Extended Forecast Discussion NWS Weather Prediction Center College Park MD 248 AM EDT Mon Apr 12 2021 Valid 12Z Thu Apr 15 2021 - 12Z Mon Apr 19 2021 ...Central Rockies/High Plains Heavy Snow Threat... ...Guidance/Predictability Assessment... The WPC medium range product suite was primarily derived from a blend of well clustered guidance of the 18 UTC GFS/GEFS mean, 12 UTC ECMWF/ECMWF ensemble mean and the 01 UTC National Blend of Models (NBM). Guidance is in better agreement than yesterday, bolstering forecast confidence. Applied most blend weight to the models days 3-5 (Thursday-Saturday). Shifted blend weights in favor of the ensemble means and NBM days 6/7 (Sunday-next Monday) amid growing forecast spread. 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 mean trough/low over the West and lead cold Canadian high pressure (with temp anomalies ~10-20 degrees below normal) this week should allow a potentially heavy upslope snow event over the North-Central Rockies/High Plains linger into mid-late week. Some deformation and terrain enhanced precipitation still remains possible over parts of the Great Basin. Confidence is also increasing on a moderate rain focus then gradually shifting into the Plains Thursday-Friday. Meanwhile, a wavy frontal boundary settles and lingers this period into 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 through the forecast period. Timing and details remains uncertain with ejecting focus from southern stream impulses, but the Central Gulf Coast states and vicinity have had much above normal rainfall recently, so there may be flooding concerns. Farther north, recent guidance has trended stronger and less progressive with a main upper trough/low over the Mid-Atlantic and Northeast Thursday-Friday. Moderate to locally heavy rainfall can be expected to accompany an associated front and surface low trek across the region. There may also be some mountain snows over the cooled Northeast. Ample coastal low genesis will also present some maritime and coastal threats later week into the weekend. 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