Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1249 AM EDT Tue Aug 14 2018 Valid 12Z Fri Aug 17 2018 - 12Z Tue Aug 21 2018 ...Pattern Overview and Preferences... The models show the large scale pattern becoming more amplified with time over the weekend into early next week as a high amplitude trough in the north Pacific south of Alaska amplified the east Pacific to west coast of Canada ridge, in turn favoring the development of the trough over the central US early next week. At the start of the forecast period on Friday, an area of low pressure moves from near Lake Huron to near or over northern Maine Sat morning and across the Canadian maritimes Sunday. Attention then turns upstream to the pattern amplification out west late Sat into Sunday. The 12z ECMWF becomes more amplified than other solutions with the 500 mb trough and associated surface that develops further south Sun night into Monday in the mid Ms Valley at a time when the means have low pressure in the upper MS Valley. The 18z GFS was the strongest with the lowest pressure on 12z Mon with its low in MN. Due to the big difference among the GFS and ECMWF and much better agreement on both the location and intensity of the low among the 18z GEFS/12z ECMWF ensemble means, greater weighting was given to the ensemble means, with less to the ECMWF in case it is correct on the greater amplification of the trough. The higher heights out west persist over AZ and CA with lowering heights providing relief from some of the more extreme temperatures in the northwest. By the end of the forecast period, there is good agreement among the 12z ECMWF , the 12z ECMWF ensemble mean, and GEFS mean on the anticyclone center over AZ 00z Wed 22 Aug. ...Weather Highlights and Hazards... It should be quite warm across the north-central U.S. through Friday as high temperatures are forecast to be 10-15 degrees above normal in much of northern ND and MN. A building upper ridge along the West Coast will allow temperatures to persist above normal by 5-10 degrees across most areas inland from the coast across the Pacific northwest and northern Rockies. Above normal readings also persist across interior CA across the Great Basin. With the developing central US trough early next, temperatures will switch from being above normal over the weekend in the northern Plains to upper MS Valley to moderate to normal or even a few degrees below normal by Tue 21 Aug. Multiple rounds of showers and thunderstorms are likely from the middle of the country eastward into the Great Lakes and New England. The best focus will initially be near the lifting surface wave with widespread showers across northern New York and New England Fri/Fri night. With the flow expected to amplify this weekend across the Rockies, robust QPF amounts are noted in many model solutions with a particular focus rainfall across the Central Plains into the middle or lower Mississippi Valley. As the trough develops early net week, showers increase across the northern Plains to upper MS Valley and then move out of the mid MS Valley into the Ohio and TN Valleys. Across the southwest, terrain aids in the development of diurnal showers/storms in the central to southern Rockies and Mogollon Rim of AZ, with heavier amounts further south over southern Arizona where higher precipitable water air is available. Petersen/Santorelli 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