Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 PM EST Tue Jan 19 2021 Valid 12Z Fri Jan 22 2021 - 12Z Tue Jan 26 2021 ...Additional heavy rainfall appears likely for portions of the south-central U.S. by Sunday and Monday as unsettled weather overspreads the West Coast... ...Overview... The recent shift in the synoptic pattern toward trough amplifications along the West Coast appears to continue through the medium range period as model consensus forecasts multiple vigorous shortwaves digging across the region. This pattern will also encourage additional heavy rain to develop across the Mid-south as these shortwaves exit into the southern Plains. ...Guidance/Predictability Assessment... Overall, model guidance this morning was in good to very good agreement on the evolution of the synoptic scale pattern across the U.S. through the medium range period. The 00Z ECMWF and 06Z GFS solutions appeared most compatible with one another through this weekend. By early next week, both models indicate the likelihood of a low pressure system developing in the central to southern Plains as broad troughing dips into the northern tier states. Both models show significant run-to-run variability regarding the speed and intensity of this system as it moves toward the East Coast by about next Tuesday. The 00Z EC mean and the 06Z GEFS mean show only modest development of this system with a relatively flat upper trough. Therefore, the days 6 and 7 grids were based mainly on a blend of these ensemble means with some 06Z GFS and the least on the 00Z ECMWF. The 00Z CMC was not as compatible with the 00Z EC/06Z GFS blend and thus was not taken into account in the forecast package. ...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 with 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 addition, unsettled weather appears likely for the Desert Southwest by this weekend and into early next week. Wintry precipitation appears possible for the higher elevations of Arizona while rain appears likely for the southern half of the state into southern California. 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. 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