Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
201 AM EDT Sun Oct 28 2018

Valid 12Z Wed Oct 31 2018 - 12Z Sun Nov 04 2018

...Pattern Overview...

A moderately progressive flow pattern across the U.S. will become
increasingly amplified during the extended forecast period. The
upper ridge which has been prevalent off the West Coast over the
past few months is expected to become reestablished by midweek,
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...

While models show broad agreement on the general evolution of the
large scale central U.S. trough through the period, they continue
to struggle with the details of smaller scale short waves
traversing the flow. A number of solutions (including a sizable
number of ensemble members) have trended toward a more amplified
shortwave exiting the central/southern Rockies into the plains on
day 3 (Wed). After lagging this trend, the 12Z ECMWF is now among
the more amplified deterministic solutions, especially by Thu (day
4) by the time the system reaches the Mississippi Valley, showing
a relatively deep surface low across the Lower Mississippi Valley
by 12Z Thu. At this time, a blend of deterministic guidance was
favored during days 3-4 to smooth out these differences, including
the GFS/ECMWF/CMC.

From day 5 (Fri) onward, things become even more murky as
additional shortwave energy traverses the trough across the north
central U.S. and amplifies as it reaches the Ohio/Tennessee
Valleys. The 12Z ECMWF made a significant change from previous
runs (and other guidance), ejecting the main upper-level trough
northeastward into eastern Canada much more quickly, with heights
rising again across much of the southeastern U.S. The GFS has
shown a greater degree of run-to-run consistency by this time
frame, and has a significant degree of ensemble support for a
slower progression of this second wave, keeping the energy near
the northeast U.S. coast into day 7 (Sun). Also by next weekend,
additional fast-moving/low amplitude shortwave energy arriving
from the Pacific should reach the central U.S., perhaps
reinforcing the large scale trough. Given the time range and the
small scale of these features, model spread is, as would be
expected, large. Given the described differences shown by the 12Z
ECMWF, it was excluded from the WPC forecast starting on day 5,
and weighting of the ECENS/GEFS ensemble means was gradually
increased through day 7 given the quickly increasing spread.


...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
and upper shortwave energy. The result will be potentially
widespread rain and thunderstorms, with the possibility for heavy
multi-inch rainfall totals from the Lower/Mid-Mississippi Valley
to the Ohio Valley Tue-Wed. Widespread rains are also expected to
reach the Eastern Seaboard, with amounts likely not as heavy as
farther west. The Pacific Northwest and northern Rockies will also
see multiple rounds of precipitation (rain and mountain snow) as
weakening shortwaves/frontal systems traverse the northern
periphery of the Pacific ridge. A chilly, polar air mass will
accompany the amplifying upper trough across the central U.S. and
portions of the Rockies, with high temperatures from 5 to 15 deg F
below average from mid to late week. Meanwhile, areas east of the
front will see warmer conditions, with high temperatures expected
to reach 5 to 10 degrees above average across the Southeast and
Mid-Atlantic Wed-Thu. Cooler temperatures will arrive in these
areas by late in the week as the surface cold front passes.
Temperatures will also be above average along much of the West
Coast through the week just downstream of the Pacific upper ridge.

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