Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
223 AM EDT Sat Sep 25 2021
Valid 12Z Tue Sep 28 2021 - 12Z Sat Oct 02 2021
...Overview...
The medium range period begins on Tuesday with a mid/upper-level
trough axis along the Pacific Northwest, troughing across the
Northeast stemming from an upper low in eastern Canada, with
mainly ridging in between in the central U.S., with the exception
of a small vort max in the Southern Rockies. This general
trough-ridge-trough pattern should maintain itself through the
rest of the week while amplifying, as additional energy
strengthens both troughs and narrows the ridge, and potentially
creates closed lows over portions of both the West and the East by
the end of the week.
...Guidance Evaluation/Preferences...
While model guidance is generally agreeable with the overall
trough-ridge-trough pattern described above, differences in
placement of the features as well as their evolution certainly
remain. In the East, models are well clustered with the upper low
position in Canada and subsequent troughing on day 3/Tuesday, but
the 12Z UKMET became an outlier by Wednesday by dropping a
shortwave into the northeastern U.S. unlike other guidance. For
the latter part of the week, most deterministic guidance is now
suggesting that an upper low will close off somewhere along the
Eastern Seaboard/western Atlantic. There is low confidence with
the positioning and movement of this upper low, however, with the
12Z ECMWF and CMC showing the upper low centered over the
Mid-Atlantic on Friday and retrograding on Saturday, while GFS
runs have the low's centroid remaining offshore. Cluster analysis
of the multi-suite ensemble members shows that these are both
possibilities, along with showing latitudinal variations and even
some solutions without a closed low. So attempted to take a middle
ground approach for now in the upper low position and strength,
with changes expected in future forecasts with newer guidance.
In the West, there is good model agreement through midweek as the
trough pushes through the West. By around Thursday, differences
begin to arise with the potential northern/southern stream
separation as energy spins into the Southwest, with some guidance
forming a closed low at times. Then additional shortwaves stemming
from the northeast Pacific vary with their timing and tracks by
Friday/Saturday, indicating lower predictability by then.
The WPC forecast utilized a blend of the 12/18Z GFS and 12Z ECMWF
and CMC deterministic guidance for the early part of the period,
and reliance on the GEFS and EC ensemble means increased by the
end of the period to reduce some of the differences between
individual models.
...Weather and Potential Hazards...
Showers and thunderstorms are likely across the Southern Plains
beginning around Tuesday, but rain should become heavier and more
widespread there for the latter part of the week, as moisture from
both the Gulf of Mexico and the Pacific streams into the region
ahead of the second round of energy in the Southwest. Some rain is
also forecast to spread farther north in the Plains especially
around midweek focusing along and ahead of a cold front. Scattered
showers in parts of the Four Corners states may also become more
widespread and heavier through the period as well. The Pacific
Northwest should see periods of orographically enhanced
precipitation next week, with snow possible in the highest
elevations. Meanwhile, a round of light to moderate rain is
possible along a front in the Mid-Atlantic on Tuesday into
Wednesday. Then depending on the eastern upper low's track,
moisture may spread into the Northeast by the end of the week.
Temperatures are expected to be warmer than average for the
north-central U.S. throughout the period underneath the
upper-level ridge. The Northern Plains in particular could see
temperatures around 15-25 degrees above normal, with highs in the
mid to upper 80s on Tuesday. Troughing across the western U.S.
will result in below normal temperatures, with highs 10-20 degrees
below average spreading from the Pacific Northwest into the Great
Basin. Meanwhile, temperatures in the East will generally be right
around average.
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