Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
201 PM EDT Sat May 08 2021
Valid 12Z Tue May 11 2021 - 12Z Sat May 15 2021
...Heavy rain possible over parts of the South early-mid week...
...Well below normal temperatures over the central-southern High
Plains with some snow in the central Rockies early next week...
...Overview...
Expect a moderately progressive upper pattern over the lower 48
during the period. A Great Lakes/Northeast upper trough will
depart after midweek, followed by a trough emerging from the
Rockies and likely reaching near the East Coast by next Saturday.
A trailing upper ridge will cross the West mid-late week. By
early next weekend the ridge is likely to be over the Plains while
an amplifying trough arrives at the West Coast. Central
Rockies/High Plains precipitation associated with the initial
upper trough and low-level upslope flow will be trending lighter
Tuesday but chilly temperatures will remain before a moderating
trend. Progression of the upper trough and interaction with a
front over or just south of the Gulf Coast region should promote
rain/thunderstorms over the South with some heavy activity
possible. Some of this moisture may extend farther
north/northeast after midweek but with uncertainty in
amounts/coverage. The trailing upper ridge will bring a brief
period of warmth to the West followed by a return back toward
normal readings as the next upper trough reaches the West Coast.
...Guidance Evaluation and Preferences...
Most guidance has been finding a way to achieve a similar
trough-ridge-trough configuration for the upper pattern by the end
of the period next Saturday but the embedded details vary and
there is continued divergence for the eastern U.S./western
Atlantic surface pattern after midweek.
From the Rockies eastward, combined details of the departing
Northeast trough and specifics of the upstream trough emerging
from the Rockies differ enough to produce widely different
evolutions at the surface. 00Z Canadian held onto more
southwestward elongation of the Northeast trough Tue-Thu (and was
not preferred) while the ECMWF/UKMET and especially GFS were
quicker. Utilized that trio plus a minority ensemble mean
weighting for a starting point. The result is much greater
dominance of surface high pressure over the eastern U.S./western
Atlantic after midweek--in contrast to recent ensemble mean
consensus for a weak wave to track near the Mid-Atlantic/New
England coast. On the other side of the spectrum recent GFS runs
have been the strongest with this development but there are still
quite a number of ensemble members from each system showing low
pressure just off the East Coast next Fri-Sat. Will maintain a
system with the day shift update amid limited confidence. Upper
pattern should support a system in the vicinity based on a digging
western trough, but it could trend farther offshore.
Near the West Coast, there is still a period of time mid-late week
when models/ensembles suggest a very low-predictability regime
with small-scale shortwaves progressing from the Pacific into
western North America before upper troughing amplifies near the
coast. Question remains how upstream energy evolves and at what
speed into the West -- which has yet to be resolved. Model spread
continues through the latest runs, so an ensemble consensus was
preferred. The 00Z/06Z GFS may have been closest in trough timing,
rather than the much slower ECMWF, but were slower over the East
(via the coastal system) so both models remain a bit suspect.
Blend of the GEFS mean and ECMWF ensemble mean along with
continuity offered a reasonable placeholder until the models can
resolve the upstream flow (perhaps not for a couple days).
...Weather Highlights/Hazards...
Precipitation over the central Rockies and High Plains should be
tapering off on Tuesday, with some high elevation snow still
possible. Most of the West will be dry during the period though
the Northwest may begin to see scattered precipitation toward the
end of the week as the upper trough moves in. Farther east, parts
of the South may see some heavy rainfall as the upper trough
progressing out of the Rockies interacts with a wavy front that
should be over or just south of the Gulf Coast region.
Uncertainty remains over exactly where the highest rainfall totals
will be as the models try to resolve small-scale features atop the
surface front. The most likely scenario of a frontal wave
ultimately tracking near the East Coast would spread rain of
varying intensity northeastward over the eastern U.S., but
confidence in this evolution is lower than normal. Early in the
period the upper trough and surface system affecting the Northeast
should produce mainly light precipitation with a little snow
possible in the highest elevations.
Unseasonably cool temperatures will persist over the central
Rockies and parts of the central-southern Plains into Tuesday with
highs up to 20-25F below normal (perhaps not quite near records).
The southern half of the Plains may still see highs 10-20F below
normal on Wednesday but a quick warm-up should bring highs up to
5-10F above normal over central parts of the Plains
Friday-Saturday. Many other portions of the central/eastern U.S.
will likely see one or more days with temperatures 5-15F below
normal through Wednesday or Thursday, followed by a moderating
trend that should bring readings closer to normal by Saturday.
Meanwhile continue to expect the main emphasis for 10-20F above
normal highs to progress from parts of California and Oregon into
the Great Basin during Tuesday-Friday. The upper trough nearing
the West Coast should bring readings back toward normal over the
West Coast states by Saturday.
Fracasso/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