Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1159 AM EDT Tue Sep 11 2018 Valid 12Z Fri Sep 14 2018 - 12Z Tue Sep 18 2018 ...Hurricane Florence could bring catastrophic flash flooding and significant river flooding to portions of the Carolinas and Mid-Atlantic states late this week into early next week... ...Hurricane Florence... Based on the 1500 UTC National Hurricane Center advisory, powerful Hurricane Florence is most likely to make landfall as a major hurricane along the southern half of the North Carolina by Friday (near the start of the medium range period). The models and ensembles continue to waver on the expected landfall location (or near-landfall) and timing of when Florence may slow down and stall/meander just inland. In spite of ongoing guidance spread for track/timing specifics, confidence is gradually increasing that an extreme rainfall and flooding event may unfold along and east of the track of Florence, aided by inflow off the Gulf Stream. Currently expect an area of 15-20 inch accumulations with isolated local maximum totals to 30 inches. Upslope/terrain/coastal enhancement would enhance 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 more consistent. This will be a long-duration event, continuing even 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. Over the past couple days guidance has been showing steady trends toward holding trough energy over the Pacific Northwest for a longer time before ejecting across Canada and the extreme northern U.S. As a result latest consensus continues to adjust slower with the eastward progression of a wavy front over the northern Plains--finally becoming a fairly strong cold front by day 7 Tue. Downstream this slower trend allows more upper ridging to build from the Midwest through the Great Lakes/Northeast during the weekend, helping to keep Florence suppressed. Albeit with a bit of a delay, the pattern still appears to start to breaking down early next week as troughing (or at least flatter mean flow) replaces ridging over Alaska. Toward the end of the period ECMWF and GFS/GEFS-based guidance diverges somewhat on how quickly lingering West Coast trough energy ejects in response to upstream flow. Prefer a compromise/blend approach, given offsetting considerations--the overall slower trend to eject energy in general, versus teleconnections relative the evolving upstream pattern suggesting development of broad northeastern Pacific troughing that would serve to kick out leading West Coast energy. The starting blend used ideas from 00Z parallel GFS/ECMWF/UKMET early in the period followed by a transition toward a blend of models (00Z GFSP, 00-06Z GFS, 00Z ECMWF) and means (06Z GEFS/00Z ECMWF mean). The initial blend was adjusted for the NHC track of Florence as needed. ...Weather Highlights/Hazards... Regarding precipitation away from Florence, portions of Texas (most likely the southern half of the state) may see significant rainfall in association with a surface trough/upper low emerging over the southern Gulf. Consult latest NHC tropical weather outlooks as conditions could become more favorable for development as the feature heads into the western Gulf. Slower guidance trends with the West Coast upper trough have delayed the drying trend over the Northwest, with light to moderate activity likely to persist through most of the weekend. Temperatures over/near the northern Rockies may be cold enough to support some higher elevation snow but the best snow potential will be to the north of the US/Canadian border. Aside from a modest delay per guidance trends with the upper pattern, expect the emphasis for coolest temperatures (highs 5-15F below normal) to transition from the West Coast states into the northern Plains. Over the latter area there may be decent coverage of minus 10-15F anomalies for highs by next Mon-Tue. Otherwise most of the central/eastern states should see above normal temperatures, especially for overnight mins. Warmest morning lows versus normal (some 15F+ anomalies) should extend from the central Plains into upper Great Lakes, ahead of the northern Plains front, and over middle/northern latitudes of the East as Florence pulls in warm/humid air. Some lows may be near or exceed record warm values on 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