Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
214 AM EDT Sat May 15 2021
Valid 12Z Tue May 18 2021 - 12Z Sat May 22 2021
...Heavy rain threat for the Southern Plains/Lower Mississippi
Valley next week...
...Guidance Evaluation and Preferences...
The WPC medium range product suite was primarily derived from a
composite blend of reasonably well clustered mid-larger scale
guidance of the 18 UTC GFS/GEFS, 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
much less certain. Latest 00 UTC models and ensembles are overall
in line with this favored composite plan.
...Weather Highlights/Hazards...
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 to lift 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 over-top, the Midwest eastward 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 underneath a building
Southeast upper ridge may support some heavy downpours for
southern Florida and into the Gulf of Mexico. The 12/18 UTC GFS
runs seemed too aggressive developing an organized low, but the 00
UTC run backed off.
Mid-later next week digging of amplified upper troughing over the
West will support widespread moderate precipitation from the
Pacific Northwest through the north-central Intermountain
West/Rockies, including some mountain snows. Ahead of the Pacific
front that moves into the West, maximum temperatures 10-20F above
normal will progress from the Northwest U.S. though the northern
Rockies/Plains early next week. A post-frontal cooling trend may
bring 10-20F below normal temperatures into 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