Extended Forecast Discussion NWS Weather Prediction Center College Park MD 256 AM 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... ...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. Over the past day or so there has been a gradual trend toward maintaining better definition of 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. The new 00Z ECMWF dampens out the leading wave though. Some timing differences exist with the latter energy but an average is reasonable considering some of the greater spread and variability seen in previous days. Farther upstream a northeastern Pacific trough/embedded upper low should arrive into the Northwest U.S. and western Canada by next Tuesday. Ensemble means have been fairly consistent with this trough and provide a good anchor for its forecast. Recent GFS/ECMWF runs have varied somewhat faster or slower but with an overall average near the means. UKMET runs have persistently strayed out of sync with consensus, while the 00Z CMC compares better after the 12Z run became faster than the majority. A general majority of guidance has been maintaining continuity for a fairly southward track of possible tropical development extending from the northwestern Caribbean into the western Gulf of Mexico. However there are enough minority solutions showing a farther northward track over the western Gulf to temper confidence in the forecast. Interestingly the 12Z ECMWF jumped to the northern scenario versus prior runs that were well south (and maintained in the new 00Z run), and in contrast latest CMC runs have stayed south after some earlier runs had been farther north. A number of 12Z CMC ensembles still track north though. GFS runs have been consistently south. Consult the National Hurricane Center's Tropical Weather Outlooks for the latest information on this development potential over the coming days. The updated forecast started with a composite of latest operational models early in the period. Deteriorating comparisons of the 12Z CMC led to its phaseout after mid-period and ECMWF input switched more toward yesterday's 00Z run due to the 12Z run straying north of continuity for its western Gulf tropical system. The 18Z GFS maintained its contribution through the forecast while 18Z GEFS/12Z ECens components reached 60 percent total weight by day 7 Tuesday. ...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. Expect rainfall to increase along the western half of the Gulf Coast Sunday onward as possible tropical development most likely reaches the southwestern Gulf of Mexico. A lesser but meaningful probability still exists for a farther north track that would bring greater rain/wind effects to the Gulf Coast. Daily convection over the southern Rockies and Arizona may gradually spread westward and become a little heavier over the course of the period. Well 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. 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