Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
238 AM EDT Fri Mar 19 2021
Valid 12Z Mon Mar 22 2021 - 12Z Fri Mar 26 2021
...Overview...
The weather pattern will become more active during the medium
range period as a large scale mean trough becomes established over
the Plains/west-central U.S. through much of next week. Ahead of
this trough, portions of the Plains and Mississippi Valley (and
possibly a little eastward) should see increasing potential for
significant rainfall while Pacific features dropping into the mean
trough will bring episodes of rain and mountain snow from the
Northwest into the Rockies. Meanwhile, an upper trough and
possible embedded low will support a western Atlantic system, with
continued uncertainty for its exact track and how much
precipitation it may bring to areas along the East Coast. Guidance
is still fairly agreeable with the evolving pattern but there are
remaining differences in the details.
...Model Guidance/Predictability Assessment...
The system off the East Coast continues to be troublesome in terms
of model spread, with predictability on the mid-lower side of the
spectrum as the upper support essentially becomes embedded within
a developing large-scale ridge. The GFS continues to be the
fastest/farthest east model. The latest run of the UKMET still
shows a northern outlier solution, just off Cape Cod by early
Wednesday. The ECMWF/CMC are in a more middle ground position.
A vigorous short wave ejects out of the Rockies on Monday, lifting
into the Upper Midwest/Great Lakes by Wednesday while another
follows on its heels (maybe a bit farther south) Wednesday-Friday.
Both systems exhibit notable timing/intensity differences, with
the GFS consistently faster than the other models (GFSp is fast
too, but still slower than the operational GFS). A third system
enters the Pacific Northwest next Thursday, with a possible closed
low over the Southwest by Friday. However, there is significant
model spread with how deep this system gets by then, so confidence
is low.
For early period, WPC used a blend of the deterministic models
with more weighting towards the ECMWF. Due to increasing
uncertainty late in the week, the blend incorporated more of the
ensemble means (ECENS and NAEFS) Thursday-Friday. This overall
solution worked well across the board, and also fit with previous
shift continuity as well.
...Weather/Threats Highlights...
The developing mean trough aloft with periodic reinforcement will
tend to favor below normal temperatures over much of the West into
the Rockies, moderating somewhat by the end of the week. The
reinforcing shortwaves will bring multiple episodes of rain and
mountain snow to areas from the Pacific Northwest
eastward/southeastward through the West into the Rockies. Most of
this activity should be of light to moderate intensity but some
localized heavier areas will be possible.
Consolidating upper dynamics ejecting from the West should promote
a central Plains through Upper Midwest system early-mid week. This
system may produce a focused area of enhanced snowfall over or
near the Colorado Rockies with some snow possibly extending into
the central High Plains and then northeastward on the cold side of
the surface low track. Farther south, improving Gulf inflow ahead
of the trailing cold front should promote areas of heavy rainfall
from the central Plains/Midwest into the Lower Mississippi Valley
with lighter rainfall extending northeastward. The system could
produce brisk winds depending on its exact strength. The next
shortwave digging into the West by midweek should hold the frontal
boundary over the southern Plains/Lower Mississippi Valley for a
time, with continued flow of moisture from the Gulf promoting a
threat of heavy rainfall over the Lower Mississippi Valley and
vicinity into Thursday. Areas along the East Coast may see a
period of rainfall and strong winds with the western Atlantic
system. However confidence is low for magnitude and duration given
current guidance spread, as possibilities for some locations range
between dry conditions and fairly heavy rainfall.
Expect the warmest temperature anomalies to progress eastward from
the Northern Plains/Upper Midwest on Monday, reaching the eastern
states by mid-late week. The northern third of the central/eastern
U.S. will be most likely to see highs of 10-20F above normal on
one or more days. Anomalies for morning lows should be somewhat
higher with broader southward coverage of readings 10F or more
above normal, along with some northern locations seeing some plus
20-25F anomalies. Sunday lows over parts of the Dakotas and
Minnesota could be near daily record warm values if they hold for
the calendar day.
Santorelli/Rausch
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