Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
253 AM EDT Tue Apr 13 2021

Valid 12Z Fri Apr 16 2021 - 12Z Tue Apr 20 2021

...A lingering Central Rockies/High Plains Heavy Snow Threat into
late week...

...Guidance/Predictability Assessment...

The WPC medium range product suite was derived from a composite
blend of well clustered guidance of the 18 UTC GFS and 12 UTC
ECMWF/UKMET/Canadian days 3-5 (Friday/weekend) in a pattern with
above normal predictability. A composite blend of the 18 UTC GEFS
mean, 12 UTC ECMWF ensemble mean and the 01 UTC National Blend of
Models for seems best consistent with growing forecast spread days
6/7 (Monday/next Tuesday), in a pattern with average
predictability for these longer time frames. This acts to maintain
good WPC continuity in an overall forecast scenario that seems in
line with latest 00 UTC guidance.

...Weather/Hazards Highlights...

A closed trough/low working out from the West and lead cold
Canadian high pressure (with temp anomalies ~10-20 degrees below
normal) should allow a potentially heavy upslope snow event over
the Central Rockies/High Plains linger Friday into Saturday as a
moderate rain focus spreads across the Plains. Upper trough
reinforcement seems likely back through an unsettled Rockies and
Plains into early next week considering upstream upper ridge
amplitude and blocky nature of the Pacific. This seems best shown
by ensemble means versus the more varied model solutions that
suffer from uncertain shortwave wavelength spacing.

Meanwhile, recent guidance offers a stronger and less progressive
main upper trough/low over the Mid-Atlantic and Northeast Friday
into Saturday. Lingering moderate to locally heavy wrapping
rainfall can be expected to accompany an associated front and
surface low. There should also be some mountain snows over the
cooled Northeast as potent coastal cyclogenesis presents maritime
and coastal high wind and wave threats.

Well south, a wavy frontal boundary settles and lingers this
period over the northern Gulf of Mexico and vicinity to focus
several periods of moderate to heavy rains over Florida/Gulf
Coast/Lower Mississippi Valley/Southern Plains. Heavy rain chances
appear highest from the central Gulf Coast to Florida as hard to
time low pressure waves work along the front and focus moisture
convergence and instability. The central Gulf Coast states and
vicinity have had much above normal rainfall recently and
continued flooding concerns.

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