Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1159 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 heavy rainfall threat centered over 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 from the Pacific Northwest into the eastern Pacific as well as over 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. 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. Depending on the exact position of the surface low, there may also be terrain enhancement along the eastern slopes of the Appalachians. 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... For the overall forecast across the contiguous U.S. and vicinity, a blend of operational models (00Z parallel GFS, ECMWF, UKMET, CMC) provided a reasonable starting point to represent the most common ideas of guidance during the first half of the period. From the weekend into early next week model/ensemble spread increases fairly quickly with respect to how individual shortwaves within the Canada/Pac NW/Pacific upper trough eject eastward, due in part to uncertainty over details of how the Pacific into Alaska upper ridge weakens. This spread and uncertainty favor a fairly rapid trend toward majority input from the 06Z GEFS/00Z ECMWF ensemble means by days 6-7 Sun-Mon. The most noticeable trend over the past 24-36 hours is toward slower progression of height falls across southern Canada, which holds back northern U.S. frontal progression by day 6 Sun. The 06Z GFS has reversed that trend somewhat, while the 00Z ECMWF eventually ejects more West Coast trough energy than consensus. Regarding Hurricane Florence, there is still greater spread than desired considering the magnitude of impacts involved and differences in these impacts relative to exact track/motion of the system. The ECMWF continues to lie toward the western/left side of the spread and most GFS runs on the east side, however the 00Z GFS pulls the surface low to the middle and then the western side of the envelope toward Sun-Mon. At the very least there is increasing confidence toward the idea that deep layer ridging to the north of Florence will likely lead to significant deceleration of Florence after arrival and a track that may not reach much farther than the western half of the NC/VA border by day 7 Mon. Meanwhile most guidance shows an area of weak troughing/possible embedded low pressure reaching and possibly lingering near the Texas coast. This feature could promote another area of significant rainfall in its vicinity. Exact depiction will be coordinated in today's NHC/WPC extended range call. ...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 extreme northern Plains into upper Great Lakes in the vicinity of a wavy frontal zone, aligned with the southwesterly upper flow continuing into the weekend. Also the western Gulf Coast region may be at risk for areas of enhanced rainfall with the combination of an arriving (and possibly lingering) tropical wave/upper weakness. Precipitation over the Northwest will tend to be light to locally moderate, with a drier trend likely by Sun-Mon. Temperatures over/near the northern Rockies may be cold enough to support some higher elevation snow. 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-Mon. The majority of the central/eastern states should see above normal temperatures, especially for 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. Rausch/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