Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
259 PM 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. Early in the week, upper
shortwave energy (probable closed 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...

For the first few days of the forecast period (Mon-Wed) a blend of
the 06Z GFS and 00Z ECMWF/Canadian offered a reasonably
well-clustered starting point. Differences in timing of the
western trough/closed low became evident by the early Tue forecast
time with the 00Z GFS/UKMET quicker/farther east than the larger
ensemble consensus (including the 06Z GFS and 00Z ECMWF/Canadian)
that aligned with continuity and was the continued preferred
evolution. For next Thu/Fri, uncertainty increases in fairly
typical fashion with timing/track differences of the weakening
Southwest upper low as well as the incoming/reinforcing troughing
along the British Columbia coast. However, by the end of the
period into next weekend (days 8-9), the ensembles show better
than average agreement on a deepening western trough and increased
heights in the east (see the Climate Prediction Center's forecast
for days 6-10 and 8-14 for more details).


...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 toward the 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 this may
trend heavier and more widespread at the end of the week as the
next upper trough digs southward. This trough will likely 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 with an unusually strong signal continuing into
next weekend. 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
with many valley locations only in the 50s/60s. 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 (5-15F above normal).


Fracasso/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

Hazards:
- Heavy precipitation across portions of the Northern Rockies,
Fri, Sep 27.
- Heavy rain across portions of the Central Plains and the Middle
Mississippi Valley, Tue-Thu, Sep 24-Sep 26.
- Heavy rain across portions of the Southwest, Mon-Tue, Sep 23-Sep
24.
- Flooding possible across portions of the Plains and Mississippi
Valley.
- Flooding occurring or imminent across portions of the Plains and
the Upper Midwest.
- Flooding likely across portions of the Middle Mississippi Valley.
- Much above normal temperatures across portions of the Southeast
and Southern Appalachians, Thu-Fri, Sep 26-Sep 27.

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