Extended Forecast Discussion NWS Weather Prediction Center College Park MD 158 AM 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... Guidance continues to show that a narrow but very amplified ridge aloft moving into the eastern Pacific/northwestern North America, and eventually reaching the West Coast next week, will encourage a deepening mean trough to develop over the central U.S. by next Tue. Meanwhile a deep upper low initially off the West Coast will likely open up by the weekend with the surrounding trough moving into the West. Some of the eastern Pacific trough energy will eventually interact with the amplifying Canadian troughing (which will push a cold front into the northern tier during the weekend) while the rest should lag behind across the southwestern states into northern Mexico. From the big-picture perspective there was better than average agreement among most 12Z/18Z models and means through the period. The one questionable solution was the 12Z CMC which brought the eastern Pacific trough into the lower 48 at a much faster pace than consensus (still a bit fast in the new 00Z run but better than the 12Z version). Thus the updated forecast emphasized the 18Z GFS and 12Z ECMWF/UKMET for about the first half of the period and then trended toward about even weight among the GFS/ECMWF and their ensemble means. Recent trends have favored slower progression of eastern Pacific trough energy that moves into the southwestern U.S./northern Mexico, with latest GEFS means/UKMET runs plus a slower adjustment in the 00Z GFS adding to that theme. Then later in the period there has been a general slower trend with the wavy front that reaches the eastern U.S.--which is hard to dispute considering the amplifying evolution aloft. 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. Upstream expect some energy to approach the eastern Pacific/West Coast ridge aloft toward Tue. Majority cluster of the GEFS/ECMWF means and GFS runs along with the new 00Z CMC suggest that the 12Z ECMWF may be too quick to bring the energy into the ridge. ...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. Best probability for highest totals extends from near the Tennessee Valley into the southern-central Appalachians/upper Ohio Valley but confidence in specifics is not yet very high. Snow potential should be confined to northern areas including 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 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