Extended Forecast Discussion NWS Weather Prediction Center College Park MD 258 AM 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... The main band of westerlies will carry along two primary features during the period. The first, a shortwave that should progress from the northern Rockies into the northeastern U.S./eastern Canada, will push along a frontal system from the Plains/Upper Midwest into the East with accompanying rain/storms. The heavy rain potential expected for multiple days over the Upper Midwest should extend into Saturday and then taper off as the front moves eastward. 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. Upper ridging will likely prevail across the southern tier for most of the period plus much of the East during the weekend. Over the past day or so guidance has been changing rapidly for a possible tropical system forecast to originate from a Caribbean tropical wave, track into the Gulf of Mexico, and find a weakness in the southern tier mean ridge aloft. Trends are steadily increasing the threat of a heavy rainfall event somewhere along the central/western Gulf Coast early next week but with limited confidence in specifics at this time. ...Guidance Evaluation and Preferences... The full range of model and ensemble guidance provides good support for a consensus approach with respect to the northern stream shortwaves and associated surface systems. Most guidance has been stabilizing and clustering better for the system emerging from the Rockies/Plains and continuing eastward thereafter. Models suggest that the upper trough reaching western North America next week may consist of multiple pieces of energy, favoring a trend toward some ensemble weight to downplay lower confidence details. Multiple UKMET runs through the 12Z cycle had been out of sync with most solutions but the new 00Z run finally agrees better with consensus. Meanwhile the models and ensembles have displayed considerable spread and trending for the tropical system that may develop from the wave initially over the Caribbean. Just 24-36 hours ago a majority cluster was showing a track into Mexico south of the Rio Grande but with a minority showing a farther north scenario. Then the average of 12Z-18Z guidance more emphatically brought the track into southern Texas with the 12Z ECMWF even reaching over or east of the Texas/Louisiana border. The new 00Z GFS/CMC have now trended toward the latest ECMWF runs (with timing differences). The 00Z UKMET is in line with the previous farther west majority through the end of its run. Based on guidance available through the 18Z cycle, the manual forecast emphasized an average among the 12Z CMC/12Z-18Z GFS and old 00Z ECMWF plus a compromise with continuity that was similar in principle. 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... Latest trends are increasing the threat of a heavy rainfall event somewhere along the central/western Gulf Coast around early next week in association with tropical development that may track over the Gulf and into the southern U.S. The majority cluster of guidance has been shifting rapidly to the right over the past couple days so confidence is fairly low in where the ultimate track may lie. 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 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 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