Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
253 PM EDT Sat May 01 2021
Valid 12Z Tue May 04 2021 - 12Z Sat May 08 2021
...Heavy rainfall threat across the South into Tuesday/Wednesday...
...Weather Pattern and Model Guidance...
Guidance remains in above average agreement days 3-7 and offer
good run to run continuity, bolstering forecast confidence. Broad
cyclonic flow over much of the lower 48 states early next week
will feature numerous embedded shortwaves in two streams. The
active pattern will then significantly amplify as upper troughs
work into the West Coast and the East to sandwich a warming
west-central U.S. upper ridge. Variance remains with the timing
and strength of smaller scale systems. Composites tend to mitigate
these differences consistent with predictability. Accordingly, the
WPC medium range product suite was primarily derived from a blend
of well clustered guidance of the 06 UTC GFS/GEFS mean, the 12 UTC
ECMWF/Canadian/ECMWF ensemble mean and the 13 UTC National Blend
of Models. Latest 12 UTC models and ensembles remain in line.
...Weather/Hazards Highlights...
A threat of widespread moderate to heavy rain and strong
thunderstorms remains mainly into Tue/Wed for portions of the
South, especially from eastern TX and the lower MS/TN Valleys to
the southern Appalachians. This may cause some flooding,
particularly where convective training develops or across areas
that will have already received noteworthy prior rains. Depending
on the downstream evolution and track of developing low pressure
off the East Coast later next week as the flow amplifies, moderate
rain can be expected for portions of the Mid-Atlantic and the
Northeast. Trough amplification into the West Coast may meanwhile
support enhanced precipitation over the Pacific Northwest that
seem slated to work inland to the northern Rockies next weekend.
In terms of temperatures, highs are expected to be about 5-15
degrees above average for much of the eastern U.S. early next
week. This will be followed by a return to below normal
temperatures to close out the week. Across the western U.S., heat
is expected to build in response to the upper level ridge, with
highs 10-20 degrees above normal possible for the middle to end of
the week for portions of the Intermountain West and the Desert
Southwest. This will equate to some 100+ degree high temperatures
for the lower elevations of eastern CA and southern AZ.
Hamrick/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