Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 AM EST Sun Feb 09 2020 Valid 12Z Wed Feb 12 2020 - 12Z Sun Feb 16 2020 ...Midweek system to bring a heavy rain threat from the Lower Mississippi Valley to southern-central Appalachians through Wednesday night... ...Overview and Guidance/Predictability Assessment... Expect a continuation of the large scale pattern that features shortwaves rounding an eastern Pacific mean ridge and feeding into a long-term western U.S. mean trough with downstream ejection around a mean ridge that should tend to meander over or near the Bahamas and Cuba. Resolving the details of individual shortwaves dropping into the mean trough, as well as flow across Canada and the northern tier U.S. plus any possible interaction between the two, still provide various forecast challenges through the period. Guidance finally appears to be in the process of converging for the Wed-Thu system supported by the combination of the upper low forecast to track near the western U.S.-Mexico border in the short range and northern stream troughing that develops over the Plains/Upper Mississippi Valley. The 12Z ECMWF/ECMWF mean maintained continuity from their previous run that had adjusted faster, while slower GFS trends with northern stream flow have allowed low pressure to trend northwestward toward the ECMWF. Other solutions varied and the full range of ensemble guidance was still quite broad. Recent trends through the 18Z cycle favored an average among the 12Z ECMWF/ECMWF mean and 18Z GFS/GEFS mean as the best starting point for the updated forecast. This represents a faster trend from continuity and thus far the new 00Z guidance is adding confidence for a solution fairly close to the preferred scenario. While the spread for the midweek eastern system has greatly improved, guidance over the past day or so has diverged quite widely regarding the North Pacific trough expected to reach the West Coast by around late Thu. Just among operational models, solutions by Fri include the 00Z UKMET/12Z ECMWF with a fairly deep closed low near the California coast versus latest GFS runs that are open and quite progressive. The 00Z CMC pulls off enough energy farther southwest that it ends up with a ridge over the West by day 7 Sun--seemingly unlikely given the overall pattern. These late-week differences will translate downstream with time, leading to considerable uncertainty over surface details as well as timing/extent of moisture return over the east-central U.S.. The wide spread currently seen in guidance and teleconnection support for at least a moderate degree of western troughing recommend trending the early-period blend of the GFS/ECMWF and their means toward mostly the ensemble means while maintaining only modest ECMWF input by days 6-7 Sat-Sun. This yields a solution between the GFS and ECMWF/UKMET extremes over the West while allowing more east-central U.S. moisture return than recent ECMWF runs. Even with the heavier weight on the means late in the period, the blend does well in defining a modest shortwave and surface frontal system reaching the Northwest next weekend. ...Weather/Hazard Highlights... Improved guidance clustering is raising confidence for details of the system forecast to track from near the Lower Mississippi Valley into the Northeast Wed-Thu. Currently expect heaviest rainfall to extend along an axis from near the Lower Mississippi Valley into the southern/central Appalachians, falling mostly during Wed-Wed night. Recent heavy rain over portions of this area increases the potential for rainfall from this event to produce flooding/flash flooding concerns. Some snow should fall to the northwest of the storm track, with areas from the Midwest into the Northeast seeing the best potential. Accumulations may be meaningful but fast motion of the system should temper amounts somewhat. Convergence of solutions in latest guidance may provide some refinement of winter weather probabilities in the next issuance. The West will see periods of rain and mountain snow with multiple systems coming around the eastern Pacific ridge aloft. Highest five-day totals should be over favored terrain in the Pacific Northwest with lower totals extending into the northern/central Rockies. Most activity should be of light-moderate intensity though some brief localized enhancement could occur. Very wide spread among forecasts for flow aloft from late week through the weekend leads to low confidence in coverage and amounts across California and the Great Basin/Southwest. This uncertainty extends to the forecast east of the Rockies by next weekend. Expect well above normal temperatures over the East ahead of the Wed-Thu Lower Mississippi Valley into Northeast system. Southeastern areas will be particularly warm for morning lows with some plus 20-30F anomalies, which could approach or exceed daily records. Highs may reach as high as 10-15F above normal. System passage will pull a modified form of cold Northern Plains/Upper Mississippi Valley air (some areas 15-25F below normal Thu into Fri) into the East with the Great Lakes and New England seeing a day or so of readings at least 10F below normal. Temperatures may rebound to above normal levels again by Sun. The Rockies should see below normal temperatures most of the period with pockets of double-digit anomalies most likely Wed-Thu. 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