Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
152 AM EST Wed Nov 21 2018

Valid 12Z Sat Nov 24 2018 - 12Z Wed Nov 28 2018

...Overview...

Strong positive height anomalies via a closed high currently north
of England is forecast to retrograde westward across southern
Greenland later this week and settle over northern Quebec,
favoring blocked troughing over the eastern third of the lower 48
and elongated negative height anomalies across the mid-latitudes
of the Atlantic. In the west, ridging will attempt to reassert
itself on Sunday ahead of another system or two next
Tuesday/Wednesday.


...Guidance/Uncertainty Assessment...

The 18Z GFS/GEFS and 12Z ECMWF/ECMWF ensemble mean (and 12Z
FV3-GFS) offer good clustering through the period. The
Canadian/UKMET were each displaced away from the consensus and
given no weight after Saturday. Continue to favor a wavy pattern
trending toward blocked flow in the east with slowing systems as
troughing gets reloaded into the Great Lakes region. After a
couple quieter days in the west Sun/Mon, troughing will attempt to
push into the re-established ridge but prefer to keep the rain
threat to mainly coastal areas of WA/OR as an upper low over the
eastern Aleutians should be the main player in keeping the
downstream ridge mostly in tact. This would still allow some
weakening fronts into the Pac NW but likely not into much of
CA/NV.


...Weather Highlights/Threats...

Lead system in the Southeast on Saturday will ride along the coast
and turn east off the DelMarVa Sunday spreading rain and maybe
some isolated wintry precipitation through the Mid-Atlantic into
parts of the Northeast. Northern extent and intensity (and ptype)
of the precipitation shield remains in question due to uncertainty
with how the weakening system through the Great Lakes interacts.
Still favor the I-81/95 corridors between NC and PA/NJ and maybe
southeaster NY for the heaviest rain (over an inch to an inch and
a half possible, primarily on Saturday evening into Sunday
morning).

Potent shortwave in the west on Saturday and attendant surface
front will take the widespread rain/snow eastward through the
Plains into the mid-Mississippi Valley where precipitation ahead
of the front will expand in coverage as Gulf moisture gets drawn
northward. Cold front out of Canada to the north will provide
enough cold air to support snow or a mixed bag of precip to the
north/northwest of the surface low Sunday from NE/SD eastward to
the central Great Lakes. As that low lifts toward Lake Erie by
early Monday it will get blocked from continuing northeastward and
instead likely wrap its triple point low out through southeastern
New England as its front pushes off the coast. This pivot could
yield a significant rain and inland snow event for the Northeast
depending on the evolution of the system.

Temperatures will generally be below average over most of the
lower 48 by about 5-10F. A couple brief exceptions would be across
the southern Plains Saturday ahead of the western system and
southern Florida until about Tuesday when a cold front should
settle into the Florida Straits.


Fracasso


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