Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 AM EST Mon Dec 02 2019 Valid 12Z Thu Dec 05 2019 - 12Z Mon Dec 09 2019 ...Southern tier system to affect the Lower Mississippi Valley/Southeast late week... ...More moisture to spread into the West late this week/weekend and the East by next Monday... ...Pattern Overview and Guidance/Predictability Assessment... The best clustering of guidance has been stable over recent runs in showing an eastward progression of the large scale pattern during the period. This evolution will be highlighted by a late week eastern Pacific trough (with embedded closed low) moving inland by next weekend. As this occurs expect cyclonic flow over Canada to begin interacting with at least a portion of the trough, by next Mon ultimately leading to a mean trough extending from Hudson Bay through the central U.S. and into northwest Mexico. At that time an upstream ridge axis should reach near the West Coast. D+8 multi-day means continue to highlight a core of negative height anomalies over the North Pacific with the current day 7 consensus forecast providing a plausible lead-in to the teleconnection-favored pattern. For the strong eastern Pacific trough moving into the lower 48, guidance has been showing steady improvement in clustering--by way of the GFS trending slower with parts of the trough and recent ECMWF runs (with some oscillation) faster than some cycles from a couple days or so ago. An average of the GEFS/ECMWF means, with the latest GEFS actually a little slower than the ECMWF mean during the weekend, has provided the most stable forecast thus far. Among data from the 12Z/18Z cycles the 12Z GFS appeared a bit fast with leading height falls and the UKMET strayed a little fast as well. New 00Z runs of the operational models agree fairly well through day 5 Sat. By Sun-Mon guidance agrees on some northern stream interaction with a combination of the Pacific and Canadian dynamics supporting a wavy front reaching into the Plains/Mississippi Valley/Great Lakes. There is fairly good confidence in the front's existence but wave details and frontal timing will take a while to resolve due to dependence on lower-predictability shortwave details. Amplification of Canadian flow should reflect at the surface as a separate front that drops into the northern Plains during the weekend. Elsewhere, there are still modest timing differences for the weakening southern tier shortwave tracking east from the Rockies. The UKMET has been the laggard recently with most other 12Z/18Z solutions close together. The 00Z CMC has joined the UKMET over the southern Plains/Mississippi Valley though. In the northern stream one vigorous shortwave aloft will lift away from the Northeast after Thu and an upstream amplifying shortwave will cross the Great Lakes/Northeast Fri-Sat. Faster CMC/slower GFS trends in the new 00Z cycle for the surface wave associated with the second feature bring those solutions toward the 12Z ECMWF/ECMWF mean. The new 00Z ECMWF is holding serve with its timing. Strong southward push of the trailing cold front should ultimately suppress the weakening southern tier system into the eastern Gulf of Mexico by Sat. Based on guidance comparisons/continuity the updated forecast started with the 18Z GFS/12Z ECMWF and lesser weight of the 12Z CMC for about the first half of the period. Then 18Z GEFS/12Z ECMWF mean weight gradually increased, reaching 50 percent total by day 7 Mon. Comparison of various 12Z/18Z GFS details to consensus become more mixed by the latter half of the forecast so GFS input was split between both runs at that time. ...Weather Highlights/Threats... Expect relatively light southern Rockies precipitation to taper off Thu with the responsible southern tier system bringing a brief period of locally moderate/isolated heavy rainfall to the Lower Mississippi Valley and parts of the Southeast Thu night-Fri before being suppressed to the south. Moisture ahead of the eastern Pacific system should reach the West Coast late this week and then spread inland over the West through the weekend. Consistent signals from the guidance point to the greatest rainfall/higher elevation snowfall totals focusing over favored terrain in the Sierra Nevada and across northern California, possibly extending into the extreme southwest corner of Oregon. The Pacific Northwest as well as windward terrain across the northern two-thirds of the Interior West/Rockies will see less extreme but still meaningful totals. The upper trough and leading wavy cold front should reach far enough eastward for precipitation (mostly rain) to expand over the eastern half of the country by next Mon. Some activity may be locally heavy if the orientation and timing of the upper trough allow for sufficient input of low level Gulf moisture. Earlier in the period expect one episode of lake effect snow to taper off to the lee of the eastern Great Lakes on Thu followed by a modest event over the northern/eastern Great Lakes and New England approximately Thu night-Fri night. Locations near the East Coast should see near to moderately below normal temperatures from late week into the weekend. New England may see readings as much as 10-15F below normal Sat-early Sun. Clouds/precipitation associated with the upper trough progressing inland from the Pacific will spread above normal morning lows across the West into the weekend while maintaining moderately below normal highs over the Great Basin/Southwest. Cool highs will expand into the Rockies by the start of next week. Meanwhile expect a pronounced warming trend over the eastern half of the country Sun-Mon, with a broad area of plus 10-20F anomalies for morning lows and pockets of plus 10F or so anomalies for highs. 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