Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1200 PM EDT Wed Sep 12 2018 Valid 12Z Sat Sep 15 2018 - 12Z Wed Sep 19 2018 ...Dangerous Florence to bring catastrophic flash flooding and prolonged significant river flooding to portions of the Carolinas and for the Southern to Central Appalachians through the weekend and well into next week... ...Hurricane Florence and Guidance/Predictability Assessment... Powerful Florence is forecast by the National Hurrican e Center to make landfall as a major hurricane into southern North Carolina/northeastern South Carolina Friday. In spite of ongoing guidance spread for track/timing specifics, confidence is increasing that an extreme rainfall and flooding event may unfold mainly along and north/east of the track of Florence, aided by deep tropical moisture inflow. Currently expect an area of 15-25 inch accumulations with isolated local storm total maximum totals up to 40 inches. Upslope/terrain/coastal enhancement would exacerbate the seriousness of the rainfall/flooding threat. These forecast rainfall amounts have rarely been recorded in the past century and can result in flooding where it has not been observed from past events. The areas of greatest impact and highest rainfall will be refined over the next couple of days, but the signal for an extreme/catastrophic event has been more consistent. This will be a multi-day and multi-faceted event as Florence is forecast to be a coherent entity in weakened form into next midweek. Please consult the National Hurricane Center and your local NWS forecast office for latest Florence information. ...Pattern Overview... Evolving upper ridging near Alaska favors positively tilted and reasonably amplified troughing aloft from the Pacific Northwest/eastern Pacific into central Canada/Hudson bay. Upper ridging is forecast to stretch from the Four Corners region northeastward into New England and into the NW Atlantic, helping to guide Florence. A composite of reasonably clustered guidance from the 00 UTC GFS/ECMWF and the track of Florence as per the National Hurricane Center was used for this weekend into early next week, quickly tranitioning into a combination of ECMWF ensembles and the NHC track thereafter. ...Weather Highlights/Hazards... Regarding precipitation away from Florence, southern Texas may see significant rainfall in association with a surface trough/upper low emerging over the southern Gulf, crossing between short range and medium range. Consult latest NHC tropical weather outlooks as conditions could become more favorable for development as the feature heads into the western Gulf. Slower guidance trends with the West Coast upper trough have delayed the drying trend over the Northwest, with light to moderate activity likely to persist through most of the weekend. Temperatures over/near the northern Rockies may be cold enough to support some higher elevation snow but the best snow potential will be to the north of the US/Canadian border. Expect the emphasis for coolest temperatures (highs 5-15F below normal) to transition from the West Coast states into the northern Plains as high pressure builds in the wake of frontal passage. Over the latter area there may be decent coverage of minus 10-15F anomalies for highs by next week (maximum temperatures into the 50s). Warmest morning lows versus normal (some 15F+ anomalies) should extend from the central Plains into upper Great Lakes, ahead of the northern Plains front, and over much of the East as Florence pulls in warm/humid air. Some minimum temperatures may near or exceed record warm values. Maximum temperatures would be held down near Florence and its cloud shield. 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