Extended Forecast Discussion NWS Weather Prediction Center College Park MD 156 AM EST Thu Jan 21 2021 Valid 12Z Sun Jan 24 2021 - 12Z Thu Jan 28 2021 ***Additional heavy rainfall appears likely for portions of the south-central U.S. for the beginning of the week, and heavy snow returns to the higher terrain of the West*** ...Weather Pattern Overview... An active weather pattern is expected to be in place across the West Coast region and also the southeastern U.S. for the early to middle part of next week. The upper level trough that will be in place over the Desert Southwest Sunday morning is progged to track eastward and support another round of unsettled weather across the interior Southeast states. Additional shortwave energy tracking southward along the West Coast will fuel additional rounds of heavy rain and mountain snow for much of California by Tuesday and beyond. ...Guidance/Predictability Assessment... 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. 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