Extended Forecast Discussion NWS Weather Prediction Center College Park MD 257 AM EDT Mon Aug 23 2021 Valid 12Z Thu Aug 26 2021 - 12Z Mon Aug 30 2021 ...Upper Midwest heavy rain potential late week into the weekend... ...Overview... Latest models and ensembles continue to agree fairly well on the large scale upper pattern. Northeastern Pacific flow should extend into the Northwest U.S./southwestern Canada and then east-northeast around the north side of a broad mean ridge that covers areas from the Atlantic through the eastern and southern parts of the lower 48. Shortwaves within the westerlies will support at least a couple waves/frontal systems whose associated rainfall is most likely to be heaviest over the Upper Midwest. Precise details of these shortwaves have been problematic over recent days so there is still some uncertainty over specifics. 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... By late in the week there are still some differences on the exact relative strength of the shortwave that crosses the Northeast and a western Atlantic ridge that slowly retrogrades toward the southern Mid-Atlantic or nearby, leading to some differences for the position of the front that drops into the Mid-Atlantic. Recent guidance behavior suggests an intermediate solution will provide the best option at this time. Shortwave details upstream continue to be troublesome with varying degrees of spread among models/ensembles and some trends oscillating from day to day. Consensus is starting to hint that initial shortwave energy over the Northwest on Thursday will eject and be followed by additional energy reaching the Northwest Friday-Saturday. Yesterday's 00Z GFS/ECMWF provided a significant continuity change with their slow/amplified depiction of this second shortwave but since then they have generally trended somewhat faster to eject it due to another system approaching western North America around the start of next week (which actually has decent clustering for a days 6-7 forecast). The new 00Z GFS did nudge its second shortwave a bit slower but otherwise faster model trends since 24 hours ago and modestly slower means yield decent relative agreement in the big-picture sense. The 12Z UKMET began to compare less favorably to other guidance toward the end of its run and now the 00Z run is even more at odds with consensus from the Pacific into western North America by Saturday. The forecast over the western Gulf during the latter half of the period is fairly close to yesterday's solution coordinated with the National Hurricane Center aside from the latest consensus exhibiting a slightly faster trend. This trend was incorporated about halfway compared to the previous issuance. Preferences followed an operational model blend early and then transitioned to a model/mean mix including the 18Z GFS/GEFS mean and 12Z 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 large majority of guidance still highlights the Upper Midwest and vicinity for heavy rain potential from late this week into the weekend, with multiple episodes of rain/storms and possible training that could produce significant rainfall totals. The first focus for activity will be a stalled Plains/Mid Mississippi Valley front lifting northeast as a warm front. Then a trailing cold front may exhibit some waviness and be slow to move for a while depending on shortwave details aloft. Rainfall should become somewhat lighter and less focused by around Sunday or Monday. Parts of this region have been dry recently so some of the rain may initially be beneficial. However the multi-day totals that some guidance is showing would likely become problematic over some areas. Central and southern parts of the East should see areas of diurnally-favored convection within a humid and warm to hot 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. As of Thursday an axis of warmest temperature anomalies will extend from the central Plains through the Northeast with highs up to 10F or so above normal and somewhat more coverage of morning lows 10-15F above normal. Thereafter the Plains portion of this area will gradually erode while the Northeast will trend cooler with a Friday frontal passage, leaving the Midwest into the Great Lakes/central Appalachians as the most persistently warm area into the weekend. The best potential for record warm lows (even away from the highest anomalies) will be Thursday-Friday but there may be some continuing through the weekend. Meanwhile highs of 5-10F or so above normal should migrate from the Southwest into California and western Oregon. Temperature forecasts here have become uncertain due to guidance indecision over whether an east-west upper ridge could extend across the region versus the southern periphery of a Northwest trough having greater influence. The northern Rockies/Plains will be rather cool during the period with some locations seeing highs 10-15F below normal for one or more days between Thursday and Sunday. Readings should moderate some by Monday. 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