Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1148 AM EDT Mon Sep 24 2018

Valid 12Z Thu Sep 27 2018 - 12Z Mon Oct 01 2018

...Early-season cold air expected to gradually spread into areas
from the northern Rockies to the Upper Midwest later this week...


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

Expect an amplified upper-level flow pattern to persist across
North America during the medium range period. An
anomalous/blocking upper ridge will likely shift northward from
the North Pacific into Alaska from late this week into the
weekend, with 500-hPa heights reaching +3.5 standard deviations
across interior Alaska by late this week. The ridge along with
downstream persistent troughing just west of Hudson Bay will
promote southward transport of polar air through central Canada
and into the northern Plains and vicinity. Meanwhile upper ridging
initially centered off the Southeast should gradually build
westward across the southern U.S. and Gulf of Mexico. The
combination of these features will favor the setup of a persistent
and relatively strong baroclinic zone across the central Plains by
later this week. Farther west over the Pacific, upper-level energy
will begin to undercut the strengthening Alaska ridge, with a
number of model solutions suggesting the potential for rather
strong height falls off/along the Pacific Northwest coast by early
next week. Specifics of this undercutting flow and western Canada
flow downstream from the ridge amplifying into Alaska will play a
role in determining the timing of an upper low that should be
approaching the West Coast as of early Sat.

Through about days 5-6 Sat-Sun latest model/ensemble guidance
maintains reasonable agreement and continuity aside from typically
unavoidable subtle adjustments. Features of interest include low
pressure that should track northeastward from western Ontario
early Thu onward, bringing an Upper Midwest/Plains front into the
East by Fri-Sat, a trailing front dropping into the northern tier
states by Fri and settling into the central Plains/middle MS
Valley by the weekend (slightly faster timing than continuity),
and the upper low that should be nearing northern California by
early Sat. If anything, guidance consensus has adjusted a tad less
amplified with elongated troughing over/near British Columbia
which yields a trimming of the fast side of prior guidance spread
(some ECMWF runs and the 12Z/23 GFS) for the aforementioned upper
low.  Also worth noting, the latest ECMWF/GEFS means are fairly
similar over the contiguous U.S. during the period but the ECMWF
mean has a leader among the means (along with a number of
operational model solutions) in depicting the closed upper high
over Alaska.

The primary guidance spread from late weekend into early next week
involves the specifics of the complex eastern Pacific/western
North America evolution.  The 00Z ECMWF pulls the Alaska upper
high farther northwest than most other guidance including its
ensemble mean. Teleconnections relative to the associated positive
height anomaly indicate the 00Z ECMWF is internally consistent in
bringing the eastern Pacific/West Coast trough farther eastward.
However assuming the majority scenario of keeping the upper ridge
over Alaska favors the slower timing of the mean trough.
Operational runs show a variety of ideas for exactly where an
embedded upper low may be by day 7 Mon so preference leans
increasingly toward the ensemble means at that time. Farther
eastward there is better than average clustering/consistency for
some frontal waviness to migrate into the Great Lakes early next
week in response to the shortwave energy embedded within the low
amplitude mean flow across the northern half of the lower 48.

Based on the latest array of guidance, a blend of operational
guidance continues to represent consensus well through day 5 Sat
and 70 percent operational weight (00Z-06Z GFS/00Z ECMWF with a
little 00Z CMC) looks good into day 6 Sun. Considerations by day 7
Mon favor a more pronounced shift to the 06Z GEFS/00Z ECMWF means
(65-70 percent) at that time.


...Sensible Weather Highlights/Threats...

Locally heavy convection will be possible from the Gulf Coast to
the southern Mid-Atlantic Thu-Fri, focused along a couple cold
fronts. The second of these fronts may also produce areas of
showers from the northern Rockies to the Upper Midwest. By the
weekend, as this second front settles across the central U.S. and
sharpens, while additional upper shortwave energy approaches,
expect the potential for a round of more widespread precipitation
to develop from the northern Rockies into the northern/central
plains and portions of the Midwest/Great Lakes. Sufficient cold
air may be in place in the polar air mass north of the frontal
boundary to support areas of snow mixing into the rain at times
across the northern Rockies and perhaps even eastward into the
northern High Plains. Within this precip area guidance is still
signaling the best potential for heaviest rainfall within the
Midwest/Great Lakes region. Farther west expect increasing
cloudiness and generally light precipitation to begin spreading
into northern California and Pacific Northwest in advance of the
upper low heading into the West during the weekend and trailing
system that may approach the coast early next week.

Cool temperatures will increasingly become a story through the
week as a couple cold fronts spread successively cooler air into
areas from the northern Rockies to the Upper Midwest. Expect high
temperatures Thu-Fri to be 5 to 15 deg F below average across
these areas, with colder temperatures, 15 to 25 deg below average,
arriving by the weekend. High temperatures should remain in the
40s by next weekend across a wide area of the northern Plains and
Upper Midwest while lows drop to near or below freezing over many
of the same areas. Meanwhile much of the Southeast will remain
relatively warm with highs 5 to 10 deg F above average. Locations
to the south of the sharp front settling over the central U.S.
will see increasingly warm min temps in particular, 10 to 15 F
above normal during the weekend/early next week. Height falls
reaching the West Coast during the latter half of the period will
lead to a cooler trend over a majority of the West after a warm
period late this week.

Rausch/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