Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
300 AM EDT Mon May 17 2021
Valid 12Z Thu May 20 2021 - 12Z Mon May 24 2021
...Southern Plains Heavy Rain/Convection Threat lingers to
mid-later week...
...Hot for the East-Central U.S. and Cold/Unsettled over the
West/Rockies...
...Guidance Evaluation and Preferences...
The WPC medium range product was primarily derived from a
composite blend of the well clustered 18 UTC GFS/GEFS mean and 12
UTC ECMWF/ECMWF ensemble mean along with the 01 UTC National Blend
of Models, with a larger focus on the more run to run consistent
ensemble means by days 6/7. This maintains good WPC continuity.
...Weather Highlights/Hazards...
Models and ensembles still agree that upper flow pattern over the
lower 48 states will significantly amplify over the next few days.
The upper pattern will be highlighted into midweek by an emerging
digging trough into the West and troughing over the south-central
Plains increasingly blocked by an amplified and warming ridge
building out from the East. 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 will be
possible as temperature anomalies build upwards to 10-20F above
normal.
An ongoing but transitional heavy rainfall event should persist
through mid-late week with continued but slowly diminishing focus
over the southern Plains. WPC QPF has this trend, but blocky flow
and deep pooled Gulf moisture suggests some downpours may produce
more hefty local amounts and runoff issues.
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
to focus over the Northern Rockies. Ahead of the Pacific front
that moves into the West, early week maximum temperatures 10-20F
above normal from the Northwest U.S. through 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.
Ejecting western trough impulses and some energy from the southern
Plains will shear northward over the central U.S. on the western
periphery of an anchoring east-central ridge. This conduit will
supports period of enhanced convective rains as moisture and
instability focus along a slow moving and wavy from over the
north-central U.S..
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