Extended Forecast Discussion NWS Weather Prediction Center College Park MD 251 AM EDT Wed Apr 14 2021 Valid 12Z Sat Apr 17 2021 - 12Z Wed Apr 21 2021 ...Heavy Rainfall Threat for Florida... ...Guidance/Predictability Assessment... The WPC medium range product suite was primarily derived from a composite blend of well clustered guidance of the 18 UTC GFS and 12 UTC ECMWF/UKMET/Canadian days 3-5 (Saturday-Monday) in a pattern with above normal predictability. Opted for a composite blend of the 18 UTC GEFS mean, 12 UTC ECMWF ensemble mean and the 01 UTC National Blend of Models days 6/7 (Tuesday/next Wednesday) amid growing forecast spread 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 models and ensembles, albeit with some varied system timing differences. ...Weather/Hazards Highlights... A frontal boundary stalling over the Gulf 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 area appears to be from central/eastern Gulf Coast to especially Florida and portions of this area has seen more than double normal April rainfall. This additional rainfall could worsen or initiate flooding. The potential also then exists for moderate coastal cyclogenesis up off the East Coast with these series of waves, but these systems seem to be primarily a maritime concern well in the wake of a windy lead coastal storm exiting New England waters this weekend. Upstream, a series of energetic northern stream shortwave troughs digging southward to the lee of an amplified and warming West Coast upper ridge are expected to combine with several cold air surges/upslope to favor periods of moderately heavy snow over the Rockies. Temperature anomalies with these surges should range from 10-20+ degrees below normal. Lead energy offers a lingering snow threat over the south-central Rockies Saturday and subsequent energy may focus snows over the north-central Rockies Sunday into Monday. 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