Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 PM EST Thu Jan 21 2021 Valid 12Z Sun Jan 24 2021 - 12Z Thu Jan 28 2021 ...Additional heavy rainfall likely for portions of the south-central U.S. for the beginning of the week; heavy mountain snow and unsettled weather moving into the western U.S.... ...Weather Pattern Overview and Guidance/Predictability Assessment... Active amplification of upper-level troughs digging down from the Gulf of Alaska will be a continued source of unsettled weather for the western U.S. through the medium range period. Meanwhile, downstream ejections of upper-level energy into the southern Plains will periodically interact with moisture returning from the Gulf of Mexico to result in heavy rainfall in the south-central U.S. A heavy rain event should be culminating across the Tennessee Valley as the medium range period begins Sunday night into Monday. Model guidance this morning has reached a better agreement on the degree of interaction between the warm and moisture air mass from the Gulf of Mexico and broad troughing sliding across the northern tier states. The GFS is now showing more northern stream cyclogenesis in better agreement with the ECMWF while the ECMWF shows slightly less robust cyclogenesis. Consensus of the 00Z EC mean and the 06Z GEFS indicate a northward shift in the heavy rainfall axis across the Tennessee Valley. Meanwhile, a swath of wintry precipitation appears likely from the Midwest to the Mid-northern Atlantic and into southern New England Monday into Tuesday morning as the system moves across and then off the Mid-Atlantic. By the middle of next week, model consensus indicates another enhanced precipitation event will develop along the Gulf Coast and then head northeastward into the East Coast. Over the Southwest, models are in good agreement on the timing of another Pacific system moving through the area Sunday through Tuesday. Overall, deterministic model guidance is in good agreement on the evolution of the synoptic scale pattern across the nation through about Monday with good ensemble mean support. By Tuesday, the 00Z GFS remains less amplified compared to the 00Z CMC and ECMWF with the shortwave crossing the east-central U.S., and this was also the case with the earlier 12Z model guidance suite. More noteworthy model differences are apparent across the western U.S. for the second half of the forecast period as additional shortwave energy tracks southward from the Gulf of Alaska region. By Tuesday, the GFS is stronger and faster with the low off the Pacific Northwest coast. Other detail differences in the evolution of that trough/storm system make the ensemble means a more prudent choice as a starting point in the forecast process for the Wednesday through Thursday time period. ...Weather/Threats Highlights... Another round of heavy rain will likely make weather headlines across portions of the south-central U.S. by late Sunday and into early Tuesday as the southwestern storm system emerges over the Gulf Coast region. 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 southern Arkansas to the southern Appalachians, and this may be enough to produce some episodes of flooding if high rainfall rates persist for any given area. 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. Heavy snow is becoming more likely for the central and southern Rockies, the Wasatch Range, and the Sierra Nevada as multiple upper level troughs pass through the region. Heavy rain can also be expected for the lower elevations of central and northern California as copious Pacific moisture is advected towards the coast. 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 will make it feel more like spring, with highs about 10-20 degrees above normal, 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. Most of the central U.S. should have temperatures close to average for late January. 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