Extended Forecast Discussion NWS Weather Prediction Center College Park MD 326 PM EDT Wed Aug 25 2021 Valid 12Z Sat Aug 28 2021 - 12Z Wed Sep 01 2021 ...Upper Midwest heavy rain threat likely to extend through Saturday... ...Heavy rain threat for parts of the central/western Gulf Coast early next week with possible tropical development... ...Overview... A strong shortwave trough pushing through the Northern Plains and Upper Midwest and its associated surface front will continue the heavy rainfall threat across portions of the Upper Midwest through Saturday. An upstream trough with embedded upper low should reach western Canada and the Northwest U.S. by next Monday-Wednesday, with a leading frontal system pushing into the Northwest and eventually the northern Plains. Ahead of these systems, a front settling over the Mid-Atlantic by the weekend may focus some rain and thunderstorms. Attention also turns to the southern U.S. where upper ridging prevails over the weekend then subsides some as a tropical wave emerges into the western Gulf. A possible tropical system could bring a host of weather hazards to the western/central Gulf Coast early next week but confidence in specifics in lower than average at this time. ...Guidance Evaluation and Preferences... The latest deterministic and ensemble guidance showed average to above average agreement with respect to the large scale pattern, particularly with the northern stream energies, over the course of the medium range period. Initial shortwave troughing will progress through the Northern Plains and Upper Midwest this weekend then head toward the Hudson Bay region. Its associated surface front, initially stalled over the Midwest giving way to a heavy rainfall threat, will gradually move across the Great Lakes and Northeast. Upstream shortwave troughing will then drop in across the Northwest and Northern Plains where some of the typical model biases were noted. But a consensus approach was favored and this trended well from continuity. There is considerable and full range of model spread with the potential tropical system that may develop over the Caribbean. The latest guidance continues on a rightward/northeast shift, particularly as the system crosses or approaches the Yucatan Peninsula into the central/western Gulf. The CMC was a faster solution while the UKMET was on the slower side of the spread in regards to timing. The GFS and ECMWF were fairly close and near the consensus, so a blend of the two (with some favoring toward continuity) was favored for this cycle. Consult the National Hurricane Center's Tropical Weather Outlooks for the latest information on this development potential over the coming days. ...Sensible Weather/Hazard Highlights... The latest forecast guidance continues to show an increasing signal for tropical development and potential impacts along the western/central Gulf Coast early next week. The rightward/northeast shift noted over the last few cycles continues again today though there remains some larger spread beyond day 5 in all the guidance, so while confidence is increasing in a system and potential impacts, there remains uncertainty in exactly where. Elsewhere across the CONUS, the heavy rain potential over parts of the Upper Midwest should extend through Saturday ahead of a wavy frontal system whose eastward progression should finally lead to a drier trend later in the weekend and early next week. Expect areas of rain and thunderstorms of varying intensity to accompany this front as it continues into the East. A leading front draped over the eastern Great Lakes and Mid-Atlantic may promote periods of rain/storms during the weekend. Farther south away from these fronts some diurnally-favored convection will be possible within the hot and humid air mass. One or more waves/fronts ahead of the upper trough moving into western North America may bring some rainfall into the northern tier by next Tuesday or Wednesday. Scattered diurnal convection is possible over the southern Rockies/Arizona through the period. Areas from the Midwest into the Great Lakes/Ohio Valley and southern Mid-Atlantic will be quite warm to hot ahead of the cold front emerging from the Plains followed by a cooling trend after frontal passage. From the weekend into early next week there should be decent coverage of morning lows 10-15F above normal while a few locations could reach plus 10F or so anomalies for highs. Some daily record warm lows are possible. The northern Rockies/High Plains will see highs up to 10-15F below normal Saturday followed by moderation of the cool air as it continues eastward. After a rebound to around normal, parts of Montana may see below normal highs once again by next Wednesday. Portions of California/Oregon/Nevada should see highs 5-10F above normal during the weekend followed by a cooling trend and then the central High Plains should trend warmer to similar anomalies early next week. Rausch/Taylor Additional 3-7 Day Hazard information can be found on the WPC medium range hazards outlook chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php Hazards: - Flooding occurring or imminent across portions of the Mid-Atlantic. - Heavy rain across portions of the Great Lakes and the Upper Mississippi Valley, Sat-Sun, Aug 28-Aug 29. - Severe weather across portions of the Central Plains, the Upper Mississippi Valley, and the Great Lakes, Sat, Aug 28. - Heavy rain across portions of the Lower Mississippi Valley, Sun-Wed, Aug 29-Sep 1. - Heavy rain across portions of the Southeast, the Lower Mississippi Valley, the Southern Plains, and the Tennessee Valley, Mon-Wed, Aug 30-Sep 1. 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