Extended Forecast Discussion NWS Weather Prediction Center College Park MD 227 AM EDT Tue Sep 11 2018 Valid 12Z Fri Sep 14 2018 - 12Z Tue Sep 18 2018 ...Hurricane Florence to bring devastating and life-threatening flooding rainfall to portions of North Carolina and Virginia later this week... ...Heavy rainfall threat may extend well inland as Florence is forecast to slow to a crawl this weekend... ...Hurricane Florence... Per the latest National Hurricane Center forecast, powerful Hurricane Florence is forecast to make landfall as a major hurricane along the North Carolina or northeast South Carolina coast late Thursday or early Friday (just before the medium range period). The models and ensembles continued to waver on the expected landfall location (or near-landfall) and timing of when Florence may slow down and stall/meander just inland. Though uncertain in exact location by Sunday, it is increasingly clear that an extreme rainfall and flooding event will unfold along and east of the track of Florence with inflow off the Gulf Stream. Rainfall will likely be measured in excess of 10" over a large area with local maxima over 2 feet. Any upslope/terrain/coastal enhancement would only increase 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 few days (in the short range) but the ongoing and increasingly loud signal from the model guidance (and ingredients-based setup for the pattern evolution) has been unwavering. This will be a long-duration event over many days long after the strong winds have subsided. Please consult the National Hurricane Center for the latest information on Florence as well as your local NWS forecast office. ...Pattern Overview and Guidance/Predictability Assessment... Upper ridging over the North Pacific will favor positively tilted troughing from the Pacific Northwest/eastern Pacific into central Canada. Farther east an anomalously strong closed upper high between Bermuda and the Northeast will linger in place and help steer Hurricane Florence into the East Coast per the latest NHC track. By the weekend into next week expect some of the energy within the overall mean trough to eject across southern Canada, just far enough north of the weakening Florence to leave the system in a region of weak steering flow. The pattern appears to start to break down early next week as troughing replaces ridging over Alaska, sending quasi-zonal flow into western Canada while ridging remains atop ex-Florence next week. Due to the complex multi-stream flow out of the Pacific into the US west coast, a blended/ensemble solution was preferred by next Mon/Tue. The ECMWF and its ensembles were slightly more preferred over the GFS/GEFS in the west and with Florence as well (though not a perfect match). ...Weather Highlights/Hazards... Southern Texas may see significant rains due to a tropical wave interacting with a lingering weakness aloft though not all models were on board. NHC is monitoring this area (now in the northwestern Caribbean Sea) for possible tropical development. Precipitation over the Pacific Northwest will tend to be light to locally moderate, with a drier trend likely next week as heights rise. Temperatures over/near the northern Rockies may be cold enough to support some higher elevation snow, but especially north of the US/Canadian border. As the upper pattern slowly evolves, expect the emphasis for below normal temperatures (minus 5-15F anomalies for daytime highs) to shift from the West Coast states/northern Rockies late in the week toward the extreme northern Plains by Sun-Tue. The majority of the central/eastern states should see above normal temperatures, especially for overnight mins. Particular emphasis will be within an axis of warm lows ahead of a wavy front extending southwest from the northern Plains, and over parts of the East which will see the arrival of tropical air associated with Florence. Some lows may be near or exceed record warm values one or more days. Maximum temperatures would be held down near Florence and its cloud shield. Fracasso/Rausch 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