Extended Forecast Discussion NWS Weather Prediction Center College Park MD 325 PM EDT Tue Aug 24 2021 Valid 12Z Fri Aug 27 2021 - 12Z Tue Aug 31 2021 ...Upper Midwest heavy rain threat likely to extend into the weekend... ...Heavy rain possible for parts of southeast Texas early next week associated with tropical development... ...Overview... Latest guidance maintains good continuity and agreement with the mean pattern, showing the main band of westerlies extending from the northeastern Pacific through the northern lower 48 and southern Canada while ridging prevails from the Atlantic through the eastern/southern U.S. A series of shortwaves emerging from western North America and associated surface waves/fronts are likely to promote a period of heavy rain potential over parts of the Upper Midwest, extending from the short range period into the weekend. Areas south of the fronts will see humid and very warm to hot weather with some late-day showers and thunderstorms. Possible tropical development over the northwestern Caribbean and western Gulf of Mexico may bring enhanced moisture to the western half of the Gulf Coast region by early next week. ...Guidance Evaluation and Preferences... Models are still in the process of refining the details across the northern tier. Models continue to show fairly good agreement on a leading north-central Plains wave that may track northeast through the Upper Great Lakes into Canada Friday into the weekend, followed by a stronger system in response to trailing energy that emerges from the Northwest. Some timing differences exist with the latter energy especially after day 4, with the 00z ECMWF and UKMET faster than the 06z GFS and 00z CMC. The ensemble means, albeit weaker, show good agreement on a more middle ground solution. Farther upstream a northeastern Pacific trough/embedded upper low should arrive into the Northwest U.S. and western Canada by early next week. Ensemble means have been fairly consistent with this trough and provide a good anchor for its forecast. Recent deterministic model runs have varied somewhat faster or slower but with an overall average near the means. The latest guidance continues to suggest the development of a possible tropical system extending from the northwestern Caribbean into the western Gulf of Mexico early next week. The CMC had been consistently more south, bringing the low into central Mexico, but the latest 12z run for today came way north bringing a low into the central Texas coast late next Monday. This northward shift is more in line with the 12z GFS (which has also trended north the past few runs) and the more consistently north ECMWF. There remains significant uncertainty in the ensemble guidance though so confidence is low, but potential for more significant impacts to parts of the Texas coast appears to be increasing. Consult the National Hurricane Center's Tropical Weather Outlooks for the latest information on this development potential over the coming days. The updated WPC forecast started with a composite of latest deterministic models days 3-4. After that, increased contributions from the ensemble means in an attempt to mitigate the differences in northern stream energy lifting into central Canada and incoming energy into the Northwest. For days 6-7, did maintain some smaller portions of the ECMWF and GFS just to get some definition in the forecast. This maintains good continuity with the previous WPC forecast as well. ...Sensible Weather/Hazard Highlights... The signal for heavy rainfall persists over portions of the Upper Midwest from the short range period into the weekend as multiple impulses move through the area around the northern periphery of a warm/moist air mass. Multiple episodes and possible training storms could lead to significant rainfall totals at some locations. Antecedent dryness in parts of the region could initially blunt the effects of heavy rain but the multi-day nature of the event and potential higher end rainfall totals in a short duration are likely to overcome that with runoff issues becoming problematic over some areas. Well into the warm sector across the central/southern parts of the East, more typical summertime diurnally-favored convection should occur within the hot and humid air mass. A front settling into the Mid-Atlantic during Friday-Sunday may provide some added focus for rainfall, while upper level impulses could enhance convection over parts of the South. A cold front advancing from the Plains should bring a more organized area of showers/storms into the East by Monday-Tuesday. For the Gulf coast, expect rainfall to increase across parts of southeast Texas Sunday onward as possible tropical development most likely reaches the southwestern Gulf of Mexico. Guidance has trended north, increasing potential for heavy rainfall and tropical impacts more north into the southern/central Texas Gulf Coast region. Confidence in exact locations of the heaviest rain remains low at best right now though. Above normal temperatures will prevail from the Midwest into the Great Lakes/Ohio Valley and Mid-Atlantic through the weekend with some moderation thereafter as a cold front emerges from the Plains. Through early Monday there should be decent coverage of morning lows up to 10-15F above normal while isolated locations could reach plus 10F anomalies for highs. Some daily record warm lows are possible. In contrast the northern Rockies/Plains will be rather cool late this week into the weekend with some locations seeing one or more days with highs 10-15F below normal. Highs of 5-10F above normal will spread from the Southwest into California and parts of Oregon Friday-Sunday. Then Southern Rockies moisture reaching the Southwest and the upper trough nearing the Northwest will support a gradual cooling trend. Santorelli/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 Hazards: - Flooding occurring or imminent across portions of the Mid-Atlantic and the Northeast. - Flooding likely across portions of the Mid-Atlantic. - Excessive heat across portions of California and the Southwest, Fri, Aug 27. - Heavy rain across portions of the Great Lakes, the Upper Mississippi Valley, and the Northern Plains, Fri-Sun, Aug 27-Aug 29. - Heavy rain across portions of the Lower Mississippi Valley and the Southern Plains, Sun-Tue, Aug 29-Aug 31. 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