Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1101 AM EST Tue Nov 20 2018

Valid 12Z Fri Nov 23 2018 - 12Z Tue Nov 27 2018

...Overview...

A very strong closed high 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. In the west, troughing will pass
through late this week and be replaced by ridging Sunday-Monday
ahead of another system next Tuesday.


...Guidance/Uncertainty Assessment...

The latest guidance remains in fairly good agreement with the
overall large-scale pattern, keeping a trough over much of the
East Coast and with ridging replacing the trough over the West.
Specifics with individual shortwaves within the mean flow continue
to vary in regards to amplitude and location. The current GFS
solution has trended toward the 00Z ECWMF, especially with the
cyclone that will develop over the central U.S. and lift as a
Nor'Ester up the Mid-Atlantic region. Even with the differences,
using a deterministic blend served well for the beginning of the
extended period- with heavier weighting of the ECWMF and GFS with
lesser weighting of the UKMET and CMC. By day 7 (Monday) the ratio
was about 70% of the above blend and 30% EC Ensemble means.


...Weather Highlights/Threats...

Continued wet for the west as the second of two systems (the first
in the short range) moves through on Friday/Saturday with plenty
of snow at higher elevations. Additional precipitation will be
likely next week ahead of the next Pacific system, but favored
over coastal areas of WA/OR by then. In the east, lead system on
Friday will tap the Gulf for moisture and even as the northern
front weakens into the Great Lakes, precipitation will expand
eastward as the Gulf surface low organizes. Rain will spread
northeastward with some wintry weather on the north and northeast
side as the cold airmass retreats (models really struggle with
this evolution). Heaviest rain may be along the I-81/I-95 corridor
from SC up to the Mid-Atlantic to the northwest of the surface low
track. Will get only a brief respite ahead of the next system
lifting into the Lakes which has the potential to be slower and
stronger than the previous system as it gets blocked from a
typical exit through southeastern Canada.

The below/well-below normal temperatures across the Northeast
portion of the country will moderate by this coming weekend. Cold
Canada air will filter into the central portion of the U.S.,
resulting in maximum temperature negative anomalies (5-15F below
average) from the Rockies eastward.

Campbell/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