Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
223 AM EDT Sun Apr 25 2021
Valid 12Z Wed Apr 28 2021 - 12Z Sun May 02 2021
...Heavy rain threat from the Southern Plains into the Middle
Mississippi Valley next week...
...Guidance/Predictability Assessment and Preferences...
The medium range period (Wed-Sun) features amplified troughing (or
a closed low) which drifts eastward with time from the Four
Corners region, possibly reaching the Southeast/Tennessee Valley
or East Coast by the end of the weekend. Upstream, strong ridging
will build across the Western U.S. the first half of the period
before it also gets kicked eastward by additional shortwave energy
reaching the West Coast day 6-7.
The largest forecast concern continues to be evolution and timing
of the initial western troughing as it moves through the Southern
Plains and points east. Although guidance has always struggled
some with the timing of this feature, the past day of
model/ensemble runs has provided a dramatic increase in spread.
This arises from a combination of timing differences for the upper
trough/embedded low, as well as various ideas for
timing/amplitude/interaction with northern stream flow. Both the
ECMWF/CMC have made the biggest shifts in the past day or so with
a faster progression of northern stream energy across the northern
tier into the Northeastern U.S., which leaves the southern stream
trough/embedded upper low hanging back and lingering in the
vicinity of the Southern Plains (with slow eastern movement) for
several days. The ECMWF does eventually kick the energy eastward
(more so than the CMC) but is still not nearly as fast at the GFS.
The GFS (and off and on runs of the UKMET) support more phasing
between the northern/southern stream energy resulting in a much
faster progression of the trough, bringing it into the Eastern
U.S. by next weekend.
Although there has been a clear trend in all of the guidance for
slower movement, prefer to land somewhere in the middle of the two
extremes as this point (close to the ensemble means). WPC favored
a quick transition to majority ensemble mean guidance by mid to
late period, although with about a third contribution from the
ECMWF for trough definition (which obviously gets washed out in
the ensembles) and because the ECMWF is the deterministic piece of
guidance closest to that of the ensemble means. This approach
provides fairly good continuity with the previous WPC forecast and
also works well for more agreeable Western U.S. troughing entering
the picture by the last half of the period.
...Weather/Hazards Highlights...
Interaction of Gulf moisture with the leading wavy front from the
Great Lakes into southern High Plains will promote increasing
coverage and intensity of rain/thunderstorms from just before the
medium range period begins and onward, with heaviest rainfall
expected from portions of Texas and Oklahoma into the Middle
Mississippi Valley. Slow movement of the wavy front for a time
could allow for training of convection and an increased flood
risk. Dependent on trough evolution, there could be some threat
for severe weather as well (see the Storm Prediction Center for
additional details). Significant divergence in the guidance for
the pattern east of the Rockies after early Thursday lowers
confidence in the coverage and intensity of precipitation from
late week into the weekend. Emerging trends suggest potential for
rainfall to become progressive as it reaches the East while it
lingers for a longer time over the southern tier and/or southern
half of the Plains.
Flow around the upper ridge building into the West and ahead of
the next shortwave may bring some moisture into the Pacific
Northwest and possibly extreme northern Rockies. Heaviest
precipitation should remain over western British
Columbia/Vancouver island. It is uncertain how far south into the
Northwest any precipitation may reach through midweek or so, but
approach of the late-period shortwave should increase the coverage
over the Northwest and northern Rockies by Saturday.
The warm sector ahead of the main front extending from the Plains
into Great Lakes and New England will contain temperatures up to
10-20F above normal from mainly parts of the Tennessee Valley into
the Mid-Atlantic/Northeast through midweek. Temperatures should
moderate back closer to normal after Thursday for the East, but
increasing pattern uncertainty over the East leads to low
confidence in the temperature forecast over some areas. Meanwhile,
continue to expect very warm to hot conditions to build over the
West after midweek as a strong upper ridge builds over the region.
Daily highs may reach 10-20F above normal over a majority of the
West by Thursday and/or Friday with some daily record highs and
warm lows possible over the Southwest and possibly other
locations. The upper trough approaching from the eastern Pacific
will begin a cooling trend over northern areas by Saturday, with
plus 10-15F anomalies most likely to extend from parts of
California and the Southwest into the north-central Plains at that
time.
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