Extended Forecast Discussion NWS Weather Prediction Center College Park MD 158 AM EST Tue Jan 19 2021 Valid 12Z Fri Jan 22 2021 - 12Z Tue Jan 26 2021 ***Additional heavy rainfall becoming more likely for portions of the south-central U.S. by Sunday and Monday*** ...Overview... A quasi-zonal upper level flow pattern is expected to be in place across the central and eastern U.S. through much of the medium range forecast period, with a broad ridge axis extending across the Gulf Coast region. After the departure of the strong southern stream trough from the southwestern U.S. during the short range period, a rather similar second trough and upper level low drops southward along the West Coast this weekend, and likely followed by a third shortwave disturbance by early next week. Shortwave energy ejecting eastward from this disturbance, in combination with a nearly stationary frontal boundary, is expected to result in another round of active weather across portions of the south-central U.S. ...Guidance/Predictability Assessment... The deterministic model guidance is in above average agreement on the evolution of the synoptic scale pattern through Sunday, and this includes the development of the next major upper level trough over the West Coast region, so a blend of the GFS/ECMWF/CMC along with some of the ensemble means sufficed as a starting point in the forecast process. By Monday, greater differences emerge with shortwave energy pivoting around the southern Canada upper low, with the 00Z GFS indicating a greater degree of northern stream amplification. By Monday night and Tuesday, there are some detail differences with the next trough to take up residence across the West, with the 00Z CMC and ECMWF showing a slower progression. The final two days of the forecast period incorporated a little bit more of the ensemble means whilst still using some of the GFS and ECMWF. ...Weather/Threats Highlights... The storm system that will be gathering strength across the Desert Southwest and southern Rockies is expected to reach the southern Plains by Sunday, and the departing surface high over the Midwest will lead to return flow from the western Gulf of Mexico and a steady increase in low level moisture. This is expected to result in more rain for the region, mainly from the ArkLaTex region to the southern Appalachians. The axis of heaviest rainfall remains uncertain for this second system on Sunday and Monday, but the potential is there for 1 to 3 inches of rain during that time period and locally higher, and could cause flooding concerns if these higher amounts fall over those areas impacted by the earlier rainfall event late this week. The upper trough amplifying over the western U.S. will advect enough Pacific moisture and produce enough upslope flow to result in significant snow for the Central Rockies and Wasatch mountains. Some of this wintry precipitation will likely spread northeastward across the northern Plains and Upper Midwest by next weekend, but the details remain uncertain with respect to the development of surface low pressure and frontal interactions. In terms of temperatures, the greatest anomalies are forecast to be across the central and western Gulf Coast region, where the southerly moist flow keeps readings about 10-20 degrees above normal for late January. The most prolonged period of below normal temperature readings are forecast to be over the western U.S., with highs around 5-10 degrees below normal, and also across the Upper Midwest on Friday 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