Extended Forecast Discussion NWS Weather Prediction Center College Park MD 143 PM EST Sun Dec 29 2019 Valid 12Z Wed Jan 01 2020 - 12Z Sun Jan 05 2020 ...Heavy rainfall potential over the Lower Mississippi Valley to the Southern Appalachians mid to late week... ...Overview... Upper pattern will transition to broad troughing over the Lower 48 as a deep upper low initially over Baja California Wed lifts toward the Southeast later this week. This will lead to potentially heavy rainfall for the Lower Mississippi Valley northeastward. An active storm track out of the northeast Pacific will sustain several frontal passages into the Pacific Northwest, causing rounds of rain. ...Guidance/Predictability Assessment... Model guidance has come into general agreement regarding the placement of the upper low over Baja California Wed. Northern stream energy from the northern Pacific should drop southward Wed/Thu, creating a trough with its axis located from the Northern Plains to Four Corners region. The original southern stream energy will wrap up into this trough around Thu night/Fri, but this transition leads to some model spread in the position/eastward extent of the trough, as well as issues with the surface low placement and strength given the 500 mb vorticity differences. The 00Z UKMET was the farthest forward, with the GFS not far behind and the 00Z ECMWF and CMC the farthest west. Leaned slightly more toward the slower solutions by excluding the UKMET by that point and weighting the 00Z EC/CMC a bit heavier than the GFS runs. By the weekend, broad troughing seems to set in across the CONUS, with embedded shortwaves in the flow coming across the Pacific and into the U.S. Confidence is low with the details in this flow, especially given the limited predictability coming from the Pacific. GEFS ensemble members generally lie south of ECMWF and CMC members in terms of 500 mb heights across the western half of the U.S. Sat. A surface low will likely spin up ahead of one shortwave somewhere in the north-central tier by Sat, but placement and strength are still uncertain. Relied on the GEFS and ECENS means to smooth out differences in details of the flow, but expect the amplitude of shortwave features to end up greater. ...Weather Highlights/Threats... As the upper low/trough lifts out of Mexico, moisture from the Gulf will get drawn across the warm front along the coast which will lead to an expanding area of rainfall and some embedded convection. Heaviest rainfall may occur from Louisiana across Mississippi into Alabama, especially Thu. The system is expected to weaken Fri, but some rainfall is expected over areas farther northeast over portions of the Tennessee Valley to Southern Appalachians. Nearly daily bouts of precipitation are likely for the Northwest into the Northern Rockies during the first several days of January, focused over coastal areas and the Cascades. Temperatures will generally be near to above average east of the Rockies and near to below average in the West. Tate/Fracasso Additional 3-7 Day Hazards information can be found on the WPC medium range hazards 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