Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
243 AM EDT Sat Apr 10 2021
Valid 12Z Tue Apr 13 2021 - 12Z Sat Apr 17 2021
...Central Rockies/High Plains Heavy Snow Threat and additional
rains for a soggy central Gulf Coast...
...Pattern Overview and Guidance/Predictability Assessment...
The WPC medium range product suite for days 3-5 (Tuesday-Thursday)
was primarily derived from a composite blend of best clustered
guidance from the 12 UTC GFS/ECMWF/Canadian in a pattern with
above normal predictability. Opted to use a composite blend of the
still compatible 12 UTC GEFS/NAEFS/ECMWF ensemble means days 6-7
(Friday-next Saturday) amid growing forecast spread. This strategy
maintains good WPC continuity and remains generally in line with
latest 00 UTC guidance, but forecast spread remains less than
stellar at longer time frames.
...Weather/Hazards Highlights...
An amplifying upper trough/closed low will dig and settle over a
cooled West next week to the lee of an amplified and slow moving
eastern Pacific upper ridge in a blocky pattern. There is some
uncertainty with trough evolution/undercutting of the shifting
ridge later next week as a closed high builds over western Canada.
Expect some deformation and terrain enhanced precipitation over
the Great Basin, but there is a protracted threat for periods of
heavy snows over the Central Rockies/High Plains as enhanced by
upslope as ample Canadian surface high pressure digs and settles
southward in an airmass inherent with generally 10-20F below
normal temperature anomalies next week. Slow system
progression/influence may shift a moderate rain focus over the
central Plains late period, but there is uncertainty at these
longer time frames.
Meanwhile, complex/main northern stream closed low energies over
the Upper Midwest/Great Lakes early next week will struggle to
consolidate and shift downstream to off the Mid-Atlantic coast
late next week, blocked by a closed upper trough/lows slowly
retrograding uncertainly from offshore the Canadian Maritimes
toward New England. Moderate rains will accompany the path of the
main system early next week before losing frontal convergence,
with some wrap-back snows possible for the Upper Midwest. A wavy
trailing front will settle over The South/Gulf of Mexico and
offers potential for some periods with moderate to heavier
downpours next week with a changing multi-day focus over the
Southern Plains, Lower Mississippi Valley and the Gulf
Coast/Florida as uncertain ejecting southern stream impulses focus
lift and moisture.
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