Extended Forecast Discussion NWS Weather Prediction Center College Park MD 414 PM EDT Wed Mar 24 2021 Valid 12Z Sat Mar 27 2021 - 12Z Wed Mar 31 2021 ...Heavy rain threat continues for the Tennessee Valley Saturday night... ...Overview... A progressive pattern is on tap for the CONUS over the next week. A northern stream trough digging over the interior western US (starting tonight) closes and becomes cutoff over Arizona by Saturday and merely drifts east across New Mexico Sunday before ejecting northeast from the southern Plains Monday ahead of the next western trough which digs to the Four Corners through Tuesday. The northern stream energy that cuts off the low in the Southwest ejects east across the northern Great Plains Saturday, spinning into a low over the Great Lakes early Sunday and further develops over northern New England into Sunday night. That next western trough shifts east across the Great Plains Tuesday, reaching the Great Lakes on Wednesday. Broad southwesterly flow ahead of the lead trough maintains a heavy rain threat for the Tennessee Valley Saturday night with snow shifting from the northern Great Lakes Sunday to northern New England Sunday night/Monday. ...Model Guidance/Predictability Assessment... The key feature with differences among the latest global guidance is the cutoff low in the Southwest Days 3/4. The 00Z ECMWF and 06Z GFS are in decent agreement for depth and placement, while the 00Z CMC is slow/farther west and the 00Z UKMET is rather open/progressive. Therefore, the preference through Day 4 is a GFS/ECMWF blend. As usual with cutoff lows, progression is uncertain with a need for ensemble means to be weighed heavier a bit sooner than normal with the 00Z ECENS/06Z GEFS becoming a significant part of the preference blend starting on Day 5 (where sometimes this can wait until Day 6). New Mexico and west Texas precip is anticipated, particularly on Day 4 from the GFS/ECMWF positions of the closed low which further weighed into the preference leaning toward those models. The other noted area of uncertainty is in the Days 5-7 with the timing and track of the closed low that develops in the next western US trough. The 00Z ECMWF keeps the low closer to the Canadian border (over the inter Northwest CONUS) while the GFS is farther south, so the GFS has more precip in the CONUS/northern Rockies than any other model, particularly on Day 6. This further emphasized the need for heavier weight to ensemble means late in the period. ...Weather/Threats Highlights... A frontal boundary will linger over the Mid-South Saturday before lifting north as a warm front ahead of the next low crossing the Great Lakes. Heavy rain continues across the Tennessee Valley Saturday night which may overlap areas of heavy rain through Thursday, further enhancing a flood threat. This northern system sends another cold front into the Northeast on Sunday with widespread light to moderate precipitation likely to impact the Eastern states. The Storm Prediction Center states this threat is too low at the time for a Day 4 outlook, so continue to refer to products from SPC for more information. This system could undergo cyclogenesis Sunday into Monday as the center nears the New England coast, and should result in accumulating wintry precipitation for northern New England. Out West, the trend should be towards warmer and drier weather this weekend, ahead of the next system into the Pacific Northwest which likely brings another round of rain and mountain snow to the region early next week. Showers and storms should also break out into the Central Plains and Midwest next Monday and Tuesday ahead of the Western U.S. trough. The Eastern half of the nation starts warm this weekend, but trends back towards normal Sunday/Monday after the cold front moves through. Above normal temperatures will also progress from the West to Central states this weekend underneath of upper level ridging. The greatest anomalies will be across the Northern Plains where daytime highs look to be 15 to 25 degrees above normal Sunday and Monday. Below normal temperatures are on tap for the Pacific Northwest and Intermountain West Monday/Tuesday as a cold front crosses the region, while above normal temperatures are anticipated ahead of this front Tuesday/Wednesday. Jackson 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 precipitation across portions of the Northeast, Sun, Mar 28. - Heavy rain across portions of the Lower Mississippi Valley, the Southern Appalachians, the Ohio Valley, and the Tennessee Valley, Sat, Mar 27. - Heavy rain across portions of the Middle/Lower Mississippi Valley, the Ohio Valley, and the Tennessee Valley, Wed, Mar 31. - Heavy snow across portions of the Pacific Northwest and the Northern Great Basin, Sun, Mar 28. - Flooding possible across portions of the Central Plains, the Southeast, the Lower Mississippi Valley, and the Tennessee Valley. - Flooding occurring or imminent across portions of the Middle Mississippi Valley and the Ohio Valley. - Flooding likely across portions of the Central Plains and the Lower Mississippi Valley. - High winds across portions of the Northern Plains, Mon-Tue, Mar 29-Mar 30. - High winds across portions of the Mid-Atlantic and the Northeast, Mon, Mar 29. 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