Extended Forecast Discussion NWS Weather Prediction Center College Park MD 256 PM 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 from the Middle Mississippi to Tennessee Valleys Wednesday into Thursday... ...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 are expected to 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... Model guidance indicates the medium-range period will begin with an energetic northern stream that had been positioned southward and impinging on a stagnant upper-level high over the southern tier of the country, especially over central/eastern portions of the U.S., retreating back northward a bit through the forecast period. All of the guidance is in relatively good agreement with the placement of these large-scale features as well as the general flow pattern, with expected differences in the timing/placement of embedded shortwaves in the flow that will impact a couple expected frontal boundaries at the surface and subsequent precipitation chances. Both the deterministic and ensemble mean guidance indicate that the pattern will also begin to amplify a bit through the period, with mean troughing remaining in place over the Midwest/Northeast as upper-level ridging expands northwestward over the central/western U.S. as ridging also builds over the northeastern Pacific on an even broader scale. All of the guidance is also in agreement towards the end of the period in the evolution of a lingering upper-level low along the California coast from the remnants of Tropical Storm Eugene, likely with some temperature and possible precipitation implications for California. Good similarity run-to-run between the GFS and ECMWF indicates good predictability in the large-scale pattern overall. Given the good clustering of the guidance a general model blend of the 00Z ECMWF/CMC/UKMET and 06Z GFS is used for the first part of the updated WPC forecast. A contribution from the 00Z ECens and 06Z GEFS means is added in the mid- to late period while decreasing the deterministic guidance a bit (including the forecast time-limited UKMET) as natural differences grow at increasing time scales, specifically with regard to shortwave placement in the upper-level flow. This generally follows the prior WPC forecast blend and subsequently did not lead to many significant differences overall except for expected adjustments of surface system positions and QPF location/amounts at the mesoscale. ...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. QPF in the guidance has continued to trend upward, both in terms of deterministic amounts and ensemble probabilities for areal average totals of 1"+ over this general area. An embedded upgrade certainly looks possible if there is further consolidation of locally more significant rainfall totals, likely dependent on how shorter-term guidance resolves mesoscale convective evolution. There is also the potential some of the heavier amounts may overlap areas of the Middle Mississippi and Lower Ohio Valleys more sensitive due to recent heavy rainfall events. By Day 5, the best model/ensemble clustering becomes more confined over the Upper Tennessee Valley and 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. Although expected totals and placement will potentially be tied to organized convection which may remain progressive, areal coverage of locally heavier rainfall still warrants a Marginal Risk. From Friday onward expect additional areas of rain and thunderstorms across the central/eastern U.S. in an overall energetic and wet pattern but with steadily decreasing confidence for specifics due to uncertainty with details/timing of individual waves/frontal systems. Shower and thunderstorm chances should remain in place more broadly next week into the weekend across the Midwest/Southeast/East Coast with lingering frontal boundaries in the area. Another area of showers/storms may develop over the Plains during the weekend. 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 does not look to have much influence on precipitation chances. 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 over the West. 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, with the expectation this will likely continue beyond the current forecast period. Putnam/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