Extended Forecast Discussion NWS Weather Prediction Center College Park MD 152 AM EDT Wed Mar 24 2021 Valid 12Z Sat Mar 27 2021 - 12Z Wed Mar 31 2021 ...Heavy rain possible across parts of the interior Deep South/Tennessee Valley on Saturday... ...Overview... A strong surface low will be exiting the Northeast as the period begins on Saturday, while a vigorous northern stream shortwave sends another deepening surface low from the Central Plains to the Great Lakes/Northeast Saturday-Monday. This should keep weather in the East active this weekend into early next week with the best chance for heavy rain across parts of the Deep South on Saturday. Meanwhile, the guidance shows southern stream energy breaking off from the main trough axis with a cutoff upper low lingering over the Southwest this weekend, before eventually getting pushed east and absorbed as the next system enters the West. This should bring another round of rain and mountain snows from the Pacific Northwest to the Rockies early next week. ...Model Guidance/Predictability Assessment... The latest models and ensembles are pretty agreeable through the period on the overall synoptic pattern, but offer continued uncertainty in the details. There continues to be some question with the Southwest cutoff low, and whether such a feature exists at all. Just 24-hours ago, this appeared to be an outlier solution in the GFS and now most of the guidance (including a fair number of ensemble members) have picked up on this feature (except, ironically, the GFS flip flopping run to run). This doesn't look to be much of a weather maker anyways, aside from some showery activity in the Four Corners region, but how the energy interacts with the next shortwave moving through the Northwest next Monday could be important. Elsewhere, models show good agreement with the shortwaves in the Northeast and the Northwest, albeit some timing differences. With both, the GFS is fast compared to the better consensus so after day 5 it was excluded from the blend. Among all the guidance, the ECMWF has been showing the best level of consistency for the past few runs so a fair amount of it was used through the whole period. The first half of the period used a majority blend of deterministic solutions (ECMWF/CMC/GFS) with more ensemble mean incorporated after day 5 to account for increasing uncertainty. ...Weather/Threats Highlights... An intense low pressure system should be off the coast as the medium range period begins, but lingering gusty winds may continue Saturday across the Northeast. The trailing frontal boundary in the Gulf of Mexico should lift northward as a warm front focusing another round of moderate to heavy rainfall across parts of the Deep South to the Tennessee Valley on Saturday. The next 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 is monitoring the severe weather potential with this system, mainly across the Southeast so 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 could result in some 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 could be 15 to 20 degrees above normal Sunday and Monday. The West Coast drops back towards normal or slightly below normal Monday/Tuesday as troughing enters the region, while above normal temperatures may again move into the East by next Wednesday. 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