Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1100 AM EST Sat Dec 15 2018 Valid 12Z Tue Dec 18 2018 - 12Z Sat Dec 22 2018 ...Significant precipitation continues over the Northwest... ...East Coast rainstorm becoming more likely late next week... ...Overview... Guidance agrees in principle that flow aloft will rapidly evolve toward a sharp and amplified western ridge/east-central trough pattern by Thu, as incoming Pacific shortwave energy (contributing to the heavy Northwest precipitation) digs southeastward after rounding the strengthening western ridge. By the late week/weekend time frame, global models are getting into better agreement that a significant cyclone will likely develop along the East Coast. ...Guidance Evaluation/Preferences... Latest models and ensembles have come into much better agreement on the overall evolution of the synoptic pattern across the U.S. through the end of next week. The most notable feature on the map is regarding the amplitude of an upper-level trough as it progresses eastward across the Plains through the middle of next week and heads toward the East Coast late next week. The ECMWF has led the trend toward a more amplified pattern since yesterday. On the other hand, the more progressive GFS solutions yesterday have gradually trended much closer to those of the ECMWF. The 00Z Canadian model has also shown a faster solution toward the ECMWF solution as well. The convergence of global models toward a higher amplitude upper-level flow gives us higher confidence that a significant cyclone will impact many areas along the East Coast late next week. It appears that rain will be the primary precipitation type under strong southerly flow ahead of the system, with only some winter precipitation near the Canadian border of new England. A consensus approach was adapted for Days 3 and 4 pressure fields using the 00Z ECMWF/EC mean and 06Z GFS/GEFS. For Day 5 onward, a combination of 00Z ECMWF/EC mean and 06Z GEFS were used. ...Weather Highlights/Hazards... A frontal system approaching/reaching the Northwest on Tue will provide one episode of enhanced rainfall and higher elevation snow to the region with activity likely extending into extreme northern California. Another front, anchored by a wave whose strength and track are not yet well-resolved in the guidance, should bring another episode of meaningful precipitation to the Northwest late in the week. Some of the moisture from these events will extend into the northern Rockies. Another area of moisture could reach the Northwest next weekend but with lower confidence. Parts of the Northwest may see periods of strong winds. The upper trough initially from the Plains into Mexico along with leading weak surface wave may produce areas of rainfall from the southern Plains eastward mid-late week, while an approaching northern stream frontal system may also provide a focus for at least light-moderate precipitation. Interaction of these features may lead to strengthening low pressure near the East Coast by Fri-Sat, spreading a sizable moisture shield across the eastern states. Currently the best signal for highest rainfall totals exist over parts of the Florida Peninsula and along the North Carolina coast. Depending on details of system evolution there may relative maxima over other parts of the East. Expect most wintry weather to be confined fairly far to the north and west. Temperatures will be above normal over a majority of the Lower 48 during the period. The most extreme readings versus normal should be over the northern Plains where anomalies may exceed plus 20F for one or more days, especially for min temps which could approach or exceed daily records at some locations. One exception to the warmth will be New England which will be on the chilly side Tue behind a departing upper low. Kong/Rausch 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