Extended Forecast Discussion NWS Weather Prediction Center College Park MD 239 PM EDT Wed Apr 14 2021 Valid 12Z Sat Apr 17 2021 - 12Z Wed Apr 21 2021 ...Heavy Rainfall Threat for Florida... ...Overview and Guidance/Predictability Assessment... The medium range period will generally be dominated by periods of longwave troughing or quasi-zonal flow across much of the central and eastern U.S. with embedded shortwaves, with a persistent upper ridge over the West Coast of the U.S. and Canada. Overall this pattern appeared well clustered in the model guidance, bolstering forecast confidence in the synoptic scale pattern, but differences in location and timing of shortwaves remained. One example of this was across the central U.S. around Sunday, with deterministic models showing shortwave energy around the Middle Mississippi Valley on average, but the 00Z UKMET appeared too far northeast with the feature, having it over the Great Lakes. But a mainly deterministic model blend of the 06Z GFS and 00Z ECMWF and CMC was able to be used for the first part of the medium range period through around Monday. Then, differences in the potency of a possible shortwave rounding the western side of the main trough arise around Tuesday/Wednesday, as well as the possibility of energy separating off from an upper low in the Pacific and entering the western U.S. then. The CMC was most aggressive with these features, with the GFS runs weaker and the ECMWF in between. Favored the 06Z GEFS and 00Z EC ensemble means by then to minimize the individual model differences. Overall this produced good continuity with previous WPC forecasts. ...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 should shift from the central/eastern Gulf Coast Saturday to particularly northern and central Florida early next week. Portions of this area have seen more than double normal April rainfall, so this additional rainfall could worsen or initiate flooding. A windy lead coastal storm is forecast to exit New England waters this weekend. Then 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. Farther west, the Southern/Central Rockies can expect moderate to heavy snow Saturday with a lead impulse. Then, as the back end of the next frontal system passes through, the Northern/Central Rockies could see at least moderate snow Sunday and Monday. 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, warmer than average temperatures are expected for the West Coast underneath the persistent ridging. Tate 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