lead to locally heavier amounts. Meanwhile, monsoonal moisture may
make its return to the Four Corners states next week supported by
the leading shortwave. Tuesday currently looks to have the best
chances for rainfall there. Then as the troughing and cold front
come into the West Coast, showers are possible for the Pacific
Northwest into northern California.
Warmer than average temperatures are forecast for the
north-central U.S. through the period underneath upper ridging
traversing the area. Highs in the upper 70s and 80s in the
Northern Plains will be 10-20F above normal for this time of year.
A mild to warm Sunday is expected for portions of the East early
in the week before temperatures moderate closer to normal with the
cold frontal passage. The West Coast will see cool temperatures
beginning around midweek as the trough approaches and the cold
front makes its way through the region.
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