Extended Forecast Discussion NWS Weather Prediction Center College Park MD 152 AM EST Thu Dec 19 2019 Valid 12Z Sun Dec 22 2019 - 12Z Thu Dec 26 2019 ...Heavy rain over parts of the Southeast exiting Monday... ...Overview... Upper troughing off the West Coast will be reinforced from the northwest and slowly move onshore next week, maintaining an unsettled pattern. Concurrently, a potent southern stream system will track across Florida late this weekend and then push slowly out to sea. Locally heavy rain will be possible around its circulation between northern Florida and coastal North Carolina. ...Guidance/Predictability Assessment... Across the West, the general troughing pattern seems well-resolved in the longwave pattern, but there remained timing differences through about Tuesday and then both feature and timing differences thereafter. A consensus position was favored, somewhat near the 12Z ECMWF/Canadian until next Wed/Thu when they may become too deep/slow (near the slowest 10% of ensembles) but not unreasonable given the pattern. The GFS/GEFS may be too quick overall by later in the period which can be their bias. In the Southeast, models were still struggling with the forward speed and track of the closed low and surface low but forecast convergence continues. The GFS continues to be on the quicker side while the 12Z UKMET lagged the more centered 12Z ECMWF/Canadian, which were close to the ensemble consensus and was the preferred solution. Did not rule out the quicker GFS given the quasi-progressive nature of the pattern--western trough would not allow too much of a slower solution though it could trend slower in tandem. Multi-day ensemble trend has mostly stabilized so will continue to favor the best-clustered models. ...Weather Highlights/Threats... Western troughing will support fairly widespread but light to modest precipitation next week, spreading inland as height lower to the east. There remains considerable uncertainty as to how much precipitation moves into the Plains around Christmas Day into the 26th, owing to how much Gulf moisture may be tapped. For now, favored the slower/drier ECMWF/Canadian ensembles vs the quicker/wetter GEFS. Temperatures will trend cooler in the West behind the front next week. The Gulf system will provide at least modest rainfall for much of the Southeast with locally heavy amounts continuing out of the short range Saturday. Several additional inches of rain is quite possible Sun-Mon with sufficient moisture in the lower atmosphere and good inflow off the Atlantic. Some light snow is possible on its northern fringe in the marginally colder air of the southern Appalachians, but the event is expected to be nearly all rain. Warmer than average temperatures are forecast to spread from the Northern Rockies and Plains eastward across much of the rest of the U.S., with high temperatures up to 20 degrees above average. Dry conditions should persist across much of the Mississippi Valley to Ohio Valley and Mid-Atlantic/Northeast. Fracasso Additional 3-7 Day Hazards information can be found on the WPC medium range hazards 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