Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1256 AM EST Mon Dec 30 2019 Valid 12Z Thu Jan 02 2020 - 12Z Mon Jan 06 2020 ...Heavy rainfall potential over the Lower Mississippi Valley to the Southern Appalachians Thu-Fri... ...Overview and Guidance/Predictability Assessment... Broad troughing will settle over the central and eastern states by next week after a reversal of the pattern at the beginning of the period this Thursday. The models and ensembles were in good agreement to start the forecast but drifted apart by the weekend and especially by next Monday. The 12Z ECMWF and Canadian offered reasonable clustering with the 18Z GEFS mean and 12Z ECMWF ensemble mean to use as a starting point, with added details from the 12Z UKMET and 12Z/18Z GFS to start. Areas of contention were first with the weakening upper low (likely just a sharp trough by 12Z Thu) over Mexico, lifting to the northeast. With three separate streams crossing 100W, the models have wavered on how each interacts (or doesn't) with each other aloft, and consequently at the surface. The 12Z/18Z GFSs were likely too quick with the Mexican trough but perhaps too slow with the trailing shortwave out of the Rockies, and it was not preferred past Thursday. Lead and trailing surface systems will attempt to merge together by the time they reach New England around late next Fri into Sat. Next area of contention lied in the west, with a large amount of uncertainty in the upstream central North Pacific flow. This has led to large timing differences in how quickly or slowly to lower heights in the West starting Sat and then progress eastward and southward next Sun/Mon. Again there, the ECMWF/Canadian paired closer to the ensemble means as the GFSs were nearly completely out of phase. By next Mon, no deterministic model really fit the lead-in pattern suggested by the means, though the degree of spread couldn't rule out any model either. Favored the ensemble means in whole at the expense of any detail. ...Weather Highlights/Threats... As the weakening upper low or sharp trough lifts out of Mexico, moisture from the Gulf will get drawn across the surface warm front along the coast which will lead to an expanding area of rainfall and some embedded convection. This may be accentuated via additional weak surface wave development along the front. 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 period, focused over coastal areas and the Coastal Range/Cascades in addition to the Idaho Panhandle/northwestern Montana. Temperatures will generally be near to above average east of the Rockies through the weekend and near to below average in the West. By next week, more of the country may see below average temperatures than above as troughing becomes established. 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