Extended Forecast Discussion NWS Weather Prediction Center College Park MD 247 AM EDT Sun Apr 11 2021 Valid 12Z Wed Apr 14 2021 - 12Z Sun Apr 18 2021 ...North-Central Rockies/High Plains Heavy Snow Threat Midweek... ...Overview and Guidance/Predictability Assessment... The WPC medium range product suite was primarily derived from a composite blend of best clustered guidance of the 18 UTC GFS/GEFS mean, 12 UTC CMC, 12 UTC ECMWF ensemble mean and the 01 UTC National Blend of Models (NBM) for days 3-4 (Wednesday/Thursday). Discounted the 12 UTC ECMWF that seemed too progressive/weak through midweek out from the Northeast considering closed system nature and upper ridging building over-top in Canada. The 00 UTC ECMWF has now strongly trended toward a more amplified/less progressive solution, as has 00 UTC guidance overall. Out West, models show better agreement with trough energy settling into the Great Basin past midweek, but become more varied with focus and energy ejection days days 5-7. Prefer the ensemble means that more bodily lingers energies. Accordingly, used the GEFS/ECMWF ensemble means and the NBM days 5-7 (Fri/next weekend) amid growing model forecast spread. This all acts to maintain good WPC continuity. ...Weather/Hazards Highlights... A combination of an amplified upper trough/closed low held over the West and lead cold Canadian High Pressure (with temp anomalies ~10-20 degrees below normal) sets the stage for a potentially heavy upslope snow event over the North-Central Rockies/High Plains through midweek. Some deformation and terrain enhanced precipitation is also possible over the Great Basin. Dependent on system progression, a moderate rain focus may shift into the Plains later in the week. Meanwhile, a wavy frontal boundary will settle/linger this week into the northern Gulf of Mexico and focus periods of moderate to heavy rains over the Florida/Gulf Coast/Lower Mississippi Valley/Southern Plains. Amounts don't look very high, but the Central Gulf Coast states have had much above normal rainfall recently, so there may be flooding concerns. Farther north, showers/modest rainfall will linger across the Mid-Atlantic and Northeast mid-late week as upper level energy slides through. Guidance is overall trending upward on system organization and the extra focus also then suggests better potential for later week moderate coastal low genesis and unsettled maritime/coastal flow. 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