Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
126 AM EDT Sat Sep 22 2018

Valid 12Z Tue Sep 25 2018 - 12Z Sat Sep 29 2018

...Pattern Overview and Guidance Evaluation/Preferences...

Confidence remains high that an increasingly amplified upper-level
flow pattern will take from the North Pacific across North America
during the medium range. An anomalous upper ridge axis is expected
to gradually strengthen through the forecast period off the North
America west coast extending north across Alaska to the arctic.
This will favor downstream troughing across the northern plains
and Midwest, which along with a persistent upper low over the high
latitudes of Canada, will favor transport of chilly polar air
southward into the U.S., from the northern Rockies to the Upper
Midwest. A series of upper shortwaves (each accompanied by surface
frontal systems) will gradually carve out the upper trough through
the period. The amplitude of a shortwave digging into the
northwestern states by Fri-Sat will likely play a significant role
in the evolution of the upper trough farther east, and introduces
a fairly significant degree of uncertainty into the forecast by
late in the medium range, particularly with respect to to the
degree that polar air is able to penetrate southward into the U.S.
northern tier.

Given above average model consensus early in the forecast period,
a multi-model deterministic blend (including the
ECMWF/GFS/UKMET/CMC) was used as a basis for the WPC forecast for
days 3-5 (Tue-Thu). For days 6-7 (Fri-Sat), weighting of ensemble
means (ECENS/NAEFS) was increased to comprise a majority of the
forecast blend. As mentioned above, despite general large scale
agreement, uncertainty with respect to the details begins to
quickly escalate by day 7, particularly with respect to the degree
of shortwave amplification across the northwestern U.S., and the
impact on heights and upper flow downstream across the northern
plains and Midwest. The 12Z ECMWF remained the most aggressive
solution with transporting cold Canadian air southward into the
U.S. northern tier by late next week (by means of an impressive
1040 hPa surface high sliding southward along the eastern slope of
the Canadian Rockies). The GFS, however, digs the shortwave a bit
more across the Northwest and keeps higher heights in place across
the central U.S. with less transport of cold air southward into
the central U.S. (but perhaps more into the northern Great Basin).


...Sensible Weather Highlights...

A cold front crossing the central/eastern states Tue-Wed (in
association with the first in the series of upper shortwaves
crossing the northern tier) will have access to relatively deep
tropical moisture, and areas of heavy rainfall will be possible
from the southern plains/lower Mississippi Valley to the
Northeast. There is some suggestion among the guidance that
additional moisture associated with a tropical disturbance off the
Eastern Seaboard could feed northward into the frontal system,
potentially enhancing rainfall across portions of the northern
Mid-Atlantic or Northeast. The frontal boundary is expected to
become stationary from the southern plains to the Southeast
Wed-Thu as the second cold front eventually approaches, continuing
to focus locally heavy rains across those areas through late week.

The passage of each cold front is expected to spread successively
cooler temperatures into areas from the northern Rockies to the
Midwest through the next week, with potentially the coolest
temperatures arriving by Fri-Sat. High temperatures as low as
15-20 deg F below average are possible by late next week
(potentially even lower if the cooler solutions were to verify).
Meanwhile, areas across the Southeast and the Southwest, away from
the influence of the amplifying upper trough, will remain 5-10 deg
above average through most of the forecast period.


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