Extended Forecast Discussion NWS Weather Prediction Center College Park MD 157 AM 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, and unsettled weather is expected across the western U.S.*** ...Weather Pattern Overview... A quasi-zonal upper level flow pattern is expected to be in place across the central and eastern U.S. this weekend and into early next week, with a broad ridge axis extending across the Gulf Coast region and the Southeast states. The western U.S. is expected to have a rather active pattern in place with an amplified southern stream trough in place through Sunday, before ejecting eastward across the Plains and creating a favorable environment for another heavy rainfall event for portions of the south-central states. This will likely be followed by another deep trough developing over the West Coast region by Monday and into Tuesday. ...Guidance/Predictability Assessment... Overall, deterministic model guidance is in above average agreement on the evolution of the synoptic scale pattern across the nation through about Monday, and have good ensemble mean support. Greater model uncertainty develops across the northeast Pacific by Tuesday, owing to differences in the Alaska/North Pacific ridge and thus timing differences in the arrival of the next closed low. The 00Z ECMWF differs the most in that it indicates more of a ridge axis off the West Coast, and it is slower to eject the upper level trough that is expected to be over the West Coast on Monday. The first half of the forecast period incorporated a mainly CMC/GFS/UKMET/ECMWF blend, and then less of the ECMWF and slightly more of the GFS, whilst increasing the ensemble means. ...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 rainfall 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. 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