Extended Forecast Discussion NWS Weather Prediction Center College Park MD 233 PM EST Wed Mar 10 2021 Valid 12Z Sat Mar 13 2021 - 12Z Wed Mar 17 2021 ...A major weekend snowstorm for the Central Rockies/High Plains and a heavy rainfall threat from the south-central Plains to the mid-Mississippi Valley... ...Overview... During the short range period, a deep upper low is forecast to split off from northern stream flow and become a potent cutoff low moving across the Four Corners region to Southern Plains over the weekend. This plus a surface low over the Central Plains will spread significant snow to the Central Rockies and central High Plains Friday through Sunday, while severe weather and heavy rain are possible across the south-central CONUS. Another trough and additional energy should come into the West around Mon and serve as a kicker for the initial closed low, pushing it eastward as it weakens. This second trough is expected to make its way into the central U.S. by midweek. ...Model Guidance/Predictability Assessment... Model and ensemble guidance is in generally good agreement with the overall pattern described above through the weekend. GFS runs appear a little fast/east with the upper low track and its surface frontal system compared to consensus, but now within reasonable spread for the medium range period, as more recent runs have slowed closer to the consensus. A multi-model deterministic blend of the 00Z ECMWF/UKMET/CMC and a small portion of the 06Z GFS was able to be used for the first part of the period. By Mon, troughing coming into the West seems generally well clustered though with some small-scale differences, with the exception of the 00Z UKMET. The UKMET shows less and faster-moving energy in the West, allowing for the initial closed low to settle south of other guidance and maintain its strength longer, becoming an outlier solution. By Tue/Wed, notable differences arise with the initial upper low weakening and ejecting out as the upstream low tracks eastward, leading to more uncertainty. The 06Z GEFS mean and 00Z EC mean were more favored for the later part of the period to minimize the differences in individual models. ...Weather/Threats Highlights... Significant heavy snow accumulating over a foot along with gusty winds creating low visibility and possible blizzard conditions are forecast across the Central Rockies and central portions of the High Plains from Friday (short range) into the weekend. Farther east in the warm sector, heavy rains/convection will also develop in earnest from the south-central Plains to the mid-Mississippi Valley where increasingly deep moisture inflow from the Gulf of Mexico will wrap around the low into a slow-moving frontal zone. SPC shows a severe weather threat for the southern Plains to the mid-lower Mississippi Valley unstable warm sector. Rounds of rainfall are possible across the southeastern U.S. through the period as well, though the details are highly uncertain at this point. Above average temperatures there should transition closer to normal by Wed. The potential for any wintry weather on the northern side across the north-central Appalachians and northern Mid-Atlantic on the northern edge of any precipitation shield is also quite uncertain. The the upstream kicker system for the West has potential to produce generally modest precipitation from the Northwest/Northern California and to a lesser extent southeastward across and Great Basin. 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