Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 AM EST Sat Jan 04 2020 Valid 12Z Tue Jan 07 2020 - 12Z Sat Jan 11 2020 ...Overview/Guidance Assessment... Today's guidance still shows a transition from a progressive pattern to a longer term amplified western/west-central U.S. mean trough aloft. A Pacific upper ridge between 140-160W and another ridge off the southeastern U.S. will help to define this trough. Climate Prediction Center 8-14 day forecasts indicate this pattern should persist well past the end of the medium range time frame. Expect various regions of the lower 48 to see active weather before and while the amplified pattern takes shape, but with enough model and ensemble spread to temper confidence in specifics of each potential significant event. Based on guidance through the 12Z/18Z cycles, there had been healthy convergence regarding the developing storm system that may affect the Northeast around midweek, with guidance trends generally leaning in the direction of an ECMWF/ECMWF mean multi-run average. Consensus blend yielded a surface low position near the southern tip of Maine as of 12Z Wed. Recent GEFS means have tended to be on the slow/weak side of the guidance spread. However new 00Z runs highlight the low predictability given the two streams of energy involved. While the GFS/CMC (with the GEFS mean adjusting closer to consensus) are not far from continuity, a couple operational models including the UKMET are almost a complete miss for New England due to a much wider track for the wave that tracks over the Mid-Atlantic on Tue. The ECMWF is adjusting a little eastward from its previous run though not to the extent of the UKMET. Moderate detail spread exists for the Pacific shortwave that reaches the West Tue-Tue night. Into day 5 Thu most detail/timing differences are generally within the typical error range for the forecast time frame, favoring a blend approach, though the new 00Z CMC is bit questionable with how it separates southern energy for a time. Later in the period GFS runs become questionably progressive with ejecting western energy--to the point of bringing height falls into a strong positive height anomaly center which D+8 multi-day means show near the East Coast. Greater west/northwest deflection of this energy appears more reasonable as reflected by the GEFS mean along with the ECMWF/CMC and their respective means. Farther upstream guidance is starting to show more of a signal for a fairly strong bundle of energy to travel around the top of the Pacific ridge around Thu and then drop southeastward toward the large scale mean trough. Some GFS/ECMWF runs had hinted at this in recent days though generally not in the same cycle, but now the GFS/ECMWF/CMC all have a robust shortwave with reasonable support from the ensemble means. Latest GFS runs hold the energy a little westward of most other solutions late in the period--a little extreme to include in the deterministic manual forecast but not to the point of being outside the realm of possibility. There is still considerable spread for surface low track with individual solutions ranging between western Canada and the Pacific Northwest. Overall an ensemble mean average provides a reasonable intermediate track. Forecast preferences led to primary emphasis on an operational model blend for the first half of the period. Question marks in the GFS after Thu favored trending the forecast to the 12Z ECMWF/ECMWF mean/CMC and 18Z GEFS mean through day 7 Sat. ...Weather Highlights/Threats... Guidance is consistent in advertising an active pattern for the Northwest during the period with coastal areas from the northwest corner of California through Oregon/Washington as well as the Cascades seeing highest five-day rainfall/mountain snow totals. The best surges of moisture are likely to occur early in the period and then again late. Depending on surface low specifics, some areas may see a period of strong winds as well. The northern Rockies will also see significant precipitation but with somewhat lower totals than areas to the west. By Sat some activity should reach into the Sierra Nevada. Other areas in the West may see one or more periods of lighter and more scattered precipitation. The spectrum of possible outcomes for the midweek Northeast storm is unusually wide for a forecast only four days out, ranging from a minimal event to a significant snowstorm with strong winds in its wake. Important details aloft are subtle enough to make it difficult to determine a "most confident" solution at this time. At least from the probabilistic/ensemble perspective there has been some consistency in the idea that best potential for meaningful snow will extend from the Ohio Valley/Appalachians through the extreme northern Mid-Atlantic into the Northeast, with highest snowfall totals over Maine. As the amplified upper pattern takes shape, locations within and near the lower half of the Mississippi Valley as well as Tennessee Valley and Lower Ohio Valley should see increasing coverage and intensity of rainfall late this week as a wavy cold front approaches from the west and a warm front lifts northward from the Gulf Coast. Low level flow from the Gulf may help to enhance rainfall totals in the warm sector with possible embedded convection. The primary question mark is how much eastward progress the cold front will make by Fri-Sat, with the slower timing in the non-GFS consensus leading to higher rainfall totals. This event will have to be monitored given the recent heavy rainfall over/near this region. Some wintry weather is possible over northern latitudes but most likely with fairly light amounts. By Thu-Sat expect a pronounced contrast in temperature anomalies over the country, with fairly broad coverage of highs 5-15F below normal over northern portions of the Plains and through a majority of the West while readings 10-25F above normal (highest for morning lows) prevail over a majority of the eastern half of the lower 48. 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