Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
300 PM EDT Fri May 14 2021
Valid 12Z Mon May 17 2021 - 12Z Fri May 21 2021
...Heavy rain threat for the Southern Plains/Lower Mississippi
Valley next week...
...Guidance Evaluation and Preferences...
Latest models and ensembles continue to support the threat of a
multi-day heavy rain event across the south-central U.S. well into
next week. Subtle changes within the associated upper trough in
recent model runs have generally resulted in a northward shift in
the frontal boundary forecast to slowly lift northward as a warm
front, which in turn resulted in a northward shift in the heavy
rain axis. The heaviest rains/convection remain generally south
of this front. The 00Z ECMWF represented the southern-most
guidance regarding the heavy rain axis in contrast with the latest
12Z run which moved the frontal boundary significantly farther to
the north later next week. The GFS has been faster than the 00Z
ECMWF and has been carrying more energy downstream to extend the
rain axis into the Ohio Valley. A consensus of the 00Z ECMWF/EC
mean, 06Z GFS/GEFS, together with the 00Z CMC was used to handle
this system.
Toward the latter part of next week, the 12Z ECMWF yesterday
started to depict a much more amplified trough digging into the
West Coast. The CMC and now the GFS have followed suit into
agreeing with this scenario. On the other hand, both the ECMWF
and CMC have backed off from forming a closed low over New England
later next week in agreement with the more progressive GFS.
The WPC medium-range forecast package was based on the consensus
of the 00Z ECMWF/00Z EC mean, the 06Z GFS/GEFS mean, as well as
some contribution from the 00Z CMC and CMC mean. This yielded a
solution close to WPC continuity but with a northward shift in the
frontal position across the Plains, and a more progressive and
deeper upper trough moving into the West later next week.
...Weather Highlights/Hazards...
A favored model and ensemble composite solution indicates that
deep layered moisture from the Gulf of Mexico will return into an
emerging and slow moving south-central U.S. frontal system next
week. This will occur in downstream response to the slow ejection
and passage of a southern stream upper-level trough/low energies
forecast to exit the southern Rockies. Expect a multi-day period
with a threat of significant rainfall along with periods of
increased instability and strong to severe convection. Locations
and magnitude of heaviest rainfall will likely vary on a
day-to-day basis and some specifics may take until well into the
short range to be resolved, but the guidance signal for a wet
pattern over the south-central U.S. has been consistent.
Meanwhile, the Midwest eastward into the Mid-Atlantic may see one
or more convective systems aided by a wavy front and uncertain
northern shortwave energy aloft to focus moisture and instability.
Well south, late period moisture/vorticity influx underneath a
building Southeast upper ridge may support some organized heavy
downpours over southern Florida.
The favored GEFS/ECMWF ensemble mean solution over the West
mid-later next week would support mainly moderate precipitation
from the Pacific Northwest out across the north-central
Intermountain West/Rockies to include some mountain snows. Ahead
of the Pacific front that moves into the West next week, lead
maximum temperatures 10-20F above normal will also progress from
the Northwest U.S. eastward though the northern Rockies/Plains
early next week. A cooling trend behind the front may bring 5-10F
below normal temperatures.
Kong/Schichtel
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