Extended Forecast Discussion NWS Weather Prediction Center College Park MD 143 AM EDT Fri Sep 07 2018 Valid 12Z Mon Sep 10 2018 - 12Z Fri Sep 14 2018 ...Heavy rainfall to exit the Northeast early next week... ...Hurricane Florence looms off the East Coast later in the week... ...Pattern Overview and Guidance/Predictability Assessment... Upper pattern over the lower 48 will feature a reloading trough in the west with ridging in the western Atlantic next week. This will carry the remnants of Gordon northeastward through the St. Lawrence Valley along a front but may open the door for Florence to track very near the East Coast later in the week. Uncertainty is high in its track as the ensemble spread spans from Florida to Nova Scotia by next Thursday. Upstream pattern remains quite amplified over the Pacific (very strong upper high in the Bering Sea and a deep upper low near 40N/180 early in the week) and teleconnectons favor a positively tilted trough in the west and ridging in the western Atlantic which is just what the ensembles continue to forecast. A blend of the recent deterministic guidance sufficed to start as the sfc low associated with Gordon accelerates northeastward. The attendant cold front will stall across the mid-Atlantic as Florence likely moves west of 70W on Wednesday. Front in the west will southward and eastward as the trough reloads, with less agreement on the trailing shortwave later in the week. That will stall over the Dakotas and back to the central Rockies/Great Basin as it aligns with the upper SW flow. For next Thu/Fri in the east, evolution of the upper high around Florence will dictate its westward extent. For now, continued with a curve to the N just offshore the DelMarVa and NJ per the central cluster mean (among the ensemble systems), roughly between the 12Z GFS and ECMWF. This aligned well with yesterday's WPC/NHC coordination call. Will need a couple more days to decrease the uncertainty from "anything is possible" (~1500 miles of spread) to something more reasonable. ...Weather Highlights/Hazards... Heavy rain associated with ex-Gordon will only slowly abate as precipitable water values and anomalies (1.5-2" and +2 sigma) surge ahead of the front. Hurricane Florence will likely provide the greatest uncertainty for sensible weather effects near the East Coast. Even if the track remains far enough offshore to keep the region mostly dry, some coastal areas may see brisk winds with lots of waves offshore -- see OPC for more information on the wind/wave potential. Other area of rainfall later in the week may be along the upper Gulf Coast with a weakness aloft in the region atop plenty of moisture. The evolving pattern over the eastern Pacific/western North America will bring moisture into the Pacific Northwest with precipitation likely persisting through the end of the period. Upper trough will bring multiple days of highs 5-10F below average next week to Washington/Oregon but about 5-15F above average from the Southwest into the northern Plains. In the east, near average high temperatures will persist along the dissipating front but will be dependent on the track of Florence as most of the rain could stay offshore with the I-81/95 corridors in an area of subsidence outside the circulation. For now, maintained a chance of rain over much of the area. 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