Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 AM EST Sun Dec 01 2019 Valid 12Z Wed Dec 04 2019 - 12Z Sun Dec 08 2019 ...Southern tier system to affect the Southwest midweek/Mississippi Valley and Southeast late week... ...More moisture to spread into the West late week/next weekend... ...Pattern Overview and Guidance/Predictability Assessment... The large scale pattern that has tended to feature mean troughs over the eastern Pacific and eastern U.S. with a mean ridge over the west-central U.S. will begin to progress eastward by next weekend as the upper trough amplifying over the eastern Pacific during the latter half of the week moves into the lower 48--albeit with some timing uncertainty. Latest D+8 multi-day means have been showing a prominent negative height anomaly center over the North Pacific and/or Aleutians, with teleconnections supporting some eastward progression of the positively tilted trough beyond the end of the medium range period to an axis from near the Great Lakes to the southern Rockies (with a modest hint of stream separation). At the same time a ridge should begin to build into western North America with best positive height anomalies over western Canada. There should be two dominant weather producers this period: a leading southern tier system that should be just offshore southern California as of early Wed and then become more suppressed with time from the southern Plains into Southeast Thu-Fri as the energy aloft becomes incorporated into progressive flow, and the amplifying eastern Pacific trough which by next weekend should progress into the western and possibly parts of the central U.S. Meanwhile a couple northern stream shortwaves may achieve a negative tilt by the time they reach the Northeast (one Wed-Thu and another around Sat). Most precipitation associated with these shortwaves will likely be on the lighter side of the spectrum though locally enhanced activity could be possible. For the southern tier system a recent slower trend in the GFS has aided guidance clustering in the 12Z/18Z cycle. The new 00Z UKMET has strayed slower than consensus but remaining solutions maintain good continuity. The most significant difference for the larger-scale eastern Pacific into CONUS trough is timing, with the GFS and ECMWF exhibiting their fast/slow tendencies respectively. The 12Z CMC was close to the GFS and adjusted a little slower in the new 00Z run. By next weekend the GEFS/ECMWF means ultimately support an intermediate timing for the upper trough, with a positive tilt that appears to lead well into the teleconnection-favored pattern expected to prevail soon after next Sun. Therefore the updated forecast trended 50-60 percent to the ensemble means late in the period (with modest input of offsetting operational runs) after reflecting the 12Z/18Z operational model consensus early-mid period. Potential for stream interaction within the overall trough contributes to ongoing uncertainty mid-late period. The preferred blend helped to downplay lower confidence detail uncertainties with northern stream shortwaves affecting the northern tier/Northeast. Consensus continues to show that low pressure with the first shortwave will track well off the Northeast coast while there has been somewhat of a weaker trend with the surface wave associated with the trailing shortwave. ...Weather Highlights/Threats... The system tracking across the southern tier will first bring a brief period of locally heavy rain (perhaps some snow at highest elevations) over favored southern California terrain around midweek with a lesser relative maximum likely over central Arizona. Also expect higher elevation snow from the Sierra Nevada into the central Rockies. Farther east guidance is starting to show a somewhat more coherent signal for locally moderate (possibly briefly heavy) rainfall over and just east of the lower half of the Mississippi Valley late in the week before a cold front approaching from the north suppresses the system. With continued uncertainty in the exact onset, expect moisture ahead of the amplified Pacific upper trough to reach the West Coast states late in the week and push eastward with time during the weekend. Thus far guidance has been consistent in highlighting northern California and Sierra Nevada for highest precipitation totals. Lower but still meaningful activity is possible over the Pacific Northwest. Lake effect snow aided by the first shortwave crossing the Northeast early in the period will tend to favor locations to the lee of Lake Erie/Lake Ontario with lighter activity to the northwest. The next shortwave aloft and surface wave/frontal system may produce some relatively light amounts across the upper/eastern Great Lakes and New England. The greatest temperature anomalies (up to 10-20F) during the period will be of the warm variety and for morning lows in particular--in association with the mid-late week southern tier system, over the northern tier Wed-Thu ahead of a cold front drifting south from Canada, and from the West into the Plains late week/next weekend ahead of the large scale upper trough moving from the Pacific into the West. Meanwhile the Great Basin into the Southwest should continue to see highs around 5-10F below normal while the East Coast states will see temperatures vary with frontal progression but on average tend to be somewhat below 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