Extended Forecast Discussion NWS Weather Prediction Center College Park MD 121 PM EST Tue Dec 03 2019 Valid 12Z Fri Dec 06 2019 - 12Z Tue Dec 10 2019 ...Moisture spreading into the West late week-weekend with heaviest rain/snow over northern California/Sierra Nevada... ...Precipitation to increase in coverage and intensity over the East early next week... ...Pattern Overview and Guidance/Predictability Assessment... Through the medium range period /Dec 6-10/ the synoptic pattern will evolve from relative zonal flow across the CONUS to a highly amplified pattern featuring a narrow ridge along the West Coast with deepening longwave troughing over much of the central U.S. and Great Lakes region. At the surface, a West Coast low will drop through southern California and eventually move across the southern/central Plains before lifting through the Great Lakes and Northeast U.S. by the end of the forecast period. Cold Canadian high pressure settles in behind that system. For this update, model guidance confidence/agreement remained at or above average for the large scale pattern. The 00Z CMC continued to be a fast outlier with the progression of the shortwave energy across the western/central U.S. while the ECMWF continued to show a slowing trend (from its 02.12Z to 03.00Z run). However, the 12Z GFS trended a bit faster with the overall progression of the system, so by Day 6/7 confidence drops to average for the time period. The model blend preference was largely an equal blend of the deterministic ECMWF/GFS/UKMET for Day 4-5 followed by increasing weight/percentage of the deterministic ECMWF/GFS and their respective ensemble means toward Day 7. Ongoing uncertainty over the timing of incoming Pacific energy and typically low predictability for important smaller scale details within the overall late-period CONUS upper trough continue to temper confidence in frontal specifics mid-late period. ...Weather Highlights/Threats... The eastern Pacific system moving into the West will spread a broad shield of moisture across the region from late this week through the weekend. Guidance continues to highlight areas from the extreme southwest corner of Oregon through northern California and the Sierra Nevada for highest rainfall/high elevation snow totals with multi-inch liquid totals likely over favored terrain. The Pacific Northwest and locations farther inland through the Rockies will also see a period of rain/snow but with less extreme amounts. Much of the West should trend drier by the first part of next week. Meanwhile expect increasing coverage and intensity of precipitation over the eastern half of the country from about Sun night onward as a wavy cold front approaches. Guidance currently suggests enough low level Gulf inflow to produce at least some areas of locally heavy rainfall. The highest probabilities for heavy rainfall appear to be across the Tennessee Valley, southern/central Appalachians, and portions of the Ohio Valley. Temperatures look to be cold enough to support some snowfall to northern areas from the Great lakes and northern/interior New England. Finally, late this week a weakening wave near the Gulf Coast will produce some light-moderate rain in its vicinity as it weakens and descends into the Gulf while a vigorous shortwave aloft and more modest surface low crossing the Great Lakes/Northeast will produce areas of mostly light snow. The system crossing the Northeast late this week will bring chilly temperatures (highs 5-15F below normal) to the East Coast for Sat. Farther westward, an area of above normal temperatures will make its way across the country from west to east corresponding to upper trough progression/amplification into the lower 48, followed by a cooling trend. One exception to the warmth over the West early in the period will be modestly below normal highs over the Great Basin/Southwest. Greatest anomalies over the central/eastern states from the weekend into early next week should be for morning lows (plus 10-20F and locally higher) while warmest highs should be 10-15F above normal. Coldest temperatures versus normal by next Mon-Tue should settle into/near the Upper Midwest and vicinity with some readings 10-20F below normal. Rausch/Taylor 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