Extended Forecast Discussion NWS Weather Prediction Center College Park MD 139 AM EST Fri Nov 29 2019 Valid 12Z Mon Dec 02 2019 - 12Z Fri Dec 06 2019 ...Significant winter storm to affect the Northeast early in the week... ...One or more precipitation events likely over the West Coast states... ...Pattern Overview and Guidance/Predictability Assessment... A strong upper low forecast to be over the Mid-Atlantic as of early Mon will progress to the east and then northeast Mon-Tue. Expect an associated low pressure system to track just offshore the Northeast and then into the Canadian Maritimes, with meaningful winter weather effects likely to the north/west of the low track. Behind this system guidance depicts a tendency for cyclonic mean flow over the eastern U.S. with the best defined shortwave likely to pass over the East around midweek. Meanwhile most solutions suggest that an upper low and surrounding positively tilted trough energy initially off the West Coast will consolidate during the first half of the week. This feature should weaken as it moves inland over the Southwest/southern half of the Rockies Wed-Thu with continued progression of the shortwave thereafter. An upstream trough will likely amplify over the eastern Pacific during the latter half of the week. This feature should eventually spread moisture into a greater portion of the West Coast region than the leading one whose precipitation will focus more over southern areas. Clustering among the guidance is decent for the storm affecting the Northeast early in the week. Latest UKMET runs have been a bit slower than the primary consensus including the GFS/ECMWF/CMC and the GEFS/ECMWF means. The upstream shortwave reaching the East by midweek will bring a modest northern Plains front southeastward into East followed by offshore development whose track should be far enough east to have limited effect on the East Coast. There is good agreement in principle on the shortwave but some details have low predictability for being a few days out in time. The system heading into the Southwest Wed into Thu shows decent agreement in the guidance considering that there are at least a couple separate pieces of energy involved. Typical timing differences develop as the weakening feature crosses the southern tier during the latter half of the week with the GFS runs showing their faster tendency versus the ECMWF/CMC. The GEFS/ECMWF means and historical verification suggest the GFS may be too fast but the mean pattern looks sufficiently progressive for an intermediate timing to be plausible. Through day 7 Fri the primary uncertainty with the amplifying eastern Pacific trough is if/where an embedded closed low may form. Thus far the operational models are generally suggesting higher potential for a closed low than the ensemble means, while GFS runs have been tracking the closed low north/northeast of the 12Z ECMWF and latest CMC runs. Over multiple runs the GFS has been somewhat more consistent than the ECMWF with the general evolution. Based on guidance available through the 18Z cycle a compromise among the 12Z ECMWF/CMC, 18Z GFS, and 18Z GEFS/12Z ECMWF means would provide a good starting point to represent somewhat more flow separation than the means without committing to lower predictability closed low details for the time being. ...Weather Highlights/Threats... The storm system tracking just offshore the Northeast early in the week will bring a period of significant wintry precipitation to areas from the extreme northern Mid-Atlantic into New England. Winds may be sufficiently strong to make travel even more difficult/hazardous. Parts of Great Lakes should see one or more periods of snow but with mostly light accumulations. Over the West, expect weekend precipitation to linger over California into Mon followed by a brief break. Then the system moving into the Southwest will bring an episode of focused precipitation with highest totals most likely over favored terrain inland from the southern California coast and a lesser maximum over central Arizona. Higher elevation snow will be possible from the Sierra Nevada into the central Rockies. This system may produce an area of mostly rain over and east of the central-southern Plains late in the week. The upper trough amplifying over the eastern Pacific late in the week should spread moisture into the West Coast states at that time. Timing and exact focus of heaviest precipitation will be sensitive to lower predictability details within the trough. Currently the best potential for highest totals exists over northern California. The eastern states will be chilly to start the week with some parts of the South seeing highs 10-15F below normal on Mon. Eastern U.S. temperatures will gravitate toward normal by the latter half of the week. On the other hand expect the central U.S. to see above normal temperatures for most of the period with some pockets of plus 10F or greater anomalies for min and/or max readings from Tue onward. The expected pattern will support moderately below normal highs over the Great Basin and at times into southern California while most of the West should see morning lows closer to or above normal. 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