Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 PM EDT Mon Aug 23 2021 Valid 12Z Thu Aug 26 2021 - 12Z Mon Aug 30 2021 ...Upper Midwest heavy rain threat Friday into the weekend... ...Overview... The latest model guidance shows average to above average agreement for the upper level pattern and trended well from continuity. This adds forecast confidence to the period. Shortwave troughing over the Pacific Northwest will gradually extend eastward toward the Northern Plains and Upper Midwest by late this weekend into early next week, around the north side of a broad mean ridge that extends from the Atlantic through eastern/southern parts of the lower 48. Periodic impulses moving through the flow will support episodes of potentially heavy rainfall across the Upper Midwest beginning Friday through the weekend. Areas to the south of the fronts will tend to be humid and very warm to hot with some diurnally favored showers and thunderstorms. Possible tropical development over the northwestern Caribbean and southwestern Gulf of Mexico may bring some enhanced moisture to the western half of the Gulf Coast region by Sunday-Monday. ...Guidance Evaluation and Preferences... The model guidance has clustered together somewhat for the expected upper level pattern late this week into early next week, particularly with the progression and impulses of shortwave troughs through the Northwest to Upper Midwest. The first wave looks to arrive late Friday with the deterministic guidance agreeing well. Perhaps some minor timing differences but all the models then advertise upstream energy carving out a deeper trough over the Northwest/Northern Rockies Saturday/Saturday night. Beyond that, as that wave ejects into the Upper Midwest, there is some amplitude and timing issues, with the CMC notably stronger while the ECMWF a bit flatter/progressive. The 00Z GFS was on the slower side of the model spread. A consensus approach, favoring the GFS and ECMWF, were chosen. Another shortwave trough or closed low will approach the Pacific Northwest early next week with some of the typical model biases seen. A tropical wave across the southwest Caribbean will be watched closely by the National Hurricane Center for possible development this weekend into early next week as the system is forecast to track into the far western Gulf. For now, the guidance is reasonably clustered for day 6 and day 7 with the coordinated position near the model consensus. Overall, preferences followed an operational model blend early and then transitioned to a model/mean mix including the 06Z GFS/GEFS mean and 00Z ECMWF/ECMWF mean/CMC. This led to reasonable continuity in principle while reflecting some adjustments as reflected in the majority of latest guidance and downplaying details that have the lowest confidence. ...Sensible Weather/Hazard Highlights... A strong heavy rainfall signal exists for portions of the Upper Midwest / Upper Mississippi River Valley Friday into the weekend as several impulses move through the area at the nose of a warm/moist air mass. Multiple episodes and possible training storms could lead to some significant rainfall totals. Despite some antecedent dryness to the soils for some parts of the area, the multiple day event and potential higher end rainfall totals in a short duration is likely to overcome that and the repeated nature is likely to become problematic over some areas. Well in the warm sector across the central/southern parts of the East, more typical summer-time diurnally-favored convection within the hot and humid airmass. 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 and/or southern Mid-Atlantic. Expect rainfall to increase along the western half of the Gulf Coast Sunday-Monday as possible tropical development reaches the southwestern Gulf of Mexico. Daily convection over the southern Rockies and Arizona may gradually increase in coverage and intensity over the course of the period. 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 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