Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 PM EST Wed Jan 20 2021 Valid 12Z Sat Jan 23 2021 - 12Z Wed Jan 27 2021 ***Additional heavy rainfall appears likely for portions of the south-central U.S. by Sunday and Monday as unsettled weather overspreads much of the western U.S.*** ...Weather Pattern Overview... Active shortwave amplifications along the West Coast will continue to be the highlight through the medium range period. The next around of unsettled weather will likely expand through the West Coast on Sunday before overspreading across the Desert Southwest and then the central/southern Rockies early next week. Meanwhile, upper-level energies ejecting eastward into the Plains will likely interact with Gulf moisture to result in a heavy rain event across portions of the south-central states. Cold air and upper-level broad troughing sliding across the northern tier states could support a swath of wintry precipitation across the Midwest to the northern Mid-Atlantic early next week although model uncertainty remains high. Another deepening trough could reach the West Coast by the middle of next week with noticeable model uncertainty. ...Guidance/Predictability Assessment... Overall, deterministic model guidance was in above average agreement on the evolution of the synoptic scale pattern across the nation through about Monday with good ensemble mean support. Thereafter, solutions begin to diverge regarding the degree of interaction between the broad upper troughing across the northern tier states and a low pressure system moving from the Deep South toward the eastern U.S. The GFS offers a flatter and more progressive solution while the ECMWF allows a greater degree of interaction between the two streams and thus a greater chance of cyclogenesis as the system is forecast to move across the Mid-Atlantic on Tuesday. However, run-to-run variability remains noticeable with this system, with the ensemble means favoring a southern track. Toward the middle of next week, another Pacific system could reach the West Coast but with noticeable model spreads. The GFS and GEFS show robust development of a closed low dipping down from the Gulf of Alaska to a position off the West Coast by next Wednesday. The ECMWF and EC mean were less certain about this scenario. The CMC was somewhere in between. This morning's WPC medium-range package was based on a consensus of the 00Z ECMWF/EC mean, 06Z GFS/GEFS, as well as a smaller contribution from the 00Z CMC and the 00Z NAEFS. More of the ensemble means were used to handle the uncertainties on Days 6 and 7. ...Weather/Threats Highlights... Another round of heavy rain will likely make weather headlines again across portions of the south-central U.S. by late Sunday and into Monday as the southwestern storm system emerges over the southern Plains. This will support a surface low along a quasi-stationary frontal boundary, with copious Gulf moisture being advected northward across interior portions of the Deep South and Tennessee River Valley. The potential exists for 1 to 3 inches of rainfall with locally higher amounts, mainly from Arkansas to the southern Appalachians, and this may be enough to produce some episodes of flooding if it happens over areas affected by the antecedent rainfall forecast for later this week. As this storm system reaches the East Coast late Monday and into Tuesday, there should be enough cold air in place to support either snow or a wintry mix for portions of the northern Mid-Atlantic and Northeast. Moderate to heavy snow is likely for the central and southern Rockies, as well as the Sierra Nevada, as the upper level trough/low pass through the region. In terms of temperatures, the greatest anomalies are forecast to be from Texas to the Deep South and Gulf Coast region, where the humid southerly flow keeps readings about 10-20 degrees above normal for highs, and up to 25 degrees above normal for overnight lows. This will particularly be the case from Sunday into Tuesday in the warm sector of the low pressure system. The most prolonged period of below normal temperature readings are forecast to be over the western U.S., with highs up to 15 degrees below normal, and also across portions of New England, with readings about 10 degrees below normal. Kong/Hamrick 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