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Prognostic Meteorological Discussion
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000
FXUS02 KWBC 181411
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1010 AM EDT Tue Sep 18 2018

Valid 12Z Fri Sep 21 2018 - 12Z Tue Sep 25 2018

***Pattern Overview and Model Guidance***

A 500 mb trough is forecast Friday just off the Pacific Northwest
coast.  As the system moves onshore and inland, timing differences
become greater, with a large spread among the 06z GFS/00z
ECMWF/00z Canadian models (ECMWF fastest, Canadian slowest), with
a wide spread in the ensemble members. The differences stem from
the handling of a trough over the Bering Sea Sat, with the 00z
ECMWF deamplifying it and moving it faster downstream, in tandem
with the ejection of the Pacific northwest trough.

As the trough moves east, the ECMWF also becomes the most
amplified with the southern Canada
trough, as the 50 ensemble members have lower amplitude than the
operational run Tue 25 Sept. With large run to run changes and
little support from the ECMWF ensembles, little weighting was
given to the 00z operational ECMWF.

Manual progs blended the 00z ECMWF ensemble mean with the 06z GFS,
with less weighting to the 06z GEFS Mean through the forecast.
Confidence is a little below normal given the large spread of
solutions early to middle portions of next week.

On Fri 21 Sept, the models track a 500 mb trough that moves out of
the upper MS Valley and Great Lakes, deamplifying as it moves
quickly across the northeast Sat. Surface high pressure drives the
cold front south through the Lakes and into the Mid-Atlantic
region and Ohio Valley, where it stalls.  The models show a 700 mb
trough developing in CO/NM as a southern extension of the northern
stream trough and move this trough into the southern Plains, with
a surface wave  resulting.


***Sensible Weather and Hazards***

The next heavy rainfall event during the medium range period is
expected across the south-central Plains for the upcoming weekend
as tropical moisture surges northward from Mexico and the western
Gulf, with lift from an approaching 700 mb trough.  The potential
exists for several inches of rain across portions of eastern
Oklahoma into adjacent portions of southeast KS, southwest MO, and
north Texas. Lesser amounts are forecast eastward across the
Midwest and the Ohio Valley region in the vicinity of a frontal
boundary.  Another area of moderate to locally heavy rainfall will
be possible across the Upper Midwest/Upper MS Valley towards the
middle of next week as a surface low develops over the northern
Plains. Likewise, as the western trough progresses across the
northern Rockies and adjacent high Plains, showers are expected in
advance of the upper trough.

Expect temperatures to be well above average on Friday across much
of the eastern U.S. ahead of the cold front, with areas of 10 to
15 degrees above normal forecast.  Temperatures moderate to near
normal after the cold front passes.

Above normal temperatures are forecast across the southwest Tue 18
Sept. and Wed Sept 19, and also over the southeast US underneath
the upper high. Once the upper trough passes, cooler than normal
temperatures develop next week from the northern Rockies to the
northern Plains.

Petersen


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

$$
National Weather Service National Headquarters
