Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
215 PM EDT Fri Oct 01 2021
Valid 12Z Mon Oct 04 2021 - 12Z Fri Oct 08 2021
...Overview...
The latest guidance continues to highlight an amplification of the
medium range weather pattern next week. The flow should become
more blocky as an upper low closes off over the east-central U.S.
while an upper ridge/high builds over the northern tier states and
southern Canada. A leading wavy front and closed upper trough/low
support will spread rainfall of varying intensity and duration
across the eastern third of the country. Meanwhile an eastern
Pacific upper trough should steadily amplify and become more
elongated through the period, pushing a cold front into the
Northwest and California. An upper low initially offshore of
California will eject northeastward ahead of the upper trough and
may produce some areas of rainfall over the Four Corners
region/Rockies.
...Guidance Evaluation/Preferences...
The latest models and ensemble means generally agree on the
overall pattern evolution, but continue to show run to run
varability in the details. For the upper low dropping from the
Ohio Valley to the Southeast, exact placement continues to be an
issue, with the last two runs of the GFS showing a farther south
solution, and the ECMWF/UKMET/CMC favoring something farther
north. The ensemble means generally follow suit with their
deterministic counterpart, but do serve as an ok middle ground
solution. Meanwhile, the amplifying eastern Pacific upper trough
nearing the West Coast and leading closed low ejection from
offshore CA offer reasonable continuity, with some spread for how
the energy will be distributed and also overall timing of the
trough axis by late in the week.
This cycle of the WPC progs used a blend of the latest
deterministic guidance (which more weighting towards the ECMWF,
UKMET, and CMC over the GFS) for days 3-5, with increasing
ensemble mean contribution thereafter. This approach maintained
good continuity with the overnight WPC forecast.
...Weather and Potential Hazards...
Areas of rain and thunderstorms will accompany the wavy frontal
system that works across the eastern U.S. early next week and then
possibly stalls near the Southeast coast by midweek. Moderate to
locally heavy rainfall should linger over the Northeast on Monday,
with an increasing focus towards the Southwest, especially as the
upper low settles over the region. This should keep rainfall over
parts of the Southeast/East through much of next week with an
uncertain but gradual focus from the Southeast into the
Mid-Atlantic. The guidance continues to signal that some locations
could see significant totals, whether from an episode of heavy
rain and/or a multi-day accumulation due to persistence of the
pattern. Guidance variability and spread up to this point keep
confidence in specifics fairly low, but one area to monitor will
be the Southern Appalachians where terrain enhancement may be a
factor.
Expect the upper low ejecting northeastward into the West to
generate a period of rainfall over parts of the Southwest, Four
Corners, and parts of the Rockies mainly around Tuesday-Wednesday.
The front pushing into the Pacific Northwest and northern
California will spread areas of rain gradually southeastward with
time mid-later next week. Most activity should be in the
light-moderate range, but a few pockets of heavier rain could be
possible with snow in the higher elevations. The West Coast will
see moderately below normal daytime highs beginning around midweek
as the upper trough approaches and the cold front makes its way
through the region. 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 moderate daytime temperatures, but should maintain decent
coverage of plus 10F and greater anomalies for morning lows.
Santorelli/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