Extended Forecast Discussion NWS Weather Prediction Center College Park MD 148 PM 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. Recent trends through the 06Z cycle favored an average among the 00Z Canadian/ECMWF/ECMWF mean and 06Z GFS/GEFS mean as the best starting point for the updated forecast. Low pressure should lift northeastward out of the Texas coast/NW Gulf toward western NYS then quickly into Atlantic Canada. 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. The 00Z Canadian/ECMWF were most aggressive (even beyond the ~100 ensemble members) with a deep low closing off west of California. The GFS was much less amplified/sharp and thus quicker/progressive to the east. However, the 00Z NAVGEM was nearly in line with the ensemble mean trough axis and was used to add some detail to the period next Sat/Sun. 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 a slower solution than the GFS. With the tendency for the ECMWF and Canadian to overamplify the flow in the medium range, preferred to stay near the ensemble consensus position, near the 00Z NAVGEM. Also incorporated the previous shift which was correlated quite well. 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 which shows, at the moment, good agreement. ...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 (straddling the short range into the medium range). Recent heavy rain over portions of this area increases the potential for rainfall from this event to produce flooding/flash flooding concerns. Severe weather is possible in this same area as the Storm Prediction Center has outlined both a d 15% and targeted 30% area from Louisiana to Alabama. 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. Increased the day 4 (12Z Wed - 12Z Thu) winter weather probabilities to around 50% over north-central Illinois given the better agreement. 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. Uncertainty in the upper flow 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. Given the preference for a modest progression eastward in the southern stream, would expect some return flow moisture Saturday and perhaps more widespread and light/modest rainfall over the Southeast next Sunday. Again, uncertainty is high as the ensembles basically range from zero to several inches of precipitation. 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 (some record highs into the mid-80s in Florida). 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. 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