Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
249 AM EDT Fri Apr 16 2021
Valid 12Z Mon Apr 19 2021 - 12Z Fri Apr 23 2021
...Multi-day Heavy Rainfall Threat for Florida...
...Overview...
Strong upper ridging over northwestern Canada (500mb height
anomalies around +3.5 sigma) as well as over the Bermuda Triangle
will favor renewed troughing over the central CONUS next week and
at times in the Northeast and Southwest via split flow. This will
bring cooler than normal temperatures to most of the lower 48 and
push most of the precipitation chances to the south; namely,
Florida. There, a wavy stationary boundary will promote periodic
rainfall which may be heavy at times.
...Guidance/Predictability Assessment and Preferences...
With the ECMWF and GEFS ensembles in relatively good agreement,
favored the deterministic models that fell within their overlap
the best during the period. This was mostly near the 18Z GFS and
next nearest to the 12Z ECMWF. Uncertainty in the northern stream
flow out of Canada later next week as well as the breakdown of the
upper ridge in the Pacific led to large differences in the upper
pattern over much of the CONUS and, as such, utilized a mostly
ensemble mean blend to mitigate future large changes. Ensemble
trends were toward stronger closed centers in the higher latitudes
suggesting punctuated zonal flow to its south (over the CONUS) and
limited predictability.
...Weather/Hazards Highlights...
A frontal boundary stalling over the Gulf of Mexico and Florida
will be the focus for several rounds of potentially heavy rainfall
as the parallel upper flow from the southwest bring in episodes of
moisture and rainfall. The greatest risk will be over northern and
central Florida where several inches are likely in some areas
during the period. ECMWF EPS EFI highlights the area along and
north of I-4 for anomalous rainfall early in the week, then
sinking southward. 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 central
Rockies/western High Plains could see modest snow on Monday as a
cold front stalls in the area, yielding upslope snow potential.
Temperatures will be much colder than normal early in the week
with sub-freezing highs (15-30 degrees below normal). The cold
airmass will spill onto the southern Plains Tuesday where highs
may only reach the mid 50s over northern TX into OK, which may
challenge record cold highs.
Farther west, warmer than normal temperatures are expected for the
West Coast underneath ridging, though that may be tempered a bit
as a Pacific frontal system digs southward over the region.
Precipitation should be minimal and mainly over the mountains. The
East will see a couple chances of light rain and maybe some wet
snow in far northern areas, with temperatures near to slightly
below normal.
Fracasso
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