Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1100 AM EST Mon Jan 07 2019 Valid 12Z Thu Jan 10 2019 - 12Z Mon Jan 14 2019 ...Overview... The active pattern across the Northeast Pacific will support three significant upper-level troughs forecast to reach the West Coast during the medium-range period. Meanwhile, greater uncertainties are indicated for this weekend along the East Coast as global models show greater discrepancies on the degree of development on a potential coastal storm or nor'easter. ...Guidance Evaluation/Preferences... Global models this morning showed very good agreement on the evolution of the synoptic pattern across the CONUS through Day 5 Saturday. The timing and the amplitude of the second upper-level trough reaching the West Coast remained agreeable this weekend, while a low pressure system is forecast to form over eastern Texas. This was when the models began to diverge on the eventual evolution of the low as it progresses eastward into the East Coast this weekend. The last two runs of the GFS took a more decisive turn toward more aggressive cyclogenesis near the Mid-Atlantic coast Sunday into Monday. On the other hand, the 00Z ECMWF remained faster/progressive and much less aggressive on developing this system near the East Coast. The 06Z GEFS showed only modest developments, which was quite agreeable with the 00Z EC mean. Therefore, the WPC sea-level pressure field was derived from a consensus of the 00Z ECMWF/EC mean with the 06Z GFS/GEFS for Days 6 and 7, which yielded good continuity from the previous WPC forecasts. More of the deterministic components were used for Days 3-5. ...Weather Highlights/Hazards... Expect the potential for multiple rounds of heavy precipitation (low elevation rain and mountain snows) across the West Coast during the forecast period as a rapid succession of Pacific systems impacts the region. As shortwave energy reaches the southern Plains/lower Mississippi Valley by Fri-Sat, areas of heavy rain will be possible, with precipitation spreading east into the Southeast/southern Mid-Atlantic by Sat night/Sun. Cold air to the north of the system could result in areas if wintry precipitation. The degree of winter weather potential from the southern Appalachians to the Mid-Atlantic Sat-Sun is highly uncertain for the reasons described above. Above average temperatures will persist through the forecast period across the north central U.S., with max temp anomalies in the +10 to +15 deg F range (even warmer for some areas) through much of the period. The coldest temperatures will be across the Northeast, where temperatures as cold as 10 to 20 deg below average will be possible from Fri through the weekend into early next week. Kong/Ryan WPC medium range 500 mb heights, surface systems, weather grids, quantitative precipitation, winter weather outlook probabilities and heat indexes are found 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