Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 PM EST Sat Dec 28 2019 Valid 12Z Tue Dec 31 2019 - 12Z Sat Jan 04 2020 ...Significant winter storm expected to continue Tue across northern New England... ...Heavy rainfall potential over the Lower Mississippi Valley to the Southern Appalachians mid to late week... ...Overview... An upper low will move from the Great Lakes region along the U.S./Canadian border on Tue into Wed, causing snow to continue across northern New England. Another upper low is expected to track across Baja California Tue-Wed and combine with northern stream energy to create a central U.S. trough by Fri. Rainfall, possibly heavy, is forecast ahead of these features. Shortwaves and a series of surface fronts should lead to persistent precipitation in the Northwest. ...Guidance/Predictability Assessment... Model guidance generally remains pretty agreeable in terms of the large scale pattern early in the forecast, with the two aforementioned upper lows. A multi-model blend of the 00Z and 06Z GFS runs and the 00Z ECMWF, UKMET, and CMC was used through midweek, as there were no significant model differences in the East at that point. For the upper low moving through Baja Califonia, the 00Z EC had a bit slower, more southwest track compared to the GFS. Took a compromise of those solutions for its placement, which seemed to work well, and the new 12Z GFS is in between the two which is a good sign. There is uncertainty with what time frame that low in the southern stream combines with the northern stream, after troughing in the northern stream gets created by energy dropping southward out of the eastern Pacific Wed-Thu. The 06Z and 12Z GFS runs keep the streams separate into Thu, while the other 00Z models have have one combined trough in the central U.S. at that point. Leaned away from the GFS but did not discount it completely, as having separate streams is not out of the question. These differences could affect the timing of the frontal movement at the surface, in turn affecting the placement of the heaviest rain in Texas to the Southeast. Broad troughing appears to set up over the CONUS by Sat, but no significant height anomalies are expected by the end of the period. ...Weather Highlights/Threats... As a surface low develops off the Northeast coast and the parent upper low persists over the Great Lakes, snow will continue from the short range period into Tue across northern New England, with high probabilities for additional plowable snowfall amounts especially over Maine. Precipitation is also likely for the Northwest over the medium range period as persistent onshore flow brings in some periods of higher moisture, while shortwaves act as lift support and a couple of fronts move through. Lower elevation rain and mountain snow are expected. As the upper low/trough lifts northeast from Mexico, moisture from the Gulf will get pulled northward, and rain should spread from the western Gulf Coast into the Lower Mississippi Valley by Thu. The rain is currently forecast to move into the Tennessee Valley and Southeast by Fri/Sat, but the placement of the heaviest rainfall is not certain at this time. Warmer than average temperatures are expected to spread across most of the CONUS by midweek, with the Four Corners region being the main cool spot. 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 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