Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
403 PM EDT Thu Apr 15 2021
Valid 12Z Sun Apr 18 2021 - 12Z Thu Apr 22 2021
...Multi-day Heavy Rainfall Threat for Florida...
18Z Update: The 12Z deterministic guidance is in very good
agreement through most of the forecast period with a few
exceptions. The UKMET is stronger and slower with the shortwave
crossing the northern Rockies on Monday, and the ECMWF becomes
more amplified with that trough across the Plains and Midwest
states by Tuesday/Wednesday. The CMC becomes more out of phase
with additional shortwave energy across southern Canada for the
second half of the forecast period, and the 00Z run was not in as
much agreement as the 12Z run. Except for some minor timing
differences, there were no major changes with this forecast cycle,
and a blend of the ECMWF and the past two runs of the GFS, along
with some of their respective ensemble means and the 13Z National
Blend of Models, was used as a baseline for the medium range
forecast. The previous discussion is appended below for
reference. /Hamrick
...Overview and Guidance/Predictability Assessment...
The WPC medium range product suite was primarily derived from a
composite blend of the very well clustered guidance of the 18 UTC
GFS/GEFS mean, the 12 UTC ECMWF/ECMWF ensemble mean and the 01 UTC
National Blend of Models in a pattern with above normal
predictability days 3-7. This solution maintains good WPC
continuity and is in overall line with later 00 UTC guidance.
...Weather/Hazards Highlights...
A frontal boundary stalling over the Gulf of Mexico and Florida
this weekend and next week will be the focus for several rounds of
potentially heavy rainfall as upper impulses induce a series of
moisture and instability-focusing waves. The greatest risk will be
over northern and central Florida. This may be a significant
multi-day event leading to flooding issues.
Generally light precipitation will be possible near another
frontal system dropping south and eastward through the central and
eastern U.S. for the first half of next week. However, the
North-Central Rockies/High Plains could see moderate snow Sunday
and Monday with the frontal surge/upslope. Supporting the snow
chances will be colder than normal temperatures surging across the
central U.S., with periods of maximum temperatures 10 to 25
degrees below average.
Meanwhile upstream, warmer than average temperatures expected for
the West Coast underneath ridging may moderate early to mid next
week as an upper trough and frontal system digs southward over the
region, but precipitation should be minimal.
Schichtel/Hamrick
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
Hazards:
- Heavy rain across portions of the Southeast, Sun-Tue, Apr 18-Apr
20.
- Flooding possible across portions of the Southeast and the Lower
Mississippi Valley.
- Flooding occurring or imminent across portions of the Lower
Mississippi Valley.
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