Extended Forecast Discussion NWS Weather Prediction Center College Park MD 305 AM EDT Sun Aug 06 2023 Valid 12Z Wed Aug 09 2023 - 12Z Sun Aug 13 2023 ...Heat wave to persist over the South into the Southwest... ...Excessive rain/flash flood threat for portions of the Middle Mississippi and Tennessee Valleys Wednesday... ...Overview... An elongated upper ridge extending from northern Mexico/southern Texas across the Gulf of Mexico and Florida Peninsula as of Wednesday should weaken a bit into Thursday but then strengthen once again over the southern Rockies/Plains and northern Mexico late week through the weekend. This ridge will maintain the ongoing southern tier heat wave with daily record high temperatures possible every day of the forecast period, with the most anomalous temperatures likely to be over Texas. To the north, expect broadly cyclonic flow with the axis of a broad trough centered over the eastern half of the lower 48. Embedded shortwaves progressing from the Pacific into/through the trough will produce a series of central-eastern U.S. waves/frontal systems that could produce episodes of locally heavy rainfall. The scale of the shortwaves is small enough to yield lower predictability for day-to-day specifics versus the larger scale pattern by the latter half of the period. In the West, upper level troughing and a weakening frontal system will bring rainfall to the Pacific northwest into Wednesday followed by rising upper heights downstream from an upper low settling over or near the Aleutians. Meanwhile the National Hurricane Center is issuing advisories for Tropical Storm Eugene in the Pacific. The surface reflection should dissipate well offshore after Wednesday but the system's upper level energy and possibly some moisture could get incorporated into an upper trough/low forecast to evolve along or a little offshore the California coast. ...Guidance Evaluation/Predictability Assessment... The latest cycle of model/ensemble guidance offers nothing new in terms of the recent theme of good pattern agreement but low confidence in various specifics due to the low predictability of embedded shortwaves. Therefore the forecast approach continues to favor an operational model blend early and then a steady shift toward more ensemble input (12Z ECens/CMCens and 18Z GEFS) toward the end of the period next weekend. After the unseasonably strong system departs from northern New England on Wednesday, there are at least some common ideas for the trailing wavy southern tier front whose eastern end should lift northward as one embedded wave reaches into the Mid-Atlantic. However shortwave differences aloft lead to a fair amount of spread for specifics along the East Coast by Friday. Meanwhile a Pacific Northwest/British Columbia shortwave will likely continue eastward and bring an area of low pressure and associated frontal system into the Upper Midwest toward the end of the week. From that point onward there is notable timing spread, with latest GFS/CMC runs tending to be on the fast side and recent ECMWF/ECens/CMCens runs slower. With modest confidence the blend has maintained somewhat more ECMWF/ECens/CMCens weight later in the period given their relative majority status with better continuity. Some GEFS runs have had difficulty depicting this system. GFS/ECMWF runs and to some degree ensemble means are suggesting another front may reach the northern tier by next Sunday. ...Weather/Hazards Highlights... During the Wednesday-Thursday night period covered by the Days 4-5 Excessive Rainfall Outlooks, the most prominent area of focus for heavy rainfall/flash flood potential will extend from the east-central Plains into the Appalachians and Mid-Atlantic. The Day 4 ERO depicts a Slight Risk area extending from eastern Kansas into parts of the Lower Ohio Valley and Tennessee Valley as a southern Plains wave should increase the flow of moisture interacting with a leading warm front while a separate front drops from the northern tier into the Midwest. Guidance signals are steadily improving with showing the potential for heavy convection over this general area, though with typical detail spread for this far out in time. An embedded upgrade may be possible depending on how shorter-term guidance resolves this convection and potential overlap with sensitive areas due to antecedent conditions. By Day 5, the best model/ensemble clustering becomes more confined over the southern Appalachians where the outlook shows a Slight Risk area. Signals are more diffuse within the surrounding Marginal Risk area. Meanwhile most guidance is showing an area of locally heavy rainfall potential over parts of the northern Plains/Upper Midwest in the Day 5 period due to the approaching northern tier system. The combination of forecast rainfall totals, moderate progression, and detail spread merit a Marginal Risk area for this activity. From Friday onward expect additional areas of rain and thunderstorms across the eastern U.S. but with steadily decreasing confidence for specifics due to uncertainty with details/timing of individual waves/frontal systems. Another area of showers/storms may develop over the Plains during the weekend. There has been some continuity recently for a potential increase of deep moisture to reach the Florida Peninsula next weekend, with corresponding higher rainfall totals. The Pacific Northwest should see some rainfall continue into Wednesday as a weakening front reaches the area. Scattered showers and thunderstorms will be possible over the Four Corners states and the Sierra Nevada. Some upper level moisture from Tropical Storm Eugene may enter the West but at this time it is not clear how much this could affect rainfall totals. The ongoing heat wave over the southern tier should continue through the week and next weekend, with greatest emphasis over the southern High Plains where highs could reach 10-15F above normal especially mid-late week. Temperatures may reach or exceed daily record highs most days during the period from the central/western Gulf Coast into the southern Rockies. The Desert Southwest should see a brief lull in the 110F+ highs mid-late week but temperatures may reach into the low 110s once again by next Sunday as the upper ridge strengthens. The Florida Peninsula could also see a few record highs. Record warm lows will be possible across a broader portion of the southern tier each night. High humidity should lead to maximum heat indices at least in the 110-115F range along and near the western half of the Gulf Coast. In contrast, the forecast pattern will tend to favor near to below normal high temperatures from the northwestern U.S. into the Midwest/Ohio Valley. The upper trough/low developing near California should keep highs over most of the state near or slightly below normal as well. Areas near the Pacific Northwest coast may be an exception with a building upper ridge helping to push readings above normal by late week and next weekend. 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 forecast, excessive rainfall outlook, winter weather outlook probabilities, heat indices and Key Messages 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/#page=ero https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml https://www.wpc.ncep.noaa.gov/#page=ovw