Extended Forecast Discussion NWS Weather Prediction Center College Park MD 131 PM EST Sun Dec 15 2019 Valid 12Z Wed Dec 18 2019 - 12Z Sun Dec 22 2019 ...Overview and Guidance/Predictability Assessment... Stormy period for the Pacific Northwest into Northern California with successive systems moving inland and a possible atmospheric river event type set-up. The East will be quieter for the end of the week but a weak system in the Central Plains may become more developed off the Southeast coast next weekend. Models and ensembles are starting to show better agreement with the incoming system into the West on Wednesday and how it travels across the CONUS through at least day 5. After that, big questions remain on interactions between northern/southern stream energies. Recent runs, like the ECMWF, tend to suggest limited seperation of the energy, while some like the GFS keep the southern stream seperate with rather significant low development off the Southeast coast next weekend. Run to run continuity in most of the deterministic runs is quite poor by day 6-7, though ensemble members do suggest at least the potential for low development somewhere off the East Coast. 00z low track plots show East Coast lows anywhere from Florida to New England at 12z next Sunday. Given the spread, WPC preferred a mostly ensemble mean blend, with some contribution from the ECMWF and GFS for a bit more definition. This resulted in a rather large shift northward with the surface low to off the Mid-Atlantic coast (which is roughly in the middle of the spread) on this cycle of the WPC progs, though it's important to note that the day 7 forecast is of very low confidence at this point. ...Weather Highlights/Threats... Precipitation will be centered on the West, particularly coastal OR and northern California. Upper trough in the eastern Pacific will allow a fetch of moisture from near Hawai'i to reach the OR/CA coast with a north/south oscillation between fronts as the trough changes shape upstream. Several inches of precipitation are expected but over a 2-3 day period with a likely break in between. Daytime temperatures will likely be below average with the rainfall but otherwise near average elsewhere and near to above average for overnight lows. East of the Rockies, temperatures may be more variable with a quick cold shot skirting through the Northeast on Thursday (10s/20s for highs and -0s/0s/10s for lows). East Coast low development may bring a period of snowfall to someone from the Mid-Atlantic to the Northeast which of course is highly variable on actual low track/existence. At this point, WPC winter weather probabilities show nothing more than a 10% chance of accumulations along the I-95 corridor. 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