Extended Forecast Discussion NWS Weather Prediction Center College Park MD 255 AM EDT Fri Sep 14 2018 Valid 12Z Mon Sep 17 2018 - 12Z Fri Sep 21 2018 ***Hurricane Florence to bring catastrophic flash flooding and prolonged significant river flooding to parts of the Carolinas and Appalachians into early next week*** ...Hurricane Florence... The National Hurricane Center forecasts Hurricane Florence to make landfall over southeastern North Carolina Friday morning. The cyclone should gradually weaken as it tracks across northern South Carolina before lifting northward, roughly along and just west of the Appalachians through Tuesday as it accelerates northeastward. Flooding will be the main threat owing to heavy and persistent rainfall as it interacts with the mountainous terrain. Although the majority of the prolific rainfall will happen before the medium range period begins, a swath of enhanced rain will follow along the track of Florence as it becomes extratropical. This will be a multi-day and multi-faceted event, and the National Hurricane Center and local NWS forecast offices have additional information pertaining to this event. ...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 is forecast to spread a large area of significant rainfall across the central and northern Appalachians early next week despite its acceleration ahead of an approaching cold front. Rainfall amounts on the order of 2 to 4 inches appear likely with isolated higher totals possible. Flash flooding will be possible for flood prone areas and this is something that WPC will continue to monitor as the event enters the short range forecast period. 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. Meanwhile, a new area of precipitation will take shape and expand over the northern Plains and Upper Midwest next week as waves and attendant precipitation focus along/near the front exits the foothills of the Rockies. Hamrick/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