Extended Forecast Discussion NWS Weather Prediction Center College Park MD 256 AM EDT Sun Sep 16 2018 Valid 12Z Wed Sep 19 2018 - 12Z Sun Sep 23 2018 ...Pattern Overview and Weather Highlights/Hazards... At the start of the period, what is currently Tropical Storm Florence will have already become extratropical in nature as it moves across the western Atlantic on Wednesday. Mid-level heights should gradually build across the Eastern Seaboard in response to a progressive shortwave digging through the north-central U.S. on Thursday. This particular region of amplification has the potential to drive a fairly impressive surface cyclone across Quebec by Friday. Ultimately, this would favor a well defined baroclinic zone across the Great Lakes and eventually into the northeastern U.S. on Days 5-7, September 21-23. A strong 594-dm mid-level ridge centered over the southeastern U.S. should help focus the eventual stalled boundary up toward the mid-Atlantic region. Back across the eastern Pacific, a broad ridge extending up into the Gulf of Alaska will help maintain lower heights along the West Coast for much of the period. An initial shortwave is forecast to slide into the Pacific Northwest late Thursday before eventual absorption into the active Canadian northern stream. On its heels should be a more impressive axis of height falls which focuses a longwave trough over the far eastern Pacific next weekend. Finally, across the Desert Southwest, an area of disturbed weather lifting out of the subtropical Pacific should enhance low/mid-level moisture across the region. The National Hurricane Center currently suggests an 80 percent chance for tropical cyclogenesis during the next 5 days. Regarding the models handling of these mentioned systems, the dynamic surface low ejecting out of the Great Lakes seems highly probable but with much spatio-temporal uncertainties. Given the extent of forcing accompanying this shortwave, the deep wave scenario seems reasonable. Run-to-run continuity remains a bit suspect with the 00Z UKMET jumping back to the north to join the 00Z CMC. Meanwhile, the 00Z/18Z GFS and 12Z ECMWF sit along the southern flank of the spread. Considerable thickness packing accompanying this frontal passage suggests a feel of autumn in the air although the boundary will stall as the better forcing lifts well into eastern Canada. Across western North America, there has definitely become a large trend in the ensemble spaghetti plots favoring a longwave trough for late in the period. While ensembles cluster well, the amount of change between runs is quite notable so future cycles could exhibit similar behavior. Through Day 4, Thursday, a multi-model consensus of the 12Z GFS/ECMWF/CMC/UKMET was utilized before gradually incorporating ensemble means into the mix. From Day 5/Friday onward, removed any contribution of the 12Z CMC/UKMET from the picture and replaced them with the 12Z NAEFS/ECMWF ensemble means. With uncertainty looming next weekend, only kept about 30 percent of operational guidance in the blend while being led by ensemble influences. ...Guidance/Predictability Assessment... A heavy rainfall threat looms across sections of the Upper Midwest/Great Lakes through early Friday as the mentioned surface cyclone traverses the region. While models keep the higher amounts just north of the area which has been wet the past month, a lot could change in the upcoming model cycles. Flash flooding issues could ensue given the magnitude of rainfall noted in many of the more reliable pieces of guidance. 2 to 3 inch 24-hour areal averages are highly possible with isolated heavier amounts of course possible. Across the southern half of the country, diurnal convection is expected each day with a better focus along the Gulf Coast given enhancement along coastal boundaries. Back across the Four Corners, a surge of moisture accompanying a tropical disturbance could spread showers and thunderstorms north of the international border with Mexico. And across the Pacific Northwest, a series of shortwave passages should keep conditions wet throughout much of the forecast. Regarding temperatures, below average numbers are anticipated from the Pacific Northwest eastward into the northern Rockies and adjacent Great Plains. This region the from the Rockies eastward could see highs around 10 to 15 degrees below climatology which would offer highs in the upper 50s to lower 60s. To the south of the baroclinic zone, expected highs should be well above average, particularly from the central Rockies eastward across the middle Mississippi Valley and into the Ohio Valley/Great Lakes region. Similar anomalies are in the forecast which supports highs in the 80s for the most part. Rubin-Oster 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