Extended Forecast Discussion NWS Weather Prediction Center College Park MD 258 AM EDT Thu Sep 20 2018 Valid 12Z Sun Sep 23 2018 - 12Z Thu Sep 27 2018 Pattern Overview and Model Evaluation _____________________________________ A quasi-zonal pattern across the northern tier of the U.S. on Sunday is forecast to transition into a more amplified pattern by Tuesday as a deep upper level trough becomes established over the north-central states. Upper level ridges are expected over both the western Atlantic and the eastern Pacific and may lead to a blocked flow pattern developing. Shortwave impulses are expected to track around the base of the synoptic scale trough and these will result in a series of surface lows and a relatively strong cold front crossing the central and eastern U.S. next week. There is also a cold air damming signature in the surface pressure pattern across the Mid-Atlantic and Appalachians early in the week. There is good overall model agreement on the synoptic scale through Monday regarding the evolution of the upper trough over the Rockies and the northern/central plains. The CMC is notably slower with the progression of the front across the plains owing to its placement of the downstream ridge over the eastern U.S., whereas the other models are farther east with the high and indicate a quicker progression. Compared to the past couple of days, the ECMWF has trended slower and closer to the model consensus regarding the main trough axis over the north-central U.S. early next week. There has been a quicker trend with the GFS with the shortwave impulse crossing the Great Lakes during the middle of the week, and the 12Z ECMWF is stronger with the surface low lifting northward across Ontario. The latest WPC forecast is more progressive than the previous one, and incorporated mainly UKMET/GFS/ECMWF through Tuesday, and then more towards the EC mean along with some of the GEFS mean and ECMWF. Sensible Weather and Potential Hazards ______________________________________ The potential for heavy rain and strong thunderstorms exists across parts of the middle Mississippi Valley through early Tuesday as deep moisture continues to surge north from the Gulf of Mexico, along with remnant moisture from former Tropical Depression Nineteen-E. Significant model differences remain with the axis of heaviest QPF, which may be on the order of a few inches in some areas. A swath of enhanced rainfall may extend eastward to the Mid-Atlantic region, albeit lighter in intensity. Behind the cold front across Montana and Wyoming, light to moderate rain is likely and some snow in the highest elevations of the Rockies. Temperatures are expected to be above normal across much of the central and southern U.S. before the cold front brings a return to more autumnal conditions for much of the nation by the middle to end of next week. Below normal readings are expected across the northern tier of the nation with highs in the 50s and 60s. 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