Extended Forecast Discussion NWS Weather Prediction Center College Park MD 155 AM EST Sun Jan 17 2021 Valid 12Z Wed Jan 20 2021 - 12Z Sun Jan 24 2021 ***Heavy rain for portions of the southern U.S., and a return to colder weather for much of the nation by next weekend*** ...Weather Pattern Overview... A split upper level flow pattern will be in place through much of this forecast period, with a well-defined and detached upper low just south of southern California for the middle of the week, followed by another amplifying trough along the West Coast by next weekend. A negative NAO (high pressure block over Greenland) and a negative EPO (ridge over Alaska) should be in place, and this will indirectly lead to quasi-zonal and broad cyclonic flow over the north central U.S. The second western trough will begin to herald a pattern change for much of the western U.S. with colder weather becoming more likely. ...Guidance/Predictability Assessment... The 12Z deterministic guidance, along with the 18Z GFS, are in very good agreement on the overall pattern on Wednesday regarding the anomalous upper level low near the Desert Southwest and the broad upper trough over the Northeast States. This also holds true with the latest 00Z guidance. By Thursday and beyond, the 12Z UKMET becomes quicker than the consensus in ejecting the upper low and its absorption into the southern stream flow, and thus not favored for this forecast cycle. The 12Z CMC has come into better agreement with this system, although still on the slower side of the guidance by Thursday night. By the Friday to Sunday time period, while there is good consensus on a second trough/closed low dropping southward along the West Coast, its timing and depth remain in question, with the CMC on the slower side. Taking these factors into account, the WPC fronts and pressures forecast was primarily based on a 12Z ECMWF/CMC/18Z GFS blend through Thursday, and then less of the CMC and more of the means whilst still incorporating the GFS and ECMWF for the second half of the forecast period. ...Weather/Threats Highlights... The main weather story next week will be the potential for a multi-day heavy rainfall event, extending from eastern Texas to the Mid-South, mainly during the late Wednesday to Friday morning time period. A prolonged period of moist southerly flow from the Gulf of Mexico ahead of a slow moving low pressure system over southern Texas, in combination with moisture convergence along a nearly stationary front, may result in 2 to 4 inches of rainfall for parts of this region and some potential for flooding where convection develops repeatedly over the same areas. Moderate rain can also be expected for southern Arizona and southwestern New Mexico. In terms of winter weather potential, moderate to heavy snow is likely for portions of the central Rockies and the higher elevations of the Great Basin by Friday and into Saturday as the next storm system arrives. Some lake effect snow can also be expected downwind of Lake Erie and Lake Ontario for the end of the week. In terms of temperatures, readings are expected to be well above normal across the northern Plains on Wednesday with highs 15 to potentially 25 degrees above normal. This will likely transition to slightly below average readings by Friday across this region, and then below normal temperatures expand in coverage across the western U.S. for next week as the upper level trough heralds a change in the weather pattern. 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