Extended Forecast Discussion NWS Weather Prediction Center College Park MD 112 PM EST Sat Jan 04 2020 Valid 12Z Tue Jan 07 2020 - 12Z Sat Jan 11 2020 ...Heavy rain potential next Fri/Sat from the Lower Mississippi Valley to the Ohio Valley... ...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 (500mb heigh anomalies around +2 sigma) 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 00Z/06Z cycles, GFS/ECMWF and their ensemble means clustered a bit quicker with the surface low near the Gulf of Maine by Wednesday morning (Canadian a bit slower but the UKMET well to the southeast). Sensitivity to the northern/southern stream intereaction may lead to continued guidance differences as the northern stream source was still over the Gulf of Alaska. Favored the GFS/ECMWF cluster which was close to continuity and favored more rather than less interaction. 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 (both speed and amplitude), favoring a blend approach. Did not incorporate the 00Z Canadian as it was a bit questionable with how it separates southern energy for a time, though its latest 12Z run appeared more in line. 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 Canadian and the GEFS/ECMWF ensemble means. Despite its earlier questionable evolution, the 00Z Canadian was preferred for its timing of the upper trough (perhaps closed low) by next Saturday but with below average confidence. This will also affect the position/latitude of the frontal boundary in the east. A slower ejection out of Texas would favor a farther southward progression of the front into or through the Northeast. 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 support has increased for a robust shortwave with reasonable support from the ensemble means. GFS runs were a little suspect (too far west) given the likely too quick progression in the east by then. 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 and ECMWF after Fri favored trending the forecast to the 06Z GEFS mean/00Z ECMWF ensemble mean/00Z Canadian into 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 but continued to favor a more defined system for now. 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 generally 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 as depicted in the WPC Winter Weather Outlook probabilities (~10% chance of low-end snowfall late in the period). 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. East of the central Plains, readings 10-25F above normal (highest for morning lows) will prevail over a majority of the eastern half of the lower 48 with highest confidence from the mid-Atlantic to the Southeast. Some record highs may be possible in the warm sector. Fracasso/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