Extended Forecast Discussion NWS Weather Prediction Center College Park MD 253 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 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