Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
200 AM EST Wed Feb 20 2019
Valid 12Z Sat Feb 23 2019 - 12Z Wed Feb 27 2019
...Excessive rainfall/flooding threat from the Lower
Mississippi/Tennessee/Ohio Valleys to the Southern/Central
Appalachians and a significant winter storm from the south-central
Plains to the Upper Midwest/Great Lakes and interior New England
this weekend...
...Pattern Overview and Weather Highlights/Hazards...
Potent upper low/trough energy lifts northeastward this weekend
from the southern Plains to the Northeast overtop an amplified and
slow to erode Bahamas/Southeast U.S. upper ridge. This brings a
threat for significant accumulating snow with some icing possible
around an associated and deepening surface low with a pressure in
the 980s by Sunday over the Great Lakes. This would likely result
in gusty winds and potential blizzard conditions across the Upper
Midwest and Great Lakes.
Underneath, a multi-day heavy rain/training episode rejuvinates
from the lower MS/TN/OH Valleys to the Southern/Central
Appalachians Saturday, exiting the East Coast Sunday with frontal
passage. Flooding/runoff issues will remain fueled by advection of
highly anomalous moisture/warm air and deep cyclo/frontogenesis.
SPC also shows a severe weather threat given system anomalous
nature/vigor.
A long-standing and wintry mean upper trough over the West will
again be reinforced Sat-Mon as energy digs to the lee of a
blocking Gulf of Alaska centered upper ridge. These systems will
be less amplified than prior systems in this recent pattern, but
will bring unsettled weather pattern through the Northwest and
north-central Intermountain West and Rockies, with the best chance
for accumulating snowfall over the Cascades. Guidance has not been
very consistent recently with the evolution of the upstream flow
next week, but now shifts the upper high/ridge northward over the
Alaskan interior. This flattens and moderates flow into the West
by Tue/Wed with emergence of undercutting but less certain
southern stream Pacific flow and pattern with northern stream
energy/height falls ejecting eastward over the n-central to
northeastern U.S. This should keep the n-central U.S cold but also
support system development that could lead to deeper development
into the East in about a week.
...Guidance/Predictability Assessment and Preferences...
Guidance is reasonably well clustered at mid-larger scales through
much of the medium range period. WPC progs were primarily derived
from a composite blend of the latest GFS/ECMWF and GEFS/ECMWF
ensemble means. This maintains reasonable WPC continuity overall.
Schichtel
WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation and winter weather outlook
probabilities 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