Extended Forecast Discussion NWS Weather Prediction Center College Park MD 155 AM 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 Wednesday 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. ...Guidance/Predictability Assessment... Despite some lingering timing differences, the 12Z/18Z models generally agreed on the upper low progression Wed/Thu over Mexico and a deterministic model consensus was used as a starting point. By Thursday, the various streams within the continental-wide trough axis (between 100-110W) differed quickly, and the UKMET/Canadian became quicker overall with the pattern. Preferred to rely on the steadier consensus of the 18Z GFS and 12Z ECMWF with their ensemble means which steadily took the southern system northeastward out through the Southeast (with trailing height falls on its heels) and the northern stream through the Great Lakes (uncertainty in speed there as well). Ensembles were still split between very slow southern stream solutions and very quick weaker solutions. Middle ground position seemed prudent. By next weekend, ensembles still show a large amount of spread in how quickly height falls will progress through the West and interior West. Degree of spread has led to a canceling of quicker/slower waves and a rather unlikely flat flow out of the Pacific into broad CONUS troughing. With some agreement among the GFS/ECMWF/Canadian in wave tracking, opted to use about 1/3 to 1/2 weighting of the GFS/ECMWF with their means to give some amplitude to the forecast. Confidence is low in timing given the quick progression out of the central Pacific by then. ...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 on Thursday. With the system weakening on Friday, generally light to modest rainfall is expected over areas farther east into the Carolinas. 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. 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