Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1118 AM EDT Sun Sep 09 2018 Valid 12Z Wed Sep 12 2018 - 12Z Sun Sep 16 2018 ...Hurricane Florence likely to impact a portion of the East Coast later this week... ...Potentially significant rainfall threat for portions of the Southeast and Mid-Atlantic... ...Pattern Overview... A rather stagnant upper pattern starts the medium range period (Wednesday) with strong ridging over the northern Pacific and the northwestern Atlantic off the New England coast. This favors the maintenance of a positively tilted trough along the west coast of the lower 48 and broad SW flow out of the central US into southeastern Canada. As the Pacific ridging breaks down later this week, a buckle to the flow will eventually work its way into the Pac NW next week. Hurricane Florence is forecast to strengthen to a major hurricane over the next few days and is likely to impact at least part of the Southeast/Mid-Atlantic starting around Thursday but perhaps carrying into next weekend, even if the center stays offshore. Please consult the National Hurricane Center for the latest information on Florence. ...Guidance/Predictability Assessment... Overall, the amplified pattern remains fairly well-handled by the ensemble guidance especially east of the Rockies with uncertainty associated with the ridge breakdown over the Aleutians. Ensemble consensus approach seems best suited in the west with the deterministic runs too varied to instill confidence. To the east, dominant ridging will help steer Florence toward the US Southeast coast and perhaps inland into the Carolinas per the latest NHC track. By next Fri-Sun, the steering flow over the east essentially vanishes which leaves Florence with no real exit strategy. Where this may happen will have enormous consequences on the wind/rain threat to coastal and then inland areas. Farther west, a weakness aloft over Texas ans the lower MS Valley will drift westward through the period. ...Weather Highlights/Hazards... Biggest impacts will be in the east surrounding Florence. With an increase in the likelihood that the system stalls just inland or just offshore, the heavy rain threat will continue via an unlimited moisture source. Maximum potential rainfall per some model guidance has been beyond extreme -- but remains just model output for now. Many factors will determine location/amounts/timing but that won't be resolved for a couple more days at the least; however, the rainfall/flooding threat is a real concern even at this time range. A second area of rain will exist over the northern and northwestern Gulf Coast under a weakness aloft that will eventually fade to the west. An inch or so of rain is possible over the earlier portions of the medium range period. Temperatures will generally be cooler than average in the Northwest under the trough but near to above average elsewhere (especially for minimum temperatures which may be near record warm values in the east). Those cooler temperatures may spread eastward through Montana this weekend which would support some higher elevation snow. Fracasso/Ryan 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