Extended Forecast Discussion NWS Weather Prediction Center College Park MD 258 AM EDT Sat Sep 15 2018 Valid 12Z Tue Sep 18 2018 - 12Z Sat Sep 22 2018 ...Pattern Overview and Guidance/Predictability Assessment... The forecast period begins on Tuesday with a trough centered near the West Coast, an upper level ridge over the south-central U.S., and a mid-upper level trough over the Appalachians that captures the remnants of Florence. The upper ridge expands across the south-central U.S. and then the southeast states by the end of the week, followed by a building trough over the northeast U.S. that breaks down the ridge and results in a cold front passage by next weekend. There is decent synoptic scale model agreement regarding the large scale pattern through the middle of the week, with the greatest differences noted in regards to the evolving upper level trough near the West Coast. The CMC appears to have more downstream ridging and its surface low tracks farther to the northwest across the central plains compared to the model consensus. The most uncertain aspect of the forecast is across the Pacific Northwest by Friday and Saturday, with widely differing model solutions pertaining to a potential surface low over the northeast Pacific. ...Weather Highlights/Hazards... In terms on what will be the remnants of former Hurricane Florence, a band of heavy rainfall is expected from upstate New York to the New England coast during the day 3 period on Tuesday after hammering the Carolinas and parts of the Appalachians. Since it will become caught up into the westerlies, the remnants are expected to progress eastward quickly and drier weather is expected to commence on Wednesday. The 12Z ECMWF was heavier and to the south of the more modest amounts depicted by the 00Z GFS. Some instances of flash flooding will be possible over this region given the potential for 2 to locally 4 inches of rain, particularly over areas that have received heavy rainfall recently. Another area of significant rainfall is becoming increasingly likely from the Dakotas to the Upper Midwest for the middle of next week and then reaching the Great Lakes by Thursday in association with a developing surface low across the central plains. The potential exists for a few inches of rainfall in some areas. Much cooler will follow by the end of the week for much of the central and northern tier of the U.S. as a Canadian surface high builds into the region and brings a pristine, autumnal airmass. Temperatures are expected to be well above normal across much of the central and eastern U.S. ahead of the cold front, with highs in the upper 80s to middle 90s likely across much of the southeastern quadrant of the nation. Cooler than normal conditions are expected from the northern Rockies to the Upper Midwest, with the potential for frost and freeze conditions by the end of the week for parts of Montana, North Dakota, and Minnesota. D. Hamrick 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