Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1142 AM EDT Tue Sep 25 2018

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

...Chilly temperatures 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 western/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 Canadian 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. Newly
formed tropical storm Rosa in the East Pacific is forecast to move
northward over the next five days per the latest NHC track with
the potential to be drawn northward/northeastward ahead of the
western trough. However, the ensembles are split between lifting
toward Baja and the Gulf of California vs remaining just far
enough south to linger in the open water. 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
solution was 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
(Sunday), 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 (between
125-130W per the GFS runs), or open into a broader wave and move
inland more quickly (per the 00Z ECMWF). Though the amplified flow
already in place could favor the slower and more closed-off
GFS-like solutions, the majority of the ensembles were farther
west than the GFS deterministic runs with at least the southern
portion of the trough which would then allow the northern portion
to slide eastward quickly (somewhat like the ECMWF). Given the
lack of consensus, opted to trend toward an ensemble mean blend by
next Monday and especially Tuesday. This would keep Rosa and the
bulk of its moisture out of the Southwest, but the potential
certainly exists for greater impacts. Please consult the National
Hurricane Center for the latest information.


...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 (40s and 50s). 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/upper Mississippi Valley to the Great Lakes.
Sufficiently 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 slowly weakening frontal boundary fails to
clear the region. Moisture may spread into portions of the
southwestern U.S. by next week but will be conservative for now
with PoPs just above climo and generally light QPF. However, heavy
rain is possible if the system takes a more northerly track ahead
of the trough to the north but confidence is low at this time as
to where that may occur (if at all).


Fracasso/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