Extended Forecast Discussion NWS Weather Prediction Center College Park MD 710 PM EDT Thu Sep 20 2018 Valid 12Z Sun Sep 23 2018 - 12Z Thu Sep 27 2018 ...Pattern Overview... A significant trough is forecast to dig into the central states early next week as upper ridging lumbers toward a position just off the Pacific Northwest coast. This will take a lead system northeastward out of the High Plains Monday and into eastern Canada as its attendant cold front moves southward and eastward, bringing in colder air in its wake and a swath of rain along and ahead of the boundary. ...Guidance Evaluation/Preferences... Model/ensemble agreement was better than average as the pattern transitions toward robust amplification. The 00Z Canadian and its ensembles departed from the GFS/ECMWF ensemble clustering in the west as the initial trough moves out of the west late this weekend being slower/deeper with the northern stream portion of the trough rather than allowing it slide eastward (as the trend has been quicker). The 00Z/06Z GFS were just a bit quicker with the trough progression than the ECMWF/UKMET and ECMWF/GEFS means but a broad blend offered a reasonable starting point. The 00Z parallel GFS was used by the end of the period with the ECMWF as the deterministic parts to add some detail to the ensemble counterpart. ...Sensible Weather Highlights... The potential for locally heavy rain and convection exists across parts of the middle Mississippi Valley early next week as deep moisture surges north from the Gulf of Mexico atop a lingering stationary surface front. As the front moves eastward, rainfall may expand across the Ohio Valley into the Northeast/Mid-Atlantic via a surge of moisture from the eastern Gulf and also off the Atlantic as a weak wave moves toward the Southeast coast. Temperatures will trend cooler than average from the Northwest/High Plains Sun/Mon to the Plains/MS Valley Tue-Thu. Ahead of the front, temperatures will remain above average, especially min temperatures (about +5 to +15F) thanks to a humid airmass, in the Southeast to the Northeast. 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