Extended Forecast Discussion NWS Weather Prediction Center College Park MD 224 AM EDT Mon Aug 24 2020 Valid 12Z Thu Aug 27 2020 - 12Z Mon Aug 31 2020 ...Tropical Storm Laura expected to spread potential heavy rain impacts inland from the Gulf Coast to the Ohio Valley later this week... ...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 Typhoon Bavi with amplifying higher latitude flow across northeastern China over the next couple 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 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. this 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, Laura is forecast to make landfall as a hurricane along the central Gulf Coast Wed evening, with the center forecast to be located across the lower Mississippi Valley early on day 3 (Thu). Model consensus surrounding the track of Laura has continued a very slow improvement, but a significant amount of uncertainty remains. The official NHC forecast track was generally between the 12Z ECMWF and 00Z GFS, with the ECMWF continuing to move the system northeastward more quickly after landfall relative to the GFS. Please refer to products issued by the National Hurricane Center for specific details on Tropical Storm Laura. For other parts of the CONUS, a blend of the 18Z GFS/12Z UKMET served as a good forecast starting point during days 3-5 (Thu-Sat), with weight shifting toward ensemble means (GEFS and ECENS) during days 6-7 (Sun-Mon). Consensus has improved compared to yesterday with the trough expected to amplify across the West over the weekend, but there are still some timing differences, and continue to favor the slower side of the spread given the overall amplifying flow pattern from Asia and the North Pacific east to North America. Farther east, solutions differ in amplitude but are similar in timing with shortwave energy moving from the Northern Plains/Midwest to the Northeast this weekend, making an ensemble-based approach ideal for depicting a consensus solution for the feature. ...Weather/Hazard Highlights... As Laura moves inland Thu-Fri and gradually weakens, the system is expected to spread heavy rainfall, with potential flooding impacts, across portions of the lower Mississippi Valley and Tennessee/Ohio Valleys, as well as portions of the central Appalachians and Mid-Atlantic. Potential interaction of the system with the shortwave crossing the Great Lakes and associated cold front could lead to areas of enhanced heavy rainfall threat, although confidence in the specifics of this interaction, and any location where heavy rainfall would be focused, remains relatively low at this time. Elsewhere, by Sat-Sun, a trailing stationary/warm front across the central U.S., and the approaching western U.S. upper trough, will focus areas of showers and thunderstorms across portions of the Central Plains and Mid-Mississippi Valley, eventually developing farther north across the Northern Plains/Upper Midwest as the stronger height falls associated with the trough spread east. Hot conditions will continue from the Southwest to much of the central U.S. Thu through the weekend (with the greatest temperature anomalies becoming more focused across the Southern Plains over time). A few record high temperatures are possible from the southern Rockies east into the Southern Plains, with temperatures surpassing 100 deg F across portions of West Texas. The trough expected to dig into the western U.S. Sun-Mon is forecast to bring much cooler temperatures to areas from the northern Great Basin to the northern Rockies, where highs Sat-Sun are expected to be 10-20 deg F below average. Morning low temperatures could drop to near or below freezing across the northern Rockies by Sun morning. 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