Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 AM EST Mon Jan 06 2020 Valid 12Z Thu Jan 09 2020 - 12Z Mon Jan 13 2020 ...Heavy Rain Threat from the Lower-Mid Mississippi Valley into Tennessee/Ohio Valleys Fri-Sat... ...Active pattern over the Northwest... ...Overview/Guidance Assessment... There are embedded details that exhibit some spread and variability but guidance agrees that strong ridges aloft over the east-central Pacific and off the southeastern U.S. coast will hold in place an amplified western U.S. trough at least into early next week. Within this pattern, low level flow of Gulf moisture ahead of an ejecting western shortwave/leading wavy cold front will produce an episode of heavy rainfall from parts of the Lower-Mid Mississippi Valley eastward/northeastward from late this week into the weekend with some wintry weather on the northern side of the precipitation shield. Meanwhile energy flowing around the northeastern side of the Pacific ridge will bring a series of systems into the Northwest. The trajectory of moisture across the Northeast Pacific is not favorable for extreme precipitation totals but the accumulations over several days may still be significant. One of the most problematic aspects of the forecast in recent days has been the timing/evolution of shortwave energy reaching the West Coast around the start of the period early Thu and ejecting from the West after Fri. For the moment solutions are much closer together than they had been, by way of the GFS hedging a bit slower/less amplified as the shortwave crosses the eastern half of the lower 48 and the 12Z ECMWF trending significantly faster/more open than a number of previous runs. CMC runs continue to hold onto a somewhat slower and more closed feature aloft until it reaches past the Mississippi Valley. Ensemble means are still a bit slower than the non-CMC cluster but provide added support for an open wave. An early look at the new 00Z ECMWF reveals a slightly slower trend, highlighting the ongoing uncertainty. The latest guidance cluster would still produce a significant heavy rain event over the east-central U.S. but a modestly shorter duration would temper the highest totals somewhat. Upstream the guidance spread appears to have narrowed somewhat for the storm system forecast to track into Washington/southern British Columbia Fri-Sat. There are still meaningful medium/smaller scale shortwave differences yet to be resolved so convergence of solutions may be a slow process. The energy aloft will overspread the western U.S. with time with some of it possibly reaching the Plains and supporting one or more surface waves by the start of next week. By the latter half of the period there is some spread over exactly how western Canada energy may descend toward the western U.S.-Canada border. The 00Z GFS backed off from the 516dm contour upper low that the prior two runs had just north of Montana on day 7 Mon but is still deeper than most other models/means over that area. A multi-model blend with greater emphasis on the 12Z ECMWF and 12-18Z GFS relative to the 12Z UKMET/CMC provided a reasonable depiction of consensus for the first half of the period, while increasing detail uncertainties by days 6-7 Sun-Mon recommended about 50-60 percent total weight of the 18Z GEFS/12Z ECMWF means along with lingering input from the ECMWF/GFS runs. ...Weather Highlights/Threats... A system tracking from the northern Plains into eastern Canada late in the week will spread precipitation across the Great Lakes and Northeast with some wintry weather possible across northern parts of these regions. Confidence is still above average for the overall heavy rainfall event expected from parts of the Mid-Lower Mississippi Valley into the Ohio/Tennessee Valleys during the late week into weekend time frame. However uncertainty with the timing of supporting dynamics ejecting from the West provides some challenge for resolving the rainfall duration and highest totals. The scenario continues to be favorable with a feed of Gulf moisture interacting with an approaching wavy front, producing areas of heavy rain/embedded convection. The Storm Prediction Center is also highlighting a potential for severe weather across the southern tier of states with this event. Some locations within the heavy rainfall threat area have recently received significant rainfall so effects from this upcoming event will require close monitoring. Meanwhile over the past day the signal for wintry precipitation on the northern periphery of the moisture shield has strengthened. Primary snow potential should extend from parts of the Midwest through the Great Lakes and Northeast. The wavy cold front will eventually cross the East Coast and extend into the Gulf. This trailing part of the front is likely to return north as a warm front around the start of next week and become one focus for another potential rainfall event near the end of the medium range period and beyond. The series of systems coming into the Northwest will focus the highest rain/mountain snow totals over the Pacific Northwest coast/northwest corner of California along with the Cascades. Expect a secondary maximum over the northern Rockies. At times moisture will extend southeastward into the Sierra Nevada/Great Basin/Central Rockies but with lighter amounts in most cases. Amounts should not be extreme given that northwesterly flow should promote only modest anomalies for deep moisture, but multi-day totals over the most favored terrain could reach a few inches liquid. From late week into the weekend expect a surge of very warm air to extend from the central through eastern U.S. with a broad area of plus 10-25F anomalies for highs and plus 20-30F and locally higher anomalies for morning lows. Some daily records are possible, especially for warm lows. Most of the West will see moderately below normal highs through the period. The extreme northern Plains/Montana will be a focus for some colder anomalies, including some readings 20-30F below normal Sun-Mon. 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