Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
245 AM EDT Sun May 16 2021
Valid 12Z Wed May 19 2021 - 12Z Sun May 23 2021
...Heavy rain threat for the Southern Plains/Lower Mississippi
Valley this week...
...Guidance Evaluation and Preferences...
The WPC medium range product suite was primarily derived days 3-5
(Wed-Fri) from a composite blend of reasonably well clustered
mid-larger scale guidance of the 18 UTC GFS/GEFS and 12 UTC
ECMWF/ECMWF ensemble mean along with the 01 UTC National Blend of
Models. Smaller scale system differences and convective rainfall
focus details remain less certain and growing forecast spread
through medium scales next weekend prompted usage of mainly just
the ensemble means and NBM. By then, the 18/00 UTC GFS runs move
to the more progressive side of solutions and the 12/00 UTC ECMWF
the less progressive side of the solution envelope. The
aforementioned WPC forecast plan focusing on the ensemble means
maintains good WPC product continuity near the envelope composite.
This seems prudent given the recent model run-run fluctuations at
these longer time scales.
...Weather Highlights/Hazards...
Deep layered moisture from the Gulf of Mexico will return into an
emerging and slow moving central U.S. systems this 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 to lift up over the Plains. 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 mainly over the south-central U.S. has been consistent.
Meanwhile, deep moisture and vorticity underneath a building upper
ridge over the East may support some heavy downpours for southern
Florida and into the Gulf of Mexico. Ridge amplification will
coincide with substantial warming this week from the Midwest to
the especially the East where some record high minimum and maximum
values are possible as temperature anomalies build upwards to
10-20F above normal.
Mid-later next week digging of amplified upper troughing over the
West will support widespread moderate precipitation from the
Pacific Northwest and Sierra through the north-central
Intermountain West/Rockies, including some mountain enhanced
snows. Ahead of the Pacific front that moves into the West, early
week maximum temperatures 10-20F above normal from the Northwest
U.S. though the northern Rockies/Plains will abruptly cool with
pattern amplification and wavy cold frontal passage that may
result in 10-20F below normal temperatures centered across the
Intermountain West.
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