Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 AM 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 portions of the Carolinas and the southern and central Appalachians into next week... ...Hurricane Florence... The National Hurricane Center forecasts Florence to make landfall near the coastal North Carolina/South Carolina border Friday. By the start of the medium range (early Sunday) a weakening Florence should begin making its large arc through upstate SC and northeastern Georgia as it finally escapes the region of weak steering flow. However, the threat from heavy and persistent rainfall will remain for flooding will remain as it interacts with the terrain of the southern Appalachians. Much of the rain will fall before the medium range period begins, but a significant area of rain will follow along and to the east of the center up the spine of the Appalachians next week. This will be a multi-day and multi-faceted event that transitions through the period. Please consult the National Hurricane Center and your local NWS forecast office for the latest information. ...Pattern Overview and Guidance/Predictability Assessment... Overall upper pattern from the Pacific across North America will mostly remain stagnant with meandering ridging over southwestern Alaska into/across the Aleutians. This will maintain the trough position to its east/southeast along the west coast (WA/OR) but by next Thursday the ensembles point to more zonal flow out of the western CONUS. There remains differences between the ECMWF ensembles and the GEFS in the amount of troughing next Tue/Wed into NorCal/OR but more weight was given to the ECMWF idea due to its slower handling of ex-Florence in the east more in line with the NHC forecast. To the east, Florence will eventually lift north/northeastward as enough height falls dip south to guide it through the central Appalachians then into the Northeast/New England. The 12Z ECMWF served as the best proxy to the NHC forecast (slower system exit given its trend/history seemed reasonable, but this system has had its share of audibles in the medium range). ...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. Temperatures will be mild in the tropical airmass in the east to start the week with cooler temperatures by the end of the period (and less humid). Warm/hot temperatures in the High Plains Sunday (mid or upper 90s) will 60s next Wed/Thu behind the cold front. The Southwest and southern tier will remain near to above average with mostly 80s/90s but lower 100s in the deserts. A new area of precipitation may start to take shape and expand in the central or northern Plains by next Wed/Thu as a wave along the front exits the foothills of the Rockies. Otherwise, the rest of the west should finally see a drier period. Fracasso 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