Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
256 AM EDT Fri Sep 20 2019
Valid 12Z Mon Sep 23 2019 - 12Z Fri Sep 27 2019
...Heavy rainfall threats in the Southwest early in the week and
Midwest/parts of the Plains during portions of next week...
...Overview...
An amplified pattern this weekend will briefly transition to
flatter flow during the early/middle part of next week. Then
toward next Fri most guidance begins to re-establish an amplified
western trough/eastern ridge regime that will likely extend for a
while beyond the medium range period. Upper shortwave
energy/possible embedded low may interact with East Pacific
tropical moisture to bring significant rainfall to parts of the
Southwest. Some of this energy/moisture extending northeastward
as well as a couple fronts (the latter potentially stalling over
the Midwest late in the week) will also provide support for areas
of heavy rainfall over portions of the central U.S.
...Guidance Evaluation/Preferences...
Overall an operational model blend provided a reasonable starting
point early in the period while the 18Z GFS/12Z ECMWF and their
respective ensemble means provided a reasonable solution cluster
later in the period--with the stronger but uncertain details of
the operational runs balancing the agreeable pattern of the means.
Regarding individual features, guidance still differs some for the
shortwave over the Great Lakes/Midwest as of early Mon. GFS runs
and ensemble means have tended to be on the open and/or
progressive side of the spread as the feature reaches New England,
with the favored model blend leaning more toward the idea of a
more separated evolution. New 00Z runs thus far, including the
GFS, provide stronger support for an upper low closing off by Tue.
On the opposite side of the country there is still a lot of
spread and run-to-run variability for the trough/possible closed
low dropping through the Great Basin and into the
Southwest/southern Rockies, then possibly ejecting northeast into
the Plains. Guidance as a whole is a bit faster than in some
earlier cycles but the new 00Z GEFS mean has shifted back to a
farther west position. The CMC and UKMET have displayed opposite
shifts in their 00Z run, the CMC slower and the UKMET faster, with
the UKMET/GFS now on the faster side. This leaves the 12Z ECMWF
near the middle of the spread. Guidance differences are not as
pronounced for troughing that evolves over southern Canada and
northern tier U.S. by midweek.
By the end of next week there is better than average
model/ensemble agreement toward the idea of a deepening upper
trough near the West Coast, between strengthening ridges over the
west-central Pacific/Alaska and southeastern U.S. Latest
operational runs suggest potential for a closed low over/near the
Pacific Northwest by Fri. While exact details of the operational
models are uncertain 6-7 days out in time, it seems reasonable to
include about half weight of their solutions to help strengthen
the ensemble means somewhat.
...Weather Highlights/Threats...
The primary areas of emphasis for heavy rainfall potential will be
over the Southwest early in the week and over parts of the
Plains/Midwest. Over the Southwest (and favored terrain over
central Arizona in particular) rainfall should be enhanced by the
combination of the upper trough/low dropping southeast toward and
into the region as well as leading flow of moisture originating
from tropical systems Lorena and Mario. Some of the
moisture/energy aloft may eventually reach into the central U.S.
Beyond those ingredients, an initial southern Plains front should
return northeast as a warm front Mon-Wed ahead of a cold front
approaching from the northern Plains. The northern part of the
cold front will continue eastward but stall over the
Midwest/central Plains, leading to a more persistent focus for
rainfall during the latter half of the week. The Northwest may
see a couple episodes of mostly light precipitation, though
pockets of moderate activity could develop depending on the
details of the late week upper trough. This trough may begin to
support some high elevation snow over the northern Rockies/Pacific
Northwest by late in the week. Meanwhile rainfall along the front
reaching the East early in the week should be trending somewhat
lighter and less broad compared to the short range time frame.
Areas of rain could linger for a time over New England depending
on specifics of the upper trough/low affecting the region.
Expect the Southeast to see consistently well above normal high
temperatures with coverage of plus 10F and greater anomalies
expanding northward by Thu-Fri as an upper ridge strengthens over
the region. Potential for some record highs will also increase in
that time frame. On the other hand the West and northern Plains
should be on the cool side of normal for highs during the majority
of the period. Coolest days should be Mon over parts of the
Interior West with the initial trough/upper low over the region
and then Thu-Fri over the Northwest as deeper upper troughing digs
along the West Coast. Some highs may be 10-20F below normal on
Fri. Morning lows will likely be near to above normal over much
of the lower 48 during the week. Warmest anomalies for mins
should be over parts of the Plains/Mississippi Valley Tue-Fri and
in parts of the East by Thu-Fri.
Rausch
Additional 3-7 Day Hazards information can be found on the WPC
medium range hazards chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php
WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation, winter weather outlook probabilities
and heat indices are 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