Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 AM EST Sat Nov 30 2019 Valid 12Z Tue Dec 03 2019 - 12Z Sat Dec 07 2019 ...Significant winter storm to affect the Northeast into Tuesday... ...Low pressure brings midweek heavy rain to southern California... ...Pattern Overview and Guidance/Predictability Assessment... Latest guidance is showing better than average agreement for the overall mean pattern during Tue-Sat as well as individual features in principle, though with some ongoing detail/timing uncertainties. From a multi-day mean perspective the flow aloft will have a tendency toward troughing just off the West Coast while a mean ridge prevails over the west-central U.S. and a mean trough resides over the East. Features within the eastern mean trough will include the strong system affecting New England into Tue before departing, an upstream shortwave reaching the East by Wed and supporting Atlantic low pressure that should track far enough offshore to have limited influence on the East Coast, and then an initial northeastern Pacific shortwave forecast to reach southwest Canada by Wed and the Northeast by Sat. Farther west an upper trough/closed low will move into the Southwest around midweek, weakening as it passes through the longer term mean ridge and further shearing downstream as it becomes increasingly embedded in progressive flow. An upstream trough will amplify into the eastern Pacific mid-late week with its axis likely approaching the West Coast by Sat. At the start of the period the system just off the New England coast exhibits good clustering in general but with some small-scale differences that would have some influence on New England but have fairly low predictability three days out in time. The upper trough reaching the East on Wed still exhibits some detail differences with corresponding spread for developing low pressure over the western Atlantic and into the Canadian Maritimes. The common theme among guidance so far is a low track sufficiently offshore to keep the system's moisture shield away from the East Coast states. Consensus shows the next shortwave in the series bringing a frontal wave into southwest Canada on Wed with increasing divergence thereafter regarding low track and strength/shape farther east. Thus far the ensembles and CMC runs have been much weaker with the surface wave than latest/recent GFS/ECMWF runs, while the new 00Z UKMET has trended somewhat deeper/northward versus its prior run. The current array of guidance argues for a defined wave but with somewhat less vigor than the GFS/ECMWF until there is better agreement. Over the past day there has been little change in the guidance distribution for the system heading into the Southwest around midweek and continuing eastward thereafter, with operational GFS runs still faster than other solutions (including the GEFS mean) to varying degrees. Early in the period the 18Z GFS compared least favorably with other solutions relative to other recent runs as it was farthest northeast with the upper low off California. An average among non-GFS guidance still appears to be the most likely scenario but the progressive nature of mean flow could still leave the door open to nudging the dominant cluster a tad faster at some point. There is still a fair degree of uncertainty over exact evolution of the surface reflection which should become increasingly weak/suppressed to the east of the Plains. Within the larger-scale upper trough amplifying off the West Coast mid-late week, confidence is steadily increasing that an upper low will close off for a while per recent operational model runs. Among the means the 12Z ECMWF mean finally closed off a low and now the 00Z GEFS mean has done likewise. Potential influence from energy to the north becomes an uncertainty by day 7 Sat with some solutions indicating the trough could open up by then. Clustering and trends favor primarily an operational model emphasis into days 5-6 Thu-Fri followed by a more even weight of models/means for day 7 Sat as detail confidence decreases. Based on guidance available through the 18Z cycle, the initial blend started with the 12Z GFS/ECMWF/CMC early-mid period and began incorporating a very modest 20 percent total input of the 18Z GEFS/12Z ECMWF means on day 5 Thu. Ensemble weight increased to 30-50 percent by days 6-7 respectively and GFS input switched from the 12Z run to 18Z run as the 12Z run strayed to the fast side with height falls reaching the West late in the period. ...Weather Highlights/Threats... Effects from the early week storm off the Northeast coast will continue over New England into Tue with Maine seeing the best potential for meaningful snow. Brisk winds making conditions more hazardous will likely taper off after Tue. Shortwave energy immediately upstream may promote a period of lake effect snow, most likely to the lee of Lake Erie/Lake Ontario in the Wed-Wed night time frame. The system heading into the Southwest around midweek should produce a brief period of enhanced precipitation with highest totals over favored terrain inland from the southern California coast and then a lesser maximum over central Arizona. High elevation snow is possible from the Sierra Nevada across the ranges of the Great Basin to the central Rockies. Expect this system to produce an area of mostly rain over and east of the central-southern Plains into the Southeast late in the week. Some pockets of locally moderate-heavy rain are possible but with low confidence in timing/location. Late in the period guidance is maintaining the signal for potentially heavy precipitation (rain and higher elevation snow) along parts of the West Coast with strongest focus over northern California and the Sierra Nevada. So far the detail differences exhibited by the models/ensemble means seem to suggest more uncertainty with the precise onset of the event (a little slower in the operational models versus ensemble means) rather than the event itself. Chilly temperatures will persist over parts of the East into Tue-Wed. Expect the greatest anomalies over the Southeast/Florida where some locations may be 10-15F below normal for max and/or min readings. Morning lows could drop to near freezing as far south as far northern Florida. Eastern U.S. temperatures should be closer to normal late in the week but another frontal passage may push highs back down to 5-10F below normal over the Mid-Atlantic/Northeast next Sat. Expect the central U.S. to see above normal temperatures on most days, more so for morning lows. Warmest such anomalies should be over the northern Plains Tue-Wed ahead of a cold front and across the southern tier Wed-Fri with clouds/precip accompanying the system crossing the region. Highs about 5-10F below normal should persist over the Great Basin into parts of Arizona/southern California while most of the West should see near to above normal morning lows. Rausch 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