The Weather Prediction Center

College Park, MD

Extended Forecast Discussion

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
342 PM EDT Tue Apr 13 2021

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

...Central Rockies/High Plains Heavy Snow Threat Lingers Into This
Upcoming Weekend...
...Heavy Rainfall Threat For Central Gulf Coast and Portions of
Florida This Weekend and Early Next Week...

...Guidance/Predictability Assessment...

A somewhat blocky pattern early on in the medium range period is
expected to evolve over the period and by early to mid next week,
much of the central/eastern U.S. could be within a troughing
regime while some sort of upper ridging remains in place over the
western U.S.. The exit of the closed low over the Northeast U.S.
this weekend and the ridging anchored over southwestern Canada
will allow for another strong shortwave to drop southward through
the Plains/northern Rockies. This feature is expected to carve out
a closed low over the Central Rockies and likely lead to a heavy
snow upslope event. The deeper troughing over the central U.S.
will keep a frontal boundary suppressed south into the Gulf Coast
region and this stationary boundary interacting with waves of low
pressure and moisture transport will be the focus for episodes of
heavier rainfall. The latest guidance showed average to above
average agreement and continuity from the previous cycle. The WPC
blend was composed of the 00Z ECMWF, 06Z GFS, 00Z CMC, and 00Z
UKMET initially then transitioned toward a blend favoring the 00Z
ECENS/00Z GEFS means by day 6/7 where there was higher model
spread. This resulted in a blend that trended well from

...Weather/Hazards Highlights...

Closed low/trough dropping southward through the Rockies and its
associated High pressure building across the Plains will favor
below normal temperatures and a heavy upslope snow event for
portions of the Central Rockies of WY/CO. As upstream ridging
builds over the western U.S. and eastern Pacific, the troughing
over the Rockies will remain in place with the threat of heavier
snow sagging southward through this weekend. Daytime high
temperature anomalies of 20F to perhaps 25F below normal are
possible Friday and this weekend. Conversely, the upper ridging
over the Pacific will lead to highs 15-20F above normal,
particularly for northern California to Oregon and Washington.

A frontal boundary stalling over the Gulf and portions of the
Florida peninsula this weekend and early next week will be the
focus for several rounds of potentially heavier rainfall. The
greatest risk area appears to be from central/eastern Gulf Coast
toward the Florida peninsula and much of this area has seen above
normal precipitation the last 14 days (200-300 percent of normal)
and this additional rainfall could worsen or initiate flooding in
the area.


Additional 3-7 Day Hazard information can be found on the WPC
medium range hazards outlook chart at:

- Heavy rain across portions of the Southeast and the Lower
Mississippi Valley, Fri-Sat, Apr 16-Apr
- Heavy rain across portions of the Southeast, Sat-Tue, Apr 17-Apr
- Flooding possible across portions of the Middle Mississippi
- Flooding occurring or imminent across portions of the Middle
Mississippi Valley and the Lower
Mississippi Valley.
- Flooding likely across portions of the Middle Mississippi Valley.
- High winds across portions of the Northeast, Fri-Sat, Apr 16-Apr

WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation, winter weather outlook probabilities
and heat indices are at:

Last Updated: 342 PM EDT TUE APR 13 2021