Extended Forecast Discussion NWS Weather Prediction Center College Park MD 448 PM EDT Mon Apr 12 2021 Valid 12Z Thu Apr 15 2021 - 12Z Mon Apr 19 2021 ...Late-week Central Rockies/High Plains Heavy Snow Threat... ...Guidance/Predictability Assessment... A highly blocky synoptic pattern will continue through much of the medium range period. The main areas of concern will be related to the progression/evolution of a couple of upper-level lows--one sliding across the Great Lakes and then off to the east of New England, and another exiting the central Rockies toward the eastern U.S. There is increasing model support for the slow-moving upper low over the Great Lakes to trigger cyclogenesis off the southern New England coast on Friday before heading out to sea on Saturday. Meanwhile, the upper low exiting the central Rockies should culminate with the highest threat of wet snow on Friday from the central Rockies to the High Plains. There is general model agreement that the system will continue to track eastward and weaken while heading toward the eastern U.S. this weekend. There is also good model indication for another shortwave to dip into the northern Plains from southern Canada late this week. The ECMWF and CMC generally take it on a more southerly track into the central U.S. while the GFS tracks it more toward the upper Midwest and the Great Lakes. Nevertheless, the GEFS mean was in better agreement with the EC mean and the CMC mean regarding this system. The WPC medium range product suite was primarily derived from a blend of well clustered guidance of the 06 UTC GFS/GEFS mean, 00 UTC ECMWF/ECMWF ensemble mean together with some contribution from the 00Z CMC/CMC mean. Guidance is in decent agreement, bolstering forecast confidence. Applied an even blend of the deterministic and ensemble means for days 4-5 (Friday-Saturday); then shifted blend weights in favor of the ensemble means for 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 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 to linger into 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 which should gradually shift 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. The heavy rain chances appear highest from the central Gulf Coast to the Florida Panhandle this weekend and possibly into Monday as a low pressure wave could form along the front. The central Gulf Coast states and vicinity have had much above normal rainfall recently with continued flooding concerns. Farther north, recent guidance has maintained a stronger and less progressive 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 to track across the region. The possibility of coastal cyclogenesis will also present some maritime and coastal threats later week into the weekend. There may also be some mountain snows over the interior Northeast as the upper low over the Great Lakes interacts with the developing coastal low. Kong/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 Hazards: - Heavy rain across portions of the Southeast and the Lower Mississippi Valley, Sun-Mon, Apr 18-Apr 19. - Heavy rain across portions of the Mid-Atlantic and the Northeast, Thu-Fri, Apr 15-Apr 16. - Heavy snow across portions of the Central Rockies, the Central Plains, and the Northern Plains, Thu-Fri, Apr 15-Apr 16. - Flooding possible across portions of the Middle Mississippi Valley. - Flooding occurring or imminent across portions of the Middle Mississippi Valley and the Lower Mississippi Valley. - Flooding likely across portions of the Middle Mississippi Valley. - High winds across portions of the Mid-Atlantic and the Northeast, Thu-Fri, Apr 15-Apr 16. 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