Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
311 PM EDT Sat Apr 03 2021
Valid 12Z Tue Apr 06 2021 - 12Z Sat Apr 10 2021
...Overview and Guidance Evaluation/Predictability Assessment...
Model guidance this morning generally agrees that a shortwave will
march across the middle to eastern portion of the country through
the medium-range period as another upper trough with embedded
shortwaves approaches/reaches the Pacific Northwest late next
week. The ensemble means from the ECMWF, GEFS, and CMC are
actually quite agreeable with one another through Day 7 regarding
the overall pattern evolution across the U.S., with excellent
timing agreement on the west-to-east motion of a low pressure
system from the Plains to the East Coast. The GEFS mean and the
CMC mean track the low slightly farther to the north toward the
lower Great Lakes than the EC mean which takes it across the Ohio
Valley. A blend of these ensemble means should yield a good
starting point. As for the deterministic solutions, the GFS has
continued to depict a deeper and larger low pressure system to
form over the central Plains during the middle of next week
compared with other models. As for the subsequent evolution of
the system, the deterministic solutions are increasingly diverged,
with the GFS tracking the system across the Great Lakes into
Canada while the ECMWF and CMC generally carry it into the East
Coast. The WPC medium-range package was based mainly on a
composite blend of the 06Z GEFS, 00Z EC mean, a smaller
contribution from the 00Z CMC mean, together with some from the
00Z ECMWF and a lesser amount from the 06Z GFS and the 00Z CMC.
This resulted in a solution that depicts a low pressure system
tracking from the central Plains to the East Coast with some
coastal development by next weekend.
...Sensible Weather and Potential Hazards...
The upper low emerging from the north-central Rockies along with
developing Plains/Midwest low pressure should promote an axis of
potentially heavy rainfall over parts of the north-central Plains
and Midwest during Tuesday or Tuesday night through at least
Wednesday. Some snow will be possible over the north-central
Rockies and High Plains. With time expect this system to spread
moisture into the East with some areas of locally moderate to
heavy rainfall possible. Increasing uncertainty over system
details after Thursday decreases confidence for determining
highest rainfall totals and their location at that time, though
existing signals suggest they would be within an area from the
Ohio Valley into Tennessee Valley through parts of the
Mid-Atlantic/Southeast. The Northwest should see one or more
periods of rain and mountain snow during the period but
uncertainty over shortwave details will keep confidence low for
specifics of coverage/timing/amounts.
From Tuesday into early Wednesday well above normal temperatures
(at least plus 10-20F anomalies) will prevail from the southern
Rockies/Plains into the Great Lakes. The greatest anomalies
should be over the central Plains and Midwest where morning lows
may be 20-25F above normal. This warmth will moderate somewhat as
it continues eastward ahead of the Plains system. The East should
generally see highs of 5-15F above normal mid-late week while
clouds and rainfall will lead to warmer anomalies of plus 10-20F
for morning lows. Farther west, parts of the north-central
Rockies/Plains may see modestly below normal highs into midweek
while the mean trough aloft nearby keeps the Pacific Northwest
somewhat below normal for highs. Most of the Southwest and Great
Basin will likely see highs 5-10F above normal from midweek onward.
Kong/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