Extended Forecast Discussion NWS Weather Prediction Center College Park MD 253 AM EDT Tue Apr 13 2021 Valid 12Z Fri Apr 16 2021 - 12Z Tue Apr 20 2021 ...A lingering Central Rockies/High Plains Heavy Snow Threat into late week... ...Guidance/Predictability Assessment... The WPC medium range product suite was derived from a composite blend of well clustered guidance of the 18 UTC GFS and 12 UTC ECMWF/UKMET/Canadian days 3-5 (Friday/weekend) in a pattern with above normal predictability. A composite blend of the 18 UTC GEFS mean, 12 UTC ECMWF ensemble mean and the 01 UTC National Blend of Models for seems best consistent with growing forecast spread days 6/7 (Monday/next Tuesday), 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 guidance. ...Weather/Hazards Highlights... A closed trough/low working out from the West and lead cold Canadian high pressure (with temp anomalies ~10-20 degrees below normal) should allow a potentially heavy upslope snow event over the Central Rockies/High Plains linger Friday into Saturday as a moderate rain focus spreads across the Plains. Upper trough reinforcement seems likely back through an unsettled Rockies and Plains into early next week considering upstream upper ridge amplitude and blocky nature of the Pacific. This seems best shown by ensemble means versus the more varied model solutions that suffer from uncertain shortwave wavelength spacing. Meanwhile, recent guidance offers a stronger and less progressive main upper trough/low over the Mid-Atlantic and Northeast Friday into Saturday. Lingering moderate to locally heavy wrapping rainfall can be expected to accompany an associated front and surface low. There should also be some mountain snows over the cooled Northeast as potent coastal cyclogenesis presents maritime and coastal high wind and wave threats. Well south, a wavy frontal boundary settles and lingers this period over the northern Gulf of Mexico and vicinity to focus several periods of moderate to heavy rains over Florida/Gulf Coast/Lower Mississippi Valley/Southern Plains. Heavy rain chances appear highest from the central Gulf Coast to Florida as hard to time low pressure waves work along the front and focus moisture convergence and instability. The central Gulf Coast states and vicinity have had much above normal rainfall recently and continued flooding concerns. 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