Extended Forecast Discussion NWS Weather Prediction Center College Park MD 343 PM 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... Upper troughing will traverse the central to eastern U.S. Thu-Sun, ahead of which moderate to locally heavy rainfall is possible in the south-central to southeastern U.S. Then flow from the North Pacific creates a messy forecast for the beginning of next week, but with only minor sensible weather concerns. ...Guidance/Predictability Assessment... Model agreement for the beginning of the medium range period (Thu) is fairly good, indicating a trough axis through the High Plains and a surface low near Oklahoma. By Fri, the 00Z and 06Z GFS runs were a bit farther east with the 500 mb trough than the 00Z ECMWF, UKMET, and CMC, but the surface low positions were not too different so did not completely discount the GFS. The trough continues eastward Sat, but spread starts to increase in both the East and the West. The 06Z GFS and 00Z ECMWF were deeper with the eastern trough than the 00Z UKMET and CMC, but at least they all have a similar pattern. Using a multi-model blend seemed to suffice. In the West, upper ridging in the Pacific Thu/Fri may shift eastward into the Southwest by Sat/Sun. Recent model runs have trended this direction, after previously indicating broad troughing across the CONUS. But confidence is low in the strength of the ridging, so used the 06Z GEFS mean and the operational GFS/ECMWF for more amplified ridging blended with the flatter 00Z EC mean to temper the ridging. In the northern stream, large operational model differences arise over the northern Pacific/Gulf of Alaska Sat which propagate into the northwestern U.S. Thus there is low confidence in this aspect of the forecast for the beginning of next week. The most substantial change from the previous forecast was to back up the surface low moving across southern Canada Sat-Mon compared to the previous forecast, given the current trend for a slower shortwave. Used mostly the 06Z GEFS mean and 00Z EC mean to create the forecast by next Mon at the expense of much detail. ...Weather Highlights/Threats... As the 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 should occur from Louisiana into Mississippi and Alabama on Thu, getting into the Southern Appalachians by Fri. 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, with lesser amounts in the Idaho Panhandle/northwestern Montana. Temperatures will generally be near to above average east of the Rockies through the weekend, with the Four Corners region seeing near to below average temperatures. Tate 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 Hazards: - Heavy precipitation across portions of the Pacific Northwest, Thu-Sun, Jan 2-Jan 5. - Heavy precipitation across portions of central Idaho into western Montana, Sun, Jan 5. - Heavy rain from across the lower Mississippi Valley into the Tennessee Valley, the southern Appalachians, into the interior Southeast as well as the interior Mid-Atlantic, Thu-Fri, Jan 2-Jan 3. - Flooding possible across portions of the Mid- and upper Mississippi Valley, the Great Lakes, and into the Ohio Valley. - Flooding occurring or imminent across portions of the Mid-Mississippi Valley and the northern Plains. - Flooding likely across portions of the central Plains and the Mid-Mississippi Valley. 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