Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1200 PM EDT Thu Sep 13 2018 Valid 12Z Sun Sep 16 2018 - 12Z Thu Sep 20 2018 ...Dangerous Hurricane Florence to bring catastrophic flash flooding and prolonged significant river flooding to parts of the Carolinas and Appalachians into next week... ...Hurricane Florence... The National Hurricane Center forecasts Florence to make landfall into North Carolina Friday morning. A weakening Florence should arc through South Carolina before lifting roughly along the length of the Appalachians Sunday-Wednesday. The main threat from heavy and persistent rainfall for flooding will remain as it interacts with Appalachian terrain. Much of the rain will fall before the medium range period begins, but a significant area of rain will follow mainly along and to the east of the Appalachians next week. This will be a multi-day and multi-faceted event. Please consult the National Hurricane Center and your local NWS forecast office for the latest information. ...Pattern Overview and Guidance/Predictability Assessment... The 00 UTC NAEFS/ECMWF ensemble means were used as a starting point for WPC surface fronts/pressures progs days 3-5 in a period of smaller forecast spread excluding Florence. This blend left room for insertion of Florence as per the latest NHC forecast that did not explicitly match any particular deterministic model. Later, the upper pattern from the Pacific across North America will mostly remain stagnant with meandering ridging from southwest Alaska to the Aleutians. This maintains a trough position to its southeast down toward the Pacific Northwest. There remains differences concerning troughing next Wednesday/Thursday into the northwestern U.S. The somewhat more amplified ECMWF ensembles seem more reasonable than the more progressive GEFS solutions given upstream flow amplitude and WPC continuity. ...Weather Highlights/Hazards... Florence will spread a large area of significant rainfall northeastward next week despite its acceleration ahead of an approaching cold front. Flooding may be a concern along the I-81 to I-95 corridors but will be a function of forward speed. Back to the west, as troughing finally moves out of the northwest, a cold front will usher in cooler fall air to the High Plains and Upper Midwest/Great Lakes Tue-Wed and then to the east coast next Thursday as ex-Florence should race well into Atlantic Canada then. Expect some record high minimum temperatures along/ahead of the moist track of Florence. Deepening tropical moisture over the Gulf of Mexico and expected track of a tropical wave into southern Texas and the Rio Grande Valley will meanwhile offer a threat of locally heavy rainfall for the area from the weekend into early next week. Meanhwile, a new area of precipitation will take shape and expand over the northern Plains and Upper Midwest next week as waves and attendant preciptation focus along/near the front exits the foothills of the Rockies. Schichtel WPC medium range 500 mb heights, surface systems, weather grids, quantitative precipitation, winter weather outlook probabilities and heat indexes are found 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