Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 AM EST Sat Dec 14 2019 Valid 12Z Tue Dec 17 2019 - 12Z Sat Dec 21 2019 ...Overview and Guidance/Predictability Assessment... Amplified but progressive pattern will maintain a rather stormy period for the West Coast. In the east, a lead system will bring some snow to the Northeast followed by a quieter period until perhaps next weekend. The models/ensembles diverge in timing/track right from Tue/D3 in the east and on Wed/D4 in the west, which carry through the period. For the lead system, the 12Z ECMWF (though slower) and UKMET/Canadian formed a more consistent cluster than the quicker 12Z/18Z GFS (though the 00Z GFS was closer to the ECMWF/UKMET-led consensus). Off the West Coast, most 12Z models/ensembles trended much quicker to bring in a weak closed low to California which left the once seemingly too quick older GFS runs even slower than the current models. 18Z GFS/GEFS could be considered too slow vs the 12Z ECMWF/UKMET/Canadian but have been pretty consistent. Nevertheless, with decreased ensemble spread vs previous runs, trended quicker with that system Wed. Thereafter, the 12Z ECMWF became much quicker with a northern track into the Great Lakes while the GFS/Canadian were farther south. Ensembles were split as well with no clear consensus. 12Z Canadian formed a middle ground solution but confidence was low in whether it may manifest itself more robustly in the northern or southern stream. Ensembles do suggest low pressure trying to organize off the Southeast coast late next Sat. Back to the west, yet another trough will slowly lurch toward the coast. ...Weather Highlights/Threats... Lead system on Tuesday will bring a swath of snow to the Northeast and parts of the Mid-Atlantic as low pressure exits off the coast. Colder air (highs 5-10 deg below average) will filter into the area midweek before a moderation next Fri/Sat. In the West, precipitation will favor far northern California northward for most of the period, but with some light rain/snow into coastal NorCal and the Sierra as the upper trough moves through. By next Fri/Sat, southwesterly upper flow will provide an atmospheric river connection back to the subtropics near Hawai'i for far northern CA into southwestern OR. Several inches of rain are possible per the latest model guidance. 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