Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
245 AM EDT Sat Oct 27 2018

Valid 12Z Tue Oct 30 2018 - 12Z Sat Nov 03 2018

...Pattern Overview...

A moderately progressive flow pattern across the U.S. early in the
medium range period will become increasingly amplified and slower
by the middle of next week. Guidance agrees that the upper ridge
which has been prevalent off the West Coast will rebuild with a
broad and amplified mean trough setting up over the central U.S.
This flow pattern will set the stage for transport of chilly air
southward into the central U.S. while wet conditions develop on
the eastern side of the evolving trough, especially from the lower
Mississippi Valley/Ohio Valley to the Eastern Seaboard


...Guidance/Predictability Assessment...

A blend of the ECMWF/UKMET/FV3 was used as a starting point for
the forecast during days 3-4 (Tue-Wed). Models showed good
agreement with respect to an upper trough exiting the Northeast
during that time period, but showed somewhat greater spread with
another shortwave moving from the northern Rockies into the
northern plains and Upper Midwest. The 12Z/18Z runs of the GFS
were on the fast side of the spread with respect to this feature,
and were excluded from the forecast as fast outliers. The 00Z GFS
has moved toward the slower consensus. From day 5 (Thu) onward, as
a large scale upper-level trough is carved out across the central
U.S. through the passage of numerous smaller-scale shortwaves,
model spread begins to increase, particularly with respect to the
details/timing/amplitude of these smaller scale features. The
GFS/FV3/UKMET have consistently been more amplified with shortwave
energy crossing the central Rockies/plains and reaching the
Mid-Mississippi Valley by Thu, while the ECMWF/CMC have been less
amplified. The exact intensity of this wave as it crosses the
plains will play a role in the generation/intensity/timing of a
wave along a surface front crossing the lower/mid Mississippi
Valley. An additional upper shortwave will dive southeastward from
the Canadian Rockies into the central U.S. Thu night/Fri. Models
show a range of solutions for this feature by Sat, ranging from a
deep cutoff upper low over the Mid-Mississippi Valley to a more
progressive trough. Due to lower forecast confidence from day 5
onward, weighting of ensemble means was boosted gradually through
day 7 (Sat).


...Weather Highlights/Threats...

The amplifying trough across the central U.S. will favor transport
of Gulf of Mexico moisture and instability northward into portions
of the central/eastern U.S., along/ahead of a surface cold front.
The result will be potentially widespread rain and thunderstorms,
with the potential for areas of heavy rainfall from the
Lower/Mid-Mississippi Valley and Ohio Valley Tue-Wed to the
Mid-Atlantic and Northeast Thu-Fri. the Pacific Northwest and
northern Rockies will also see multiple rounds of precipitation
(rain and high elevation snow) as weakening shortwaves/frontal
systems traverse the northern periphery of the Pacific ridge.
Initially above average temperatures across much of the central
U.S. will quickly decrease by the middle of next week as the upper
trough amplifies and a surface cold front spreads a polar air mass
southward. High temperatures from 5 to 15 deg F below average are
expected for much of the central U.S. and Rockies from the middle
to end of next week. Meanwhile, areas east of the front will see
warmer conditions, with high temperatures potentially 5 to 10
degrees above average across the Mississippi/Ohio Valleys on Tue
and Southeast/Mid-Atlantic on Wed.

Ryan


WPC medium range 500 mb heights, surface systems, weather grids,
quantitative precipitation, winter weather outlook probabilities
and heat indexes are found 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