Extended Forecast Discussion NWS Weather Prediction Center College Park MD 233 AM EDT Mon Sep 10 2018 Valid 12Z Thu Sep 13 2018 - 12Z Mon Sep 17 2018 ...Hurricane Florence to bring dangerous conditions to portions of the Southeast and Mid-Atlantic states later this week... ...Very serious rainfall threat for portions of North Carolina and Virginia through the weekend and perhaps into next week... ...Pattern Overview... Persistent but shifting ridging over the North Pacific will favor positively tilted troughing in the Pacific Northwest and across central/northeastern Canada. 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, any troughing coming out of the west will lift through eastern Canada just to the north of a weakening Florence which will leave it in a region of weak steering flow. This may set up a potentially extreme rainfall event for portions of North Carolina and Virginia as the inflow off the warm Gulf Stream provides copious amounts of moisture. The potentially devastating impacts from sustained rainfall over several days cannot be understated. Please consult the National Hurricane Center for the latest information on Florence. ...Guidance/Predictability Assessment... Models/ensembles continue to struggle in the west due to upstream uncertainty over the Pacific and how to handle the ridge breakdown over Alaska and subsequent shortwave trajectories/amplitudes through the trough. The GFS/GEFS were generally quicker/weaker than the ECMWF and its ensembles with such shortwaves, but there has been no defined ensemble trend as of late, so a blended solution of the recent ensembles should suffice for now until the Pacific gets sorted out. In the east, trend has been for higher heights in the east atop Florence as it moves inland later this week. This results in increased confidence that it will not get picked up into the flow but rather linger near the NC/VA border (per NHC/WPC coordination). Forecast track per NHC aligned rather well with the 18Z parallel GFS (FV3 version) through most of the period. The speed of the system toward/into the coast and timing of when the upper flow diminishes will dictate where the system will go, but the narrowing ensemble spread mostly shows the system lingering just inland into next week. ...Weather Highlights/Hazards... Aside from the initial wind/wave hazards with a potentially major landfalling hurricane, the heavy rain and flooding threat will continue for several days due to the combination of rare events -- stalled tropical system near the coast with inflow across the warm Gulf Stream at the Atlantic's warmest time of year. Though NWP rainfall should not be taken literally, information is contained within the output and the rainfall signal is striking when taken in concert with the ensembles. The location of heavy/extreme rainfall will not be known until the short range, but the probability of it occurring in this scenario is high and the probability of it occurring over land continues to increase. Any upslope/terrain enhancement would only increase the seriousness of the rainfall threat. A second area of rain will exist over the northern tier (MN/WI and the UP of Michigan) in the vicinity of a wavy frontal zone, aligned with the SW upper flow Thu-Sun. Finally, a tropical wave in the Gulf may combine with the in-situ weakness aloft over the upper Gulf Coast/TX to produce modest rain for the Texas coast later this week until upper ridging reasserts itself. 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 in the tropical airmass). Maximum temperatures would be held down closer to Florence and its cloud shield. Those cooler temperatures in the Northwest may spread eastward through Montana this weekend which would support some higher elevation snow. 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