Extended Forecast Discussion NWS Weather Prediction Center College Park MD 245 AM EDT Mon Apr 19 2021 Valid 12Z Thu Apr 22 2021 - 12Z Mon Apr 26 2021 ...Record cold likely for parts of the central U.S. Thursday morning... ...Overview... Upper ridging is forecast over Alaska and Greenland across the high latitudes and over the Greater Antilles in lower latitudes, favoring troughing over much of the CONUS. As an upper low south of Alaska splits off its southern portion toward the West Coast this weekend, additional troughing will sink southward out of Canada through the Great Lakes. Ridging will build in between these two systems next week across the Plains. The pattern is rather chilly for late April with precipitation chances focused over the Southeast, East Coast, and West. ...Guidance/Predictability Assessment and Preferences... The blocky pattern has proven tough to forecast for the models/ensembles, but especially the GFS/GEFS which again was generally quicker with the western trough this weekend into next week compared to the ECMWF/Canadian and their ensembles. In the interim, the GFS was slower than the rest with an upper low or sharp trough through the Southwest Thu/Fri though it has varied between quicker and slower over the past few days of runs. Preferred to stay closer to the more consistent ECMWF/Canadian consensus with their ensemble means. This system may wrap up into the Northeast/New England around next Monday as another robust surface low in a similar location as the Thursday system. By next week, uncertainty in how the closed low approaches California increases, and thusly favored the ECMWF ensemble mean as a better starting point than the quicker GEFS mean. ...Weather/Hazards Highlights... System coming out of the Southwest/southern Rockies late Thu/early Fri will push into the Southern Plains where rainfall will expand along and ahead of a warm front. Heavy rainfall is quite probable over the lower Mississippi Valley through the Southeast Fri-Sat. As the system moves just offshore and perhaps up the coast, it may combine with a northern stream system over the Great Lakes and bring enhanced rainfall to at least New England late Sun into next Mon. Off the West Coast, the upper low and occluded surface front will wobble closer to the coast through the period, eventually bringing lower elevation rain and mountain snow to northern CA northward. Warmer than normal temperatures are expected for the West Coast closer to or underneath upper ridging, with much milder air just ahead of the Pacific system late in the period. Areas east of the Rockies will see below normal temperatures Thursday, perhaps by 10-20 degrees which would support record cold low temperatures from Texas northeastward toward the Corn Belt/Midwest. That cold air mass will push eastward but moderate into the Northeast/Mid-Atlantic. With troughing nosing southward out of Canada, areas along the border will see below normal temperatures as well. The only sustained area of above normal temperatures will be over parts of the West/Great Basin ahead of the upper low offshore. As that moves closer, the milder air will move eastward across the Rockies to the Plains next week. Fracasso 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