Extended Forecast Discussion NWS Weather Prediction Center College Park MD 221 AM EDT Thu Mar 25 2021 Valid 12Z Sun Mar 28 2021 - 12Z Thu Apr 01 2021 ...Overview... The upper level pattern during the medium range period should be pretty progressive with two main systems of note. The first, a vigorous northern stream shortwave, will send another deep surface low through the Great Lakes and Northeast Sunday and Monday, with a trailing cold front exiting the East Coast. This sets the stage for continued unsettled weather from the Southern Plains to the Northeast, with some snowfall possible across northern New England to end the weekend. Meanwhile, the next system enters the Western U.S. on Sunday, driving a well developed system and cold front eastward coast to coast through the remainder of the extended range. ...Model Guidance/Predictability Assessment... Models and ensembles show very good agreement on the overall synoptic pattern through the medium range, with typical questions in the details. A cut off upper low over the Southwest on Sunday continues to show some uncertainty regarding timing and ejection eastward early next week/interactions with approaching Western U.S. troughing. The GFS continues to struggle run to run with handling of this energy, so a non-GFS consensus was preferred for this system. Elsewhere, the deterministic models show good agreement with both the northern stream energy through the Great Lakes and Northeast Days 3-4, as well as amplifying Western U.S. troughing progressing from the West on Sunday, across the Central Plains, and into the Northeast next Thursday. WPCs blend leaned more on the ECMWF days 3-4, with smaller contributions from the rest of the deterministic guidance. Increased the usage of the ensemble means days 5-7 just to mitigate the smaller scale differences, but continued inclusion of the ECMWF for some increased strength and definition of systems. This resulted in good continuity with the previous WPC forecast as well. ...Weather/Threats Highlights... Widespread showers and thunderstorms are likely ahead of the cold front on Sunday from the Southeast into the Mid-Atlantic, with moderate to locally heavy rainfall possible in spots. Given heavy rainfall also expected over the Southeast/Tennessee Valley during the short range, this may enhance a possible flooding threat for some areas. Expect some showers farther west into Texas as well along the trailing frontal boundary. The associated deepening surface low into the Northeast Sunday-Monday should result in accumulating snowfall across portions of northern New England, as well as gusty winds across much of the Northeast. The next system into the West will bring another round of rain and mountain snow to the region early next week. As the cold front enters the Central U.S. by Monday/Tuesday, it should tap into Gulf of Mexico moisture to produce showers and storms ahead of it initially across Texas and expanding northeastward into the Tennessee and Ohio Valleys by the middle of next week. Locally heavy rainfall is possible, but the details on this are dependent on intensity and timing of the Southwest U.S. closed low and interactions with the Western-Central U.S. trough. East Coast states begin above average on Sunday, through trend back towards normal Monday/Tuesday following the passage of the cold front. Above normal temperatures will also steadily progress eastward from the West to the Plains and the Midwest/parts of the East by middle of next week underneath of upper level ridging. The greatest anomalies are expected across the Northern Plains where daytime highs 15-25 degrees above normal are possible Sunday and Monday. The upper level trough into the Pacific Northwest will bring temperatures back down towards normal or below normal in the same regions by early to mid next week. Santorelli 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