Extended Forecast Discussion NWS Weather Prediction Center College Park MD 105 PM EST Sat Dec 14 2019 Valid 12Z Tue Dec 17 2019 - 12Z Sat Dec 21 2019 ...Overview and Guidance/Predictability Assessment... An amplified but progressive pattern will maintain a rather stormy period for the West Coast while in the east, a leading system will bring some snow to the Northeast followed by a quieter period until perhaps next weekend. For the leading system in the East, the latest model runs show good agreement on surface low placement as it quickly lifts from the northern Mid-Atlantic Tuesday to off the Canadian Maritime coast by Wednesday. In the West, timing differences with an emerging weak closed low/trough into California starts as early as Wed/day 4. Even though models seem to be trending faster, there is still enough run-to-run variability on speed to not latch onto one particular solution at this point. Amongst the deterministics, the slow 00z ECMWF appears to be an outlier, with the CMC, UKMET (through day 5), and GFS all faster especially as the shortwave lifts into the Central Plains and Great Lakes late next week. However, there is enough ensemble member support for the slower ECMWF so its hard to rule it out completely. Given the uncertainty, opted for a weighting towards the ensemble means which are just a hair faster than the 00z ECMWF. This approach also fits well with previous WPC continuity. By late Friday into Saturday, the troughing should shift into the East with ensembles suggesting another area of low pressure trying to organize off the Southeast coast late next Saturday. 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 early Wednesday. 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. Santorelli/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