Extended Forecast Discussion NWS Weather Prediction Center College Park MD 141 AM EST Fri Jan 15 2021 Valid 12Z Mon Jan 18 2021 - 12Z Fri Jan 22 2021 ***Heavy rainfall event likely for portions of the south-central U.S. next week*** ...Weather Pattern Overview... The departure of the strong storm system from the Northeast U.S. on Sunday will be followed by another shortwave disturbance crossing the Eastern U.S. for the beginning of the week, although considerably weaker. Meanwhile, an impulse sinking southward along the West Coast on Monday is expected to become a detached upper low over the southwestern U.S. in the southern stream. There will likely be some shortwave energy that ejects eastward from this low across the southern tier states, whilst the northern stream flow is more progressive. Another trough will likely set in across the Pacific Northwest by the end of the forecast period on Friday. ...Guidance/Predictability Assessment... The deterministic guidance is generally in good agreement on the split flow pattern across the continental U.S. through early next week, and are strongly signaling a cut-off low developing in the general vicinity of the Desert Southwest by Tuesday. The 00Z CMC is displaced farther west, whilst the 12Z ECMWF is more progressive in ejecting the energy to the east. The 00Z UKMET trended slower with the southern stream disturbance compared to its 12Z run, and the latest run now has more ensemble support. By the end of the forecast period Friday, the ECMWF is more robust with the next trough crossing the Great Lakes region and results in a stronger storm system compared to the other guidance. The WPC fronts and pressures forecast was primarily derived from a CMC/UKMET/GFS/ECMWF blend through Tuesday, and then transitioned to half ensemble means and half CMC/GFS/ECMWF for Wednesday through Friday. ...Weather/Threats Highlights... The main weather story next week will be the potential for a multi-day heavy rainfall event, extending from central Texas to the southern Appalachians, mainly during the Tuesday-Thursday time period. A prolonged period of moist southerly flow from the Gulf and Caribbean ahead of a slow moving low pressure system over southern Texas, in combination with moisture convergence along a quasi-stationary front, may result in several inches of rainfall for parts of this region and some potential for flooding. Moderate to heavy snow is likely for portions of the central and northern Rockies early in the week, and the chance for snow returns to the Northeast and Great Lakes region by Thursday and Friday as the next storm system affects this region. In terms of temperatures, readings should be near to slightly above normal for most areas to begin the week, and some 10-20 degree positive anomalies are possible across the northern Plains by Wednesday ahead of the next front. By the end of the week, there are some hints of an Arctic airmass affecting the northwestern quadrant of the U.S., although the core of the Arctic high should remain north of the Canadian border. 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