Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 PM EDT Sun Sep 06 2020 Valid 12Z Wed Sep 09 2020 - 12Z Sun Sep 13 2020 ...Record cold will surge further down into the central to southern Plains mid-week followed by gradual moderation... ...Heavy rain possible over southern Texas Wednesday night/early Thursday... ...Tropical moisture likely lingers near the East Coast into the weekend... ...Overview and Guidance/Predictability Assessment... Model guidance continues to show a highly amplified trough that will soon bring highly dramatic temperature plunge and significant snow from the central Rockies to the central High Plains but remains highly uncertain on the subsequent synoptic pattern evolution thereafter. The two most recent runs of the ECMWF have switched back to the much slower northeastward ejection of the deep trough/closed low from the central Rockies toward the end of the week while keeping the pattern over the eastern U.S. more progressive. Meanwhile, the GFS has kept a faster ejection of the upper low into the Plains although not as decisively so as in earlier runs. The UKMet also support this faster ejection scenario. The WPC morning prognostic charts used an even blend of the 00Z ECMWF, 00Z EC mean, 06Z GFS and 06Z GEFS which yielded an intermediate solution that fit fairly well with continuity. The resulting pattern depicts a gradually occluding low pressure system moving toward the Great Lakes by the end of the week. ...Weather/Hazard Highlights... During Wed-Thu many locations over the southern half or two-thirds of the Plains/Rockies should see record cold max/min temperatures, with readings 20-35F to 40 degrees below normal possible. Record cold highs may extend one more day into the Upper Midwest on Wed. Moderation after Thu will likely bring the lowest anomalies into the minus 10-20F range during Fri-Sat and minus 5-12F for Sun. The initial upper trough and leading surface front may generate some locally moderate/heavy rainfall over southern parts of Texas around Wed night/early Thu. Otherwise there is still a decent signal for some areas of moderate/heavy rainfall from the Plains into the Midwest and possibly northeastward but low confidence on the specifics due to guidance spread/run-to-run variability on the path of the upper low. An upper low reaching as far back as the Four Corners would also produce more precipitation over/near the southern half of the Rockies. How much rainfall occurs over the east-central U.S. will depend on the path/timing of the upper low, which will affect progression of the leading front. Meanwhile areas along the East Coast should see increasing rainfall, some locally heavy, as tropical moisture associated with an Atlantic tropical wave is forecast to reach the Carolina coast later this week. Above normal temperatures (plus 10-20F anomalies) concentrated over western Washington/Oregon as of midweek will gradually expand eastward/southeastward while greatest anomalies decrease toward the plus 10-12F or so range. Much of the East Coast from the Mid-Atlantic southward should be increasing wet/moist with slightly above normal temperatures. Slightly cooler air is now forecast to filter into the Northeast behind a cold front but it is still highly dependent on the uncertain upstream pattern evolution over the Ohio Valley to the Great Lakes. Kong/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