Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
116 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...

Amplified upper-level flow pattern expected to persist across
North America during the medium range. An anomalous/blocking upper
ridge is expected to shift northward from the North Pacific into
Alaska over the next week, 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 north central U.S. Meanwhile, upper
ridging is expected to hold strong or even intensity from the
southern plains to the Southeast. 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, 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.

Models initially showed good consensus with respect to shortwave
energy and a surface low pressure system crossing the Midwest on
day 3 (Thu), with relatively small timing/amplitude differences
for an additional shortwave amplifying as it dives south toward
the Pacific Northwest days 4-5 (Fri-Sat). Additionally, the ECMWF
has backed off a bit on its fast solution with cutoff energy
reaching the Northwest and becoming absorbed into the westerlies,
and most deterministic solutions are now in the same vicinity with
this feature through day 5. Thus, a multi-model deterministic
blend (including the ECMWF/GFS/CMC) was used a starting point for
the WPC forecast days 3-5. By days 6-7 (Sun-Mon), consensus has
improved that a relatively deep upper low will undercut the Alaska
ridge and approach the Pacific Northwest. While models agree on
the existence and general timing of this feature, there has been
significant run-to-run variability with respect to the exact
character of this feature and any potential interactions with
northern stream energy (which could play a role in the amplitude
of upper flow/potential height falls downstream across the north
central/northeastern U.S. In general, the downstream result
appears to be relatively flat upper flow farther east across the
central/eastern states, and an intensifying low-level baroclinic
zone across the central U.S. between a polar air mass across the
north central states and subtropical air farther south. Given
increased uncertainty and run-to-run variability by days 6-7,
weighting of ensemble means (ECENS/NAEFS) was boosted to comprise
a majority of the forecast blend.


...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 the aforementioned frontal zone intensifies across the
central U.S., and 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. 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. Additionally, the system approaching the Pacific Northwest
by the weekend may begin to spread precipitation across areas from
northern California northward.

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. High
temperatures Thu-Fri are expected 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 are
expected to remain in the 40s by next weekend across a wide area
of the northern plains and Upper Midwest, with low dropping below
freezing for many of the same areas. Meanwhile, much of the
Southeast and Southeast will remain relatively warm, with highs 5
to 10 deg above average.

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