Extended Forecast Discussion NWS Weather Prediction Center College Park MD 153 AM EDT Sun Aug 23 2020 Valid 12Z Wed Aug 26 2020 - 12Z Sun Aug 30 2020 ...Two tropical systems (Marco and Laura) are forecast to affect the southern/southeastern U.S. this week... ...Heat continues for the Desert Southwest while much above normal temperatures spread across much of the northern U.S. through Thursday... ...Overview and Guidance/Predictability Assessment... Ensembles show the flow pattern across the northern hemisphere undergoing a fairly significant amplification during the extended forecast period. One contributor to the pattern shift appears to be interaction of across eastern Asia of Tropical Storm Bavi with amplifying higher latitude flow across northeastern China over the next few days. This appears to trigger a downstream amplification of mid/upper-level flow across the North Pacific, eventually reaching North America next weekend, with development of relatively strong ridging (+2 standard deviations) across the northeast Pacific and the Gulf of Alaska, and troughing of similar magnitude setting up across the western CONUS. The amplification will likely continue even farther east, with an early fall-like trough expected to amplify across the Great Lakes/Northeast U.S. next weekend. This signal is noted across ensembles, and thus confidence in this large scale aspect of the forecast is at least a bit above average. Focusing in on the details, by the time the medium range forecast period begins on day 3 (Wed), Tropical Storm Marco is forecast to have made landfall along the central Gulf Coast and dissipated, and all eyes in that part of the country will then turn to (forecast to be) Hurricane Laura. Laura is forecast to make landfall as a hurricane along the central Gulf Coast Wed afternoon/evening. Model consensus surrounding the track of Laura has improved somewhat over the past day, but a significant amount of uncertainty remains, especially later in the forecast and after the system makes landfall. The GFS and ECMWF were initially both close to the NHC track on Wed. By Thu, the differences begin to grow, with the GFS taking the system farther west, while the ECMWF is farther eats but accelerates the system northeastward much more quickly than other guidance. Please refer to products issued by the National Hurricane Center for specific details on Tropical Storm Laura. A blend of the 18Z GFS/12Z ECMWF was used heavily in the WPC forecast during days 3-5 (Wed-Fri). More emphasis was placed on the GFS relative to the ECMWF, for a few reasons. The ECMWF solution accelerating Laura northeastward more quickly than other guidance was not preferred, based on the official NHC forecast. Additionally, the ECMWF was faster than the overall consensus with northern stream shortwave energy diving southeastward across the Upper Midwest and Great Lakes Fri. Later in the forecast period, emphasis on ECENS/GEFS ensemble means was increased coincident with increasing spread on the forecast details. The GFS continued to show some utility and remained the preferred deterministic solution over the ECMWF. As the flow pattern becomes more amplified, that should favor somewhat slower progression of shortwave features, and the ECMWF appeared to be too quick with progressing height falls, associated with the amplifying western trough, eastward across the northern Rockies by next weekend, while the GFS was somewhat slower and more in line with a consensus of ensemble members. ...Weather/Hazard Highlights... As Laura makes landfall along the central Gulf Coast on Wed, heavy rainfall is expected to spread across areas near the track of the system. Based on the current NHC forecast, several inches of rainfall along with potential flooding impacts are forecast Wed/Wed night from portions of the central Gulf Coast northward into the lower Mississippi Valley. By Thu, the system is forecast to move northward and begin to make a northeast turn, spreading potentially heavy rains into portions of the Tennessee and Ohio Valleys. Based on this track, as the remnants of Laura reach the central Appalachians and Mid-Atlantic on Fri, there is the potential for interaction with the amplifying Great Lakes/Northeast upper-level trough and associated frontal boundary, potentially leading to heavy rain impacts for portions of the central Appalachians/Mid-Atlantic/Northeast. This is highly dependent on the eventual track of Laura, however, and confidence in the specifics of the forecast by that time frame is below average. Above average temperatures are expected during the first half of the medium range from the Southwest to the north central U.S. Many locations in the Southwest will continue to see temperatures soar well past the 100 deg F mark, although not to the extent seen during the peak of the recent heat wave. Isolated record max temps will remain possible through Wed. Temperatures from portions of the north central U.S. to the northern Mid-Atlantic will also see max temperatures 5 to 15 deg above average Wed-Thu ahead of an incoming cold front, with many areas reaching well into the 90s. The pattern change (described above) later in the week will bring an end to the hot temperatures for many areas by next weekend. High temperatures next Sun are forecast be 5 to 10 deg below average across much of the Northwest, as well as the Great Lakes and Northeast. Ryan Additional 3-7 Day Hazards information can be found on the WPC medium range hazards chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, winter weather outlook probabilities and heat indices are 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