Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
258 AM EDT Thu May 13 2021
Valid 12Z Sun May 16 2021 - 12Z Thu May 20 2021
...Heavy rainfall possible over parts of the Southern Plains into
the Mississippi Valley from early through at least middle of next
week...
...Overview...
The majority of guidance continues to show the establishment of a
western trough/eastern ridge upper pattern and persistent flow of
Gulf moisture around East Coast surface high pressure, leading to
a multi-day threat of significant rainfall over portions of the
Southern Plains into Mississippi Valley. Locations and magnitude
of heaviest rainfall may vary on a day-to-day basis and some
specifics may take until well into the short range to be resolved,
but there is a consistent signal for a wet pattern over the
central U.S. Some variation of this pattern could extend beyond
the medium range period based on the latest 8-14 Day forecast from
the Climate Prediction Center. In addition to this area of focus,
locations from the Central Plains/Midwest east-southeastward into
the Mid-Atlantic may see one or more convective systems aided by a
wavy front and shortwave energy aloft, and improving consensus for
the upper low tracking through the Southwest toward the Plains is
suggesting greater potential that the central Rockies/High Plains
may see a period of enhanced precipitation. An upstream trough
reaching the Northwest will likely bring unsettled weather to that
region but with lighter totals.
...Guidance Evaluation and Preferences...
The preferred consensus forecast maintains good continuity with
only minor detail adjustments, though individual solutions
continue to show some divergence and run-to-run changes. The
northeastern Pacific upper trough reaching the Northwest had been
trending toward better clustering after a period of significant
spread, but the 12Z ECMWF suddenly adjusted slower to resemble
some GFS runs from a couple days ago. After day 4 Monday a
composite of latest GEFS/ECMWF means and GFS runs, and old 00Z/12
ECMWF represented the established consensus. Among the new 00Z
runs, the UKMET becomes somewhat strong/fast but the GFS/CMC and
GEFS mean continue to provide an intermediate cluster. Meanwhile
the models agree fairly well for the upper low tracking through
the Southwest into the southern Rockies early-mid period. As this
feature likely opens up as it reaches the Plains, some spread
develops due in part to differences with the upstream trough.
Farther east there are still notable differences for what becomes
of shortwave energy that progresses from the Northern
Plains/southern Canada. Among the possibilities include the Plains
energy lingering for a time over the Midwest as an upper ridge
passes by to the north, while the new 00Z UKMET uses Canadian
energy to form a Great Lakes upper low (another aspect seen in
some GFS runs from a couple days ago and then abandoned). Ensemble
means and remaining models wait until this energy reaches New
England and the Canadian Maritimes to amplify. The 12Z ECMWF is
underdone with New England troughing relative to the means.
If/where an upper low forms is still up for debate, with CMC runs
dropping a low just off the East Coast while other solutions have
been well eastward or northward. Guidance comparisons led to
following an operational model blend early, replacing the 12Z
ECMWF with the prior 00Z run by day 5 Tuesday, and then
transitioning to a model/mean mix for balancing the consensus
pattern with some operational detail where possible. In most
respects the new 00Z ECMWF has come in with a better comparison to
consensus/preference.
...Weather Highlights/Hazards...
The overall forecast is consistent in highlighting the potential
for a multi-day wet pattern over the central U.S., most likely
over parts of the Southern Plains into the Mississippi Valley, as
persistent low level flow from the Gulf transports moisture into
the region. Areas of heavy rainfall may be somewhat more
organized/focused around Tuesday-Wednesday as the upper system
crossing the Southwest approaches and reaches the Plains. Some
pockets of meaningful rainfall should continue into Thursday but
perhaps with less focus depending on how the energy from the
Southwest system weakens/progresses. Parts of this area have seen
much above normal precipitation over the past several weeks, which
may increase flooding concerns. Meanwhile the guidance signal
appears to be strengthening for a period of enhanced precipitation
over the central Rockies/High Plains during the first part of the
week, from the combination of low level upslope flow and favorable
location relative to the Southwest upper low's track. The trailing
upper trough reaching the Northwest should bring mostly light to
moderate precipitation to the region from about Monday night
onward. Highest elevations could see some snow. A wavy front and
initial Northern Plains shortwave energy may promote one or more
convective systems that could produce locally heavy rainfall from
the Central Plains/Midwest east-southeastward. However there is
still considerable uncertainty over the magnitude and location of
rainfall within this axis. On Sunday parts of the Northeast may
see one more episode of mostly light mid-late day rainfall under a
departing upper trough.
Ahead of the Pacific front that moves into the West next week, an
area of highs 10-20F above normal will progress from the Northwest
U.S. into the northern High Plains and eastward near the Canadian
border. The cooling trend behind the front should bring modestly
below normal highs. Meanwhile moderately below normal highs will
accompany the system initially tracking through the southwestern
states and then weakening as it ejects into the Plains. Most of
the East will see slightly below normal readings on Sunday
followed by a gradual warming trend to near or slightly above
normal temperatures.
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