Extended Forecast Discussion NWS Weather Prediction Center College Park MD 247 PM EDT Sat Aug 05 2023 Valid 12Z Tue Aug 08 2023 - 12Z Sat Aug 12 2023 ...Heat wave to persist over the South into the Southwest... ...Excessive rain/flash flood threat for the eastern Great Lakes into the Northeast Tuesday, and part of the Middle Mississippi and Tennessee Valleys Wednesday... ...Overview... A strong upper ridge will persist over the U.S.-Mexico border, weakening slightly mid-week then rebuilding by Friday-Saturday. This ridge will continue to fuel the ongoing southern tier heat wave with daily record high temperatures possible every day next week. The most anomalous temperatures will occur under the center of the ridge which will likely be positioned right over Texas. Upper level troughing will be in place over the north-Central and eastern U.S., and multiple shortwaves transiting around the base of the trough will support at least a couple frontal systems with the potential to create locally heavy rainfall. In the West, upper level troughing and weakening frontal system will bring rainfall to the Pacific northwest mid-week, then weak ridging and dry conditions will return late next week. The National Hurricane Center is issuing advisories for a tropical system in the Pacific, which will remain offshore but could have modest effects on sensible weather. ...Guidance Evaluation/Predictability Assessment... Latest guidance continues to show reasonable agreement on the overall mean pattern, but low predictability of embedded smaller scale details. Uncertainty in the timing, position, and strength of upper level shortwaves will likely have the most influence on the forecast, impacting frontal progressions and strength of low pressure systems. Models generally agree on timing and location of the shortwave over the Northeast early in the period, but agreement significantly declines for the following shortwaves over the rest of the period. The second wave will likely move off the Rockies and across the central U.S. with heavy rainfall potential on Wednesday. The GFS and CMC bring this feature across earlier while the ECMWF lags behind a bit. This has created significant uncertainty in the location of heavy precipitation and flash flood threat. A frontal system will closely follow the wave of energy and push precipitation into the eastern U.S. on Thursday. Late next week, another shortwave will likely move across the northern tier of the nation with yet another frontal system pushing south from Canada. Shortwave details are also relatively uncertain concerning the upper level energy associated with the surface tropical system in the Pacific. The upper level low will become nearly stationary offshore of California, and may drift near enough to the coast to have some effect on sensible weather. For this forecast, a blend of operational models was used for the first half of the period, and ensemble means were introduced to the blend for the second half. ...Weather/Hazards Highlights... The rather vigorous system forecast to track from the eastern Great Lakes through or near New England during Tuesday-Wednesday will spread potentially strong thunderstorms/heavy rainfall across portions of the Northeast. The Day 4 Excessive Rainfall Outlook for Tuesday-Tuesday night maintains a Slight Risk area from upstate New York to Maine, with the recent shift in guidance for the system's structure and slower timing leading to somewhat higher rainfall potential over the region. Some areas will be particularly sensitive given recent significant rainfall. As for other Day 4 areas, parts of the Lower Mississippi Valley and Southeast remain in a Marginal Risk with the Northeast system's trailing front settling over the region while the combination of a couple fronts may produce locally heavy rainfall within a Marginal Risk area over the north-central Plains. During the Day 5 outlook period of Wednesday-Wednesday night, expect a fairly broad area of locally heavy rainfall potential due to a northern tier front progressing southeastward into the Midwest and persistence of the front over the South. Guidance spread is still relatively large, making it difficult to target areas with the highest flash flood risk, but there was some overlap in solutions in the Mid Mississippi and Tennessee Valleys where a Slight Risk area was introduced. Additional Slight Risk areas and/or adjustments to the current Slight Risk area will likely be needed in future forecasts as details become more clear. Continued progression of the Midwest front should bring showers and thunderstorms into the East by later in the week, with some heavy activity still possible through Thursday and maybe becoming somewhat less organized by Friday-Saturday. Confidence is lower for another possible system/front that could produce some showers/storms over parts of the Plains/Midwest toward Friday-Saturday. Meanwhile the Pacific Northwest should see some rainfall Tuesday-Wednesday as a weakening front approaches. Amounts should be light to moderate in absolute terms, but still somewhat unusual for early August. Convection over the Four Corners states/Southwest should be lighter and more scattered than average given below-climatology precipitable water values through midweek or so. Activity may increase slightly toward Friday-Saturday as moisture gradually increases toward more normal levels. The ongoing heat wave over the southern tier should continue through next week, with greatest emphasis over the southern High Plains where highs could reach 10-15F above normal. Temperatures may reach or exceed daily record highs most days next week from the western Gulf Coast into the southern Rockies. The Desert Southwest should be quite hot as well with highs reaching around 110F, but may stay a bit short of record highs. 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 coolest temperatures versus normal will be over the northern Rockies/High Plains next Tuesday-Wednesday with some highs up to 10-15F or so below average. Temperatures should trend closer to normal thereafter, while other locations across the northern half of the Plains/Mississippi Valley may see one or more days with highs modestly below normal. The upper trough/low developing near California should keep highs over most of the state near or slightly below normal. Rausch/Dolan 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