Extended Forecast Discussion NWS Weather Prediction Center College Park MD 156 PM EST Fri Dec 27 2019 Valid 12Z Mon Dec 30 2019 - 12Z Fri Jan 03 2020 ...Significant winter storm expected to continue into Mon/Tue across the Great Lakes to Northeast... ...Heavy rainfall potential from the western-central Gulf Coast region northeastward mid-late next week... ...Overview... An upper low should shift slowly northeastward from the Great Lakes into southeastern Canada midweek, spreading precipitation and potentially heavy snow across the Great Lakes and Northeast. Meanwhile, another upper low is forecast to track southward across the West Coast into northern Mexico through day 5/Wed, combining with a northern stream trough around day 6/Thu. Ahead of the latter, heavy rainfall is possible from the western Gulf Coast northeastward. ...Guidance/Predictability Assessment... Overall the large-scale pattern for the mass fields is in good agreement in the models and ensembles. The upper low wrapping up in the central U.S. during the short range period should place a surface low in the Great Lakes region day 3/Mon, while a new surface low develops in the far western Atlantic at the triple point of the frontal system Mon-Tue. After yesterday's GFS runs were a bit faster/farther north with these surface lows, operational models now better agree on the general position and strength of the lows, so a multi-model blend of the 06Z GFS and 00Z ECMWF, UKMET, and CMC was used for these features. Model guidance also offers better agreement compared to previous runs for the the upper low moving across the West early in the week. While the 00Z operational ECMWF and its mean are still a bit farther south and west of other guidance Mon-Wed, differences are pretty minimal. By Thu, models have the upper low starting to combine with the northern stream to create a positively-tilted trough across the central U.S. Yesterday's EC runs were faster than other guidance by Thu-Fri, as they had the energy splitting, with one vort max quickly tracking east and then north through the trough (which created major QPF differences). Today's EC runs are now more in line with other guidance and its mean by hanging the trough back west. So the main model outlier that remains appears to be the 00Z CMC by Thu-Fri. Strong energy (as shown by 500 mb vorticity) streams into the trough from the northwest in the CMC, which wraps up an upper low as well as a strong surface low across the central U.S. Thus, used a 00Z EC/EC mean and 06Z GFS/GEFS mean blend for Thu-Fri. ...Weather Highlights/Threats... The low in the Great Lakes and the subsequent surface low near the Northeast coast will produce heavy precipitation across those regions on Mon-Tue. Currently, the precipitation looks to begin as rain and transition to snow in the Great Lakes region and interior New England, with mixed precipitation also possible in the transition. Locally heavy rainfall is possible farther south on Monday across New York City and southern New England. Ahead of the western upper low and central U.S. trough, heavy rainfall is forecast to move west to east with the upper-level support and as moisture flows in from the Gulf of Mexico. Currently the heaviest rain looks to be along the west-central Gulf Coast beginning Tue northeastward through the Lower Mississippi and Tennessee Valleys into the Southeast. The exact timing and placement of the heavy rainfall amounts cannot be determined yet--the heaviest precipitation will likely occur along the front, so any differences in the frontal position will cause shifts in the precipitation forecast. Regardless, the potential is there for heavy rainfall mid-late next week. Waves of moisture and a few pulses of upper-level energy will lead to persistent precipitation across the Northwest, with higher elevation (coastal ranges, Cascades, and Northern Rockies) heavy snow and lower elevation rain. Ahead of the Great Lakes low in the warm sector, much above average temperatures are likely across the Ohio Valley to the Great Lakes region and Mid-Atlantic on Mon, with temperatures still average or a bit above after a cold frontal passage. Warmer than average temperatures are also expected in the central to eastern U.S. Wed-Fri with some ridging ahead of the central U.S. trough. The Southwest to Four Corners will be the main cool spot through the period due to the anomalously low heights. 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