Extended Forecast Discussion NWS Weather Prediction Center College Park MD 237 AM EDT Tue Mar 23 2021 Valid 12Z Fri Mar 26 2021 - 12Z Tue Mar 30 2021 ...Another round of heavy rain possible across parts of the Deep South on Saturday... ...Overview... An unsettled and active weather pattern is forecast to affect much of the central to eastern U.S. later this week into this weekend as a couple of shortwaves move through. Most notable may be another round of moderate to heavy rainfall for parts of the Deep South. Meanwhile, much of the West should trend warmer/drier as upper ridging builds into the weekend before the next system enters the Pacific Northwest and brings another round of rain and mountain snows to the region. ...Model Guidance/Predictability Assessment... The synoptic pattern is fairly well clustered over the CONUS through much of the period, aside from the 18z/Mar 22 GFS which is a big outlier with multiple systems. The better consensus brings troughing from the West into the Central U.S. late this week, amplifying as it moves steadily east. The latest run of the GFS splits this energy sending a southern stream closed low into the Southwest and then again develops a deep closed low over the northern West Coast by next Tuesday with the next system into the Pacific Northwest. The 12z/Mar 22 GFS run seemed closer in line with the remaining models with the first system, so it was used in the WPC blend in place of the 18z run. However, the 12z run was also much faster with the next shortwave in the Pacific Northwest, so it was excluded altogether after day 6. The ECMWF seemed the most consistent with the ensemble means through the period, so WPCs blend leaned mostly in that direction, with support from the UKMET through day 5. Normal timing/detail differences arise after day 5 so the ensemble means were given the most weight towards the end of the period. ...Weather/Threats Highlights... A cold front exiting the East on Friday (day 3) should drive much of the associated precip off the coast before the medium range period begins. Exception will to the north and west of the surface low where a period of light snow from parts of the Great Lakes to interior New England is possible Friday into Saturday. 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 on Saturday. The next system sends another cold front into the East with widespread light to moderate precipitation likely to impact the Eastern states on Sunday. The trend should be towards warmer and drier weather out West this weekend, ahead of the next system into the Pacific Northwest which should bring another round of rain and mountain snow to the region early next week. The eastern third of the country will start warm and above normal through the weekend, with the greatest anomalies (+15 to +20F) across the Mid-Atlantic and Northeast on Friday. Morning lows will be even more so anomalously warm, with several record high minimum temperatures possible late this week stretching from Florida to the Northeast. Troughing entering the region early next week should bring temperatures back down towards normal next Monday and Tuesday. Out West, temperatures should trend warmer after this week as upper level ridging builds across the region. 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