Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1201 PM EDT Sun Sep 02 2018 Valid 12Z Wed Sep 05 2018 - 12Z Sun Sep 09 2018 ...Pattern Overview and Guidance/Predictability Assessment... To commence the period, a strong mid/upper ridge centered over the eastern U.S. will remain a fixture in the forecast. Initially around 2 to 2.5 sigma above climatology, it should gradually weaken in response to amplified northern stream flow grazing New England. This will allow a backdoor front to drop south into the Mid-Atlantic during the Day 5-7, September 7-9 window. This continues to be one of the more agreed upon features on the map. To the south of the persistent ridge, what is now deemed Invest 91L centered just north of Hispanola should track westward across the Florida Peninsula toward the northern Gulf of Mexico. The National Hurricane Center has raised tropical cyclogenesis probabilities to 80 percent during the next 5 days. Regarding the guidance, the past several GFS runs remain the most conservative showing very little surface reflection. While the 00Z ECMWF has trended downward with intensity relative to yesterday's run, the 00Z CMC/UKMET are nearly in line with the ECMWF solutions. At this time, the deepest solution is the 00Z UKMET which remains close to yesterday's 00Z ECMWF. Beyond Day 5/Friday, the forecast becomes complicated with the 00Z CMC/UKMET each showing further strengthening despite being inland. Suffice to say, it remains an area to monitor given the heavy rainfall potential. Across western North America, a broad longwave trough lurks offshore with the past four ensembles showing greater stability than prior days. Gradually this system will push inland on Friday with the 00Z CMC lagging behind staying with a more offshore solution. While these height falls accelerate eastward toward the north-central U.S., the synoptic-scale flow attains a more quasi-zonal appearance into the weekend. On its heels, a more powerful upper low drops down from the Gulf of Alaska with an eventual position well offshore of the West Coast by Day 7/September 9. At this point, outside of the 00Z CMC/GFS, the guidance is well clustered with this eventual configuration. With sufficient model clustering amongst the 06Z GFS and 00Z GFS-parallel/ECMWF/UKMET forecasts, was able to utilize a combination of these models through late Thursday. Decided to manually lower pressures with the tropical invest given the GFS/GFS-parallel solutions wash out the signal. Beyond this period, began incorporating a healthy amount of the ensemble means, 06Z GEFS/00Z ECMWF to be exact. Confidence is a bit above average in most sectors of the map but still more nebulous with tropical considerations along the western/central Gulf Coasts. ...Weather Highlights/Hazards... Wednesday and Thursday will prove to be rather warm days across the lower Great Lakes/Ohio Valley into the northeastern U.S. Generally speaking, departures from early September climatology sit around the 10 to 15 degree mark as highs reach 90 degrees as far north as 40N latitude. Overnight lows may be warm enough to support the breaking of several daily records. Some improvement can anticipated by Friday as the influence of the backdoor front brings cooler maritime air into the picture. Elsewhere, the center of the country should sit on the other side of the spectrum with highs around 5 to 10 degrees below average. Through Friday, daily precipitation chances accompanied by enhanced cloud cover should help reduce insolation over the region. Heavy rainfall potential exists from the Central Plains through the Middle Mississippi Valley and into the Great Lakes. A slow moving boundary will be the focal point for multiple days of rainfall which could prove to enhance the threat for hydrologic issues. Along the Gulf Coast, enhanced tropical moisture accompanying the mentioned invest will prove to be a heavy rainfall maker. Exactly to what extent remains unknown but the situation should continue to be monitored in the upcoming days. 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