Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
153 AM EST Wed Nov 20 2019
Valid 12Z Sat Nov 23 2019 - 12Z Wed Nov 27 2019
...Pattern Overview and Guidance/Predictability Assessment...
Models have continued to struggle the timing of shortwaves in the
northern stream during the medium range. Farther south, the
multi-day trend toward faster progression of a southern stream
trough eastward seems to have finally come to and end. Models now
show good agreement in a low somewhere across the Tennessee Valley
early on day 3 (Sat). The significant question now pertains more
to the timing and amplitude of northern stream energy, which
impacts the degree of phasing which occurs heading into day 4
(Sun), and the track of a low off the Northeast U.S. coastline.
The 12Z CMC/UKMET showed less phasing, and a surface low track
farther offshore, while the 12Z ECMWF and the past few runs of the
GFS have shown a greater degree of phasing sooner, with a low
track closer to the coast. With this system preferred a blend of
the 12Z ECMWF/GFS, which shows a moderately phased solution, but
not to the degree of the 18Z (or 00Z) GFS.
Farther west, guidance shows consensus that a vigorous shortwave
should reach the Pacific Northwest on Sun and then quickly amplify
across the interior West on Mon. Much colder air should spread
into the western states as the trough amplifies, with a cold front
reaching the Plains by Mon. Models show an above average degree of
consensus by days 6-7 (Tue-Wed) on developing a wave of low
pressure along the front across the southern/central Plains strong
shortwave energy rounds the base of the western trough, pushing
strong height falls east into the Plains. While models vary on
timing and track, the latest runs of the deterministic
ECMWF/GFS/CMC all deepen this surface low as it moves
northeastward into the Midwest by Tue night/Wed. Ensemble members
have shown a cluster around this solution for at least the past
few runs, indicating a surprising degree of consistency given the
day 7 time range. Lastly, models also show consensus that another
significant shortwave should reach the Pacific Northwest Tue
night/Wed, bringing significant height falls and potentially a
relatively strong area of low pressure.
The WPC forecast was based on a blend of the deterministic 12Z
ECMWF/GFS during days 3-4, with a gradual shift toward heavier
weighting of ECENS/NAEFS ensemble mean through time during days
5-7.
...Weather Highlights/Threats...
Low pressure crossing the eastern U.S. on Sat is expected to bring
showers and potentially a few thunderstorms to areas from the
Southeast and Southern Appalachians to the Mid-Atlantic, with
locally heavy rain possible. Greater uncertainty exists farther
north due to the variability surrounding the surface low track off
the Northeast coast. At this point in time, it appears that at
least some precipitation is possible across southern/eastern New
England, with thickness profiles marginal for rain/snow across the
region. Given the range of model solutions at time time, it is
possible that precipitation could stay entirely offshore if the
low tracks farther east, or that somewhat heavier precipitation
reaches the coast if the low tracks farther west than forecast.
Farther west, rain and mountain snow are expected across the
Pacific Northwest and northern Great Basin/northern Rockies
Sat-Sun as the first of the two significant shortwaves and cold
fronts move inland. As low pressure begins to develop across the
central U.S. early next week, models hit at the potential for rain
and thunderstorms to affect a large area from the Gulf Coast to to
the mid-Mississippi Valley, with heavy rain possible. Farther
north, depending on the eventual intensity and track of the
surface low across the northern Plains/Midwest/Great Lakes, winter
weather (potentially significant) is a possibility. Based on the
WPC forecast and current ensemble data, the great probability for
significant snow during the Tue/Tue night time frame is across
portions of the Upper Midwest, including southeastern Minnesota,
western Wisconsin, and portions of northern Iowa. Please refer to
WPC Day 4-7 Winter Weather Outlooks for specifics on the medium
range winter weather threat. Finally, as the next low pressure
system reaches the Pacific Northwest toward the middle of next
week, expect a renewal of potentially heavy rain/snow for that
region.
Expect chilly temperatures to overspread the eastern U.S. Sat-Sun,
with highs 5 to 10 deg F below average. A warming trend will bring
high temperatures 5 to 10 deg above average to the northern Plains
and Upper Midwest over the weekend. Meanwhile, as the upper trough
digs into the West by early next week, expect temperatures to
drop, with highs 5 to 15 deg below average across much of the
interior western U.S. Below average temperatures should spread
east into the central U.S. by the middle of next week behind the
cold front and the strengthening low pressure system.
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