Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
157 AM EST Sat Feb 20 2021
Valid 12Z Tue Feb 23 2021 - 12Z Sat Feb 27 2021
...Overview and Guidance Evaluation...
A relatively progressive upper flow pattern will be in place for
the beginning of the week as the eastern U.S. trough departs
across the western Atlantic. This will be followed by zonal flow
across much of the central and eastern U.S. ahead of an amplifying
western U.S. trough by Wednesday. A second trough builds in for
the Pacific Northwest by the end of the week.
The 00Z model guidance suite is overall in above average agreement
on the synoptic scale pattern through Wednesday, with a split
upper level flow pattern likely developing. By Thursday and
beyond, the CMC becomes a little more amplified and progressive
with the northern stream trough building in across the
north-central U.S. Similar to last night, the 00Z GFS parallel is
strong with the second trough/closed low approaching the Pacific
Northwest by Thursday night, but is competitive elsewhere across
the continental U.S.
For the fronts and pressures forecast, the 18Z GFS parallel was
preferred over the operational 18Z GFS given some timing
differences with the pattern change across the West, with the
parallel version closer in line with the model consensus. The
GEFS mean did not appear amplified enough with the southern stream
trough over the southeastern U.S., so it was not favored for this
forecast. A blend of the 12Z ECMWF/18Z parallel GFS/12Z CMC was
used through early Thursday, and then less weighting towards the
CMC by next weekend and more weighting towards the GFS
parallel/ECMWF/EC mean.
...Weather/Threats Highlights...
Things will be relatively uneventful across most regions of the
nation compared to the extreme winter weather that has been
endured by many locations this past week. The storm system
crossing the Pacific Northwest and the Rockies is expected to
produce heavy snow for the higher terrain of the central and
northern Rockies by the middle of the week, and will likely affect
much of the Interstate 25 corridor in Colorado. By the end of the
week, a corridor of light to moderate rainfall will be possible
across portions of the south-central U.S. and extending towards
the southern Appalachians, with perhaps some snow on the northern
edge of the precipitation. There is still a good deal of model
spread regarding the placement of this QPF axis, so changes in the
forecast going forward are expected.
In the temperature department, the building trough and the passage
of the cold front across the Intermountain West will herald a
change to readings falling to 10-20 degrees below average for the
western High Plains to the Rockies, along not nearly as impressive
as the recent Arctic blast based on our current forecasts.
Temperatures will likely be 10-20 degrees above average ahead of
that same storm system across the northern Plains on Tuesday. By
the end of the forecast period next Saturday, expect temperatures
to be near to slightly below normal for most of the continental
U.S.
Hamrick
Additional 3-7 Day Hazard information can be found on the WPC
medium range hazards outlook 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