Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
257 AM EDT Sat Apr 03 2021
Valid 12Z Tue Apr 06 2021 - 12Z Sat Apr 10 2021
...Overview and Guidance Evaluation/Predictability Assessment...
Some aspects of the medium range forecast continue to be
challenging. After midweek the models and ensembles still show
significant divergence for shortwave details within a larger scale
mean trough expected to settle over northwestern North America and
portions of the western U.S. coast. In addition to affecting
timing/coverage/amounts of precipitation over the West, some of
the energy emerging from this mean trough could ultimately affect
the upper low forecast to track from the north-central Rockies
into the Midwest during Tuesday-Thursday. Another factor at play
for this upper low by late week into the weekend will be the
fairly blocky pattern over the Atlantic and eastern Canada. This
system will bring the potential for significant precipitation from
the north-central Plains and Midwest through portions of the East.
The models are gradually consolidating for their depiction of the
upper low currently expected to track from near eastern Idaho into
the Midwest during Tuesday-Thursday. Among the 12Z/18Z solutions
for this time frame, the UKMET was the one that had to be excluded
due to being a fast extreme from the start of the period. The new
00Z run looks more reasonable. Recent trends have been
gravitating toward the stronger half of the spread. By
Friday-Saturday the ultimate evolution of the upper low becomes
increasingly uncertain due to the possible influence of upstream
shortwave energy as well as models/ensembles showing a variety of
possibilities for flow details over and south of eastern Canada
(with eastern Canada seeing rising heights/ridging in general but
potential for embedded shortwave/closed low energy). The 12Z and
old 00Z ECMWF runs represent the two extremes, lifting northward
or dropping southeastward. The new 00Z run has reverted to a more
southern track late in the period. The GEFS/ECMWF means provide
the most reasonable intermediate solution with the upper low
center drifting into the Great Lakes.
The forecast of details within the mean trough near the West Coast
is quite uncertain as well. The GFS/UKMET/CMC are all suggesting
in varying ways that leading energy reaching the trough axis by
midweek should eject much farther eastward than ECMWF runs have
shown thus far. GEFS/CMC means show a more subdued hint at such
energy. As has been the case recently, the GFS digs upstream
energy farther westward than most other solutions. In a favorable
trend the GEFS mean has been adjusting its somewhat westward axis
closer to the ECMWF mean. The past two ECMWF runs were somewhat
opposite near the West Coast late in the period as well, with the
12Z run a lot flatter/broader with the trough than the prior 00Z
run. The new run is sharper but strays a bit eastward by
Saturday. Again a blend of the GEFS/ECMWF means provide the best
intermediate starting point for a single deterministic forecast.
Aforementioned comparisons/preferences led to a starting blend
that consisted of the 18Z GFS and 12Z ECMWF/CMC for about the
first half of the period and then quickly trended toward majority
weight of the 18Z GEFS/12Z ECMWF means. Enough minority weights
of the 18Z GFS and 00Z/02 ECMWF were included late in the period
to enhance detail over the East slightly but not enough to have an
adverse impact on preferences farther West.
...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 seeing 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.
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