Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1053 AM EST Sun Nov 25 2018

Valid 12Z Wed Nov 28 2018 - 12Z Sun Dec 02 2018

...Pattern Overview...

The WPC medium range period begins on Wednesday with a large
closed upper level vortex centered over New England with multiple
embedded shortwaves pivoting around it, and quasi-zonal flow
across the western U.S. to the central plains.  The large closed
low slowly exits the Northeast U.S. on Thursday, and a large scale
trough is expected to move inland across the western U.S. by the
end of the week. As the trough reaches the Plains next weekend,
low pressure system genesis and deepening will likely bring deep
Gulf moisture inland to fuel widespread precipitation over the
east-central U.S. Upper level ridging is expected to develop ahead
of this developing storm system next weekend.


...Model Guidance and Uncertainty Assessment...

Deterministic models and the ensemble means are in relatively
decent synoptic scale agreement on Wednesday across the
continental U.S.  With the quasi-zonal flow pattern across much of
the continental U.S. on Thursday, the deterministic models are
indicating some differences with regard to a subtle shortwave
feature crossing the Plains, with the 00Z UKMET showing the
greatest amplitude with that. The models remain in decent
agreement regarding a Pacific storm system reaching California
late on Thursday, with the 00Z GFS slightly faster.

By the end of the medium range period next weekend, speed and
amplitude differences are apparent as the upper trough from the
western U.S. reaches the central plains, with the ECMWF and the
ECMWF ensemble mean being slower and more amplified, and the GFS
and CMC more progressive with the trough. There is good model
support for a downstream ridge east of the Mississippi River, with
the ECMWF showing a rather pronounced ridge axis compared to the
broader GFS ridge. Differences are even more apparent across
southern Canada in the northern stream flow. Given uncertainty,
The WPC forecast was primarily based on a blend of the 06 UTC
GEFS/00 UTC ECMWF ensemble means. However, also added the 00 UTC
ECMWF into the composite blend to infuse a bit more detail/system
depth. The ECMWF seems the most consistent deterministic model
with the ensembles and run to run.


..Weather Highlights/Threats...

A departing storm system over the northeastern U.S. is expected to
produce lake effect snows into Thursday. The heaviest
precipitation during this forecast period is expected from western
Washington to central CA as two Pacific storm systems move onshore
and bring enhanced moisture, with multiple inches of rain possible
for northwest California and southwest Oregon. Heavy snow is
likely for the higher mountains from the Sierra Nevada to the
central and northern Rockies, with several feet of snow possible. 
Another area of heavy precipitation will likely develop from the
Gulf Coast to the Midwest a a central plains surface low
deepens/organizes and lead flow advects copious moisture northward
next weekend. This system also presents another significant heavy
snow/ice risk on the cooled northern periphery of the enlarging
precipition shield and some threat for warm sector severe weather
that is being followed by SPC.

Temperatures are expected to be on the order of 10 to 20 degrees
below average from the Midwest to the Appalachians for Wednesday
and Thursday before a moderation trend ensues by the end of the
week. Areas with fresh snow cover across the Midwest in the wake
of the upcoming winter storm will likely have much colder
overnight lows early in the forecast period, with a few subzero
readings possible where winds subside and skies clear out. 
Temperatures should generally be within 5-10 degrees of
climatological averages for the western third of the nation.

D. Hamrick/Schichtel


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