Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1210 PM EDT Wed Sep 19 2018

Valid 12Z Sat Sep 22 2018 - 12Z Wed Sep 26 2018

...Heavy rainfall potential across the Arklatex region into the
Ozarks for Saturday through Monday...

...Considerable cool down expected in the middle of the country
next week...

...Pattern Overview/Model Guidance/Preferences...

The large scale pattern during the extended periods depicts a
trough exiting the Northeast, ridge extending from the Atlantic to
portions of the southern tier states with an easterly wave
drifting westward across the Gulf of Mexico, and a trough digging
into the central U.S. Model guidance is fairly reasonable with
typical noise of small timing and placement differences. Although
confidence on the exact amounts of qpf expected across the
Southern/Central Plains and Ozark region are still fair at best,
the signal for higher amounts have continued over the last few
guidance cycles which suggests that this region will have an
increase risk for excessive rainfall and flooding. PWs have
consistently been in the +2 to +3 std. dev. over this area. While
the guidance vary with how quickly the easterly wave will shear
across the Gulf of Mexico, its impacts are primarily restricted to
the offshore waters of the Gulf. One key feature with a great deal
of precipitation uncertainty is the slow moving shortwave across
the Southern Plains. This area bears watching during the upcoming
days given the potential for flooding rains.

The medium range forecast was led by the 00Z/06Z GFS, 00Z
ECWMF/UKMET/CMC with increasing weighting of the 00z GEFS,EC Means
and NAEFS.

...Sensible Weather and Hazards...

A strong signal for widespread, higher precipitation persists from
portions of the Southern/Central High Plains to the Ozark region.
The current anomalous moisture feed into the Desert Southwest will
merge over the South Plains with pooled moisture from the Gulf of
Mexico, triggering convection near a stalled boundary and
reinforced this weekend as a cold front crosses the Rockies and
another drops south through the Central Plains. A much cooler
airmass will usher into the northern-tier states, resulting in
temperatures within this region to be near 15 to 20 degrees below
seasonal normal by the middle of next week. If this pans out,
afternoon highs would only be in the low to mid 50s these days
allowing for a true feel of Autumn.


Campbell

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