Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
130 PM EST Sat Nov 16 2019

Valid 12Z Tue Nov 19 2019 - 12Z Sat Nov 23 2019

...Heavy rain possible for portions of the Southwest Tue-Thu...

1830z Update...

The pattern and guidance evaluation remains largely unchanged from
the previous (overnight) shift. The latest runs of the GFS
continue to be faster with ejecting the Southwest U.S. upper low
eastward, while the ECMWF/CMC both remain slow/farther west, and
the ensemble means coming in somewhere in the middle. Not seeing a
whole lot in the way of models trending one way or the other, but
the previous line of thought of favoring a slightly faster
solution continues to apply. Given this, the updated WPC medium
range progs for today were based on a blend of the latest
available guidance (ECMWF/GFS/ECENS mean/GEFS mean), along with
previous shift continuity. Slightly more weight was placed on the
faster GFS/GEFS solutions, with increasing weighting of the means
used towards the end of the period to account for the increased
model spread.

See previous discussion below for additional details along with an
assessment of sensible weather highlights and threats.

Santorelli

...Previous discussion issued at 0636 UTC...

...Pattern Overview and Guidance/Predictability Assessment...

Amplified and moderately progressive northern stream flow is
expected to persist across the CONUS through the medium range,
with a tendency for periods of split flow as energy separates and
cuts off south of the strongest westerlies. An upper-level trough
is forecast to exit the eastern U.S. days 3-4 (Tue-Wed) as a
deepening coastal low moves quickly into the Canadian Maritimes. A
second northern stream wave should quickly follow on its heels,
moving a cold front across the Midwest/Great Lakes Wed night/Thu
and the eastern U.S. by Fri. Farther west, northern stream energy
is forecast to dig southward along the West Coast Tue-Wed, and
models/ensembles show consensus that the feature should eventually
close off into an upper-level low, which should then slowly
progress eastward across the Southwest. This feature becomes the
biggest forecast problem of the medium range, as solutions differ
on exactly how far south/west the feature will dig before moving
east, and also on the speed with which it moves eastward (which in
turn introduces further complexities and potential interactions
with a northern stream wave). The GFS has consistently been
quicker to move the upper low eastward, with the ECMWF/CMC on the
slow side of the spread, and ensemble means largely in the middle.
Taking a look at the hemispheric pattern, the broader pattern
shows very little in the way of significant flow blocking, with
things remaining largely progressive from Asia across the North
Pacific to North America. Additionally, teleconnections associated
with persistent positive height anomalies becoming established
across western Canada favor colder conditions/lower heights across
the eastern half of the CONUS, suggesting that the flow should
have a tendency to remain somewhat progressive. Given these
considerations, decided to lean at least somewhat toward the
faster side of the spread with the upper low.

The WPC forecast was initially based on a blend of the 12Z
ECMWF/18Z GFS solutions during days 3-4 (Tue-Wed). After that
time, given the above train of thought, leaned a bit more toward
the GFS along with the ECENS/GEFS ensemble means. By day 7 (Sat),
spread was quite high and opted to go entirely with ensemble
means, but with a bit more weight on the slightly more progressive
GEFS mean relative to the ECENS.


...Weather Highlights/Threats...

Showers and perhaps a few thunderstorms are possible across the
Southern/Central Plains and mid-Mississippi Valley Wed-Thu in
association with the cold front crossing the region. Rain and
areas of snow associated with this system are expected to spread
into the Great Lakes/Northeast Thu-Fri. Farther west, shortwave
energy and a passing front will bring valley rain/mountain snows
to much of the northern Great Basin and northern Rockies on Tue.
As the trough digs along the West Coast and the upper low
develops, moisture will be drawn northward into the southwestern
U.S., potentially including moisture associated with the future
remnants of Tropical Storm Raymond. Widespread and potentially
heavy rains will be possible across much of the Southwest Tue-Thu,
with some areas expected to receive at least a couple inches of
rainfall. Snow will be possible at the higher elevations from the
southern Great Basin to the central/southern Rockies.

Temperatures will initially be well above average across much of
the West and the High Plains on Tue (highs 5 to 15 deg F above
average), but these will quickly cool by Wed as a cold front
crosses the central U.S. and heights fall across the West in
association with the digging trough. Expect highs to near average
to as much as 10 deg below average across much of the western and
central U.S. from Wed onward, with cooler temperatures returning
to portions of the Ohio Valley, Great Lakes, and eastern U.S. by
Fri-Sat.

Ryan


Additional 3-7 Day Hazards information can be found on the WPC
medium range hazards chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php


WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation, winter weather outlook probabilities
and heat indices are 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