Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
131 AM EDT Tue Sep 25 2018

Valid 12Z Fri Sep 28 2018 - 12Z Tue Oct 02 2018

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


...Pattern Overview...

Expect an amplified upper-level flow pattern to persist across
North America during the medium range period. An
anomalous/blocking upper ridge will shift northward from the North
Pacific across Alaska late this week into the weekend, with
500-hPa heights reaching +4 standard deviations across interior
Alaska by the weekend. 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 initially centered off the Southeast
will expand 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 late 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. One
additional feature of note during the medium range will be the
extratropical transition of Super Typhoon Trami across eastern
Asia, which should promote amplification of (or at least
maintenance of already amplified) flow downstream across the North
Pacific and North America late in the medium range and beyond.


...Guidance Evaluation/Preferences...

Deterministic model solutions showed relatively good agreement for
days 3-5 (Fri-Sun), and a ECMWF/GFS/CMC blend was used as a
starting point for the WPC forecast for that time period. Model
solutions showed some differences with respect to frontal position
by Sat-Sun as a cold front sweeps across the eastern U.S. and
becomes stationary across the central plains, and a consensus
solutions is represented well by the described blend. Solutions
have also converged reasonably well with respect to the upper low
approaching the West Coast in this time frame. After day 5,
forecast confidence begins to decrease rather quickly,
particularly across the western U.S. As additional upper-level
energy undercuts the Alaska ridge and approaches the West Coast by
Sun-Mon, model solutions begin to diverge significantly with
respect to the degree of amplification that occurs, and whether a
cutoff upper low will linger/deepen just offshore (per the GFS),
or open into a broader wave and move inland more quickly (per the
ECMWF). Given the amplified flow already in place, and the pattern
favoring continued amplification, believe that the ECMWF solution
by early next week is too quick to move strong height falls into
the interior western U.S. Whether a solution as amplified/cutoff
as the GFS verifies remains to be seen, but nonetheless, trended
heavily toward ensemble means by Sun-Mon and away from operational
solutions (especially the ECMWF). Amplification to the degree
shown by the GFS would likely steer a potential tropical system
northward toward central or northern portions of the Baja
Peninsula (while a solution more like the ECMWF would keep the
system on a westward track well out over the open water of the
Pacific).


...Sensible Weather Highlights/Threats...

Well-below average temperatures will prevail across areas from the
northern Rockies to the Upper Midwest through much of the medium
range as cold polar air is transported southward into the north
central U.S. High temperatures as cold as 20-25 deg F below
average will be possible. A couple waves of low pressure along the
strong central U.S. frontal boundary will result in potentially
widespread precipitation for portions of the central/northern
plains and Midwest. The most coherent signal for potentially heavy
rainfall in the guidance is early next week from the central
plains to the Great Lakes. Sufficient cold air in place north of
the frontal boundary may support precipitation falling as snow for
some areas from the northern Rockies to the northern High Plains.
Expect showers and thunderstorms with locally heavy rainfall
across the Gulf Coast and Southeast through much of the period as
a moist air mass remains in place and a frontal boundary fails to
clear the region. Depending on the eventual track of a potential
east Pacific tropical system, moisture may spread into portions of
the southwestern U.S. by early next week. Heavy rain is possible
if the system takes a more northerly track, although confidence in
the specifics is low at this time.


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