Extended Forecast Discussion NWS Weather Prediction Center College Park MD 431 PM EDT Fri Aug 04 2023 Valid 12Z Mon Aug 07 2023 - 12Z Fri Aug 11 2023 ...Heat wave to persist over the South and work back into the Southwest... ...Excessive rain/flash flood threat for the Great Lakes into the Northeast early next week... ...Overview... Expect a strong upper ridge to remain centered over the U.S.-Mexico border, maintaining a southern tier heat wave with daily record high temperatures possible most days next week from the western Gulf Coast region through the southern Rockies and possibly into Arizona. Initial upper ridging across the Florida Peninsula should weaken to the south of an eastern North America trough whose embedded shortwaves (from Pacific through northern tier U.S. flow along with some influence from southern Canada) should support at least a couple frontal systems that may focus areas of locally heavy rainfall over portions of the central/eastern U.S. Meanwhile a northeastern Pacific upper trough and weakening frontal system could bring some rainfall to the Pacific Northwest around the middle of next week. ...Guidance Evaluation/Predictability Assessment... Guidance is pretty dispersed with respect to the 500mb pattern across the lower 48. This uncertainty is reflected in the model blends throughout the period. A general model blend consisting of the 00z EC/UKMET/CMC and 06z GFS was utilized on day 3. Minimal weighting was given to the UKMET because it is weaker and more progressive than the rest of the model suite with respect to the upper trough swinging through the Great Lakes region. The 06z GFS becomes an outlier on day 4 as it is more progressive with a Northern Plains shortwave and slower/weaker with an eastern U.S. trough. The 00z ECE and 06z GEFS were introduced on day 4 to mitigate some of the aforementioned uncertainty in the deterministic guidance. The mix of deterministic and ensemble mean guidance is continued through day 5, but it is worth noting that the 06z GFS and 00z EC diverge again from the rest of the guidance with respect to the shortwave propagating through the Northern Plains and Upper Midwest. The blend is reduced on days 6 and 7 to just the ensemble means and the 00z EC due to good clustering between those models. ...Weather/Hazards Highlights... The unseasonably strong system forecast to track out of the central Great Lakes during the first half of the week will spread potentially strong thunderstorms/heavy rainfall from the Great Lakes/Ohio Valley into the Northeast. The Days 4-5 Excessive Rainfall Outlooks covering the Monday through Tuesday night period depict a Marginal Risk area corresponding to this rainfall threat. Continue to monitor forecasts as shorter-range guidance may eventually provide enough refinement to support a Slight Risk area, especially over areas with wet ground conditions. General progression of this system may be one mitigating factor that could keep rainfall totals from becoming too extreme. By Day 5 Tuesday the trailing front may take on a more west-east orientation as it drops into the Lower Mississippi Valley/Southeast, which combined with sufficient moisture/instability, could lead to an axis of heavy rainfall over this region. Guidance signals are not extremely well clustered yet though, meriting a Marginal Risk area at this time. Finally, there is a Marginal Risk area proposed for parts of the central Plains on Monday when a northwest-southeast band of convection may develop. The southern tier rainfall should persist into mid-late week as the front lingers over the area and then gradually lifts north as a warm front. During the latter half of the week, the interaction of Gulf moisture with this front as well as front dropping south from the northern tier should lead to the best shower/thunderstorm focus extending from the eastern Plains into the Ohio/Tennessee Valleys with some heavy rainfall possible. Meanwhile the Pacific Northwest should see some rainfall Tuesday-Wednesday and perhaps lingering a bit into Thursday as a weakening front approaches. Organized rainfall over the northern Rockies during the short-range time frame (through Sunday) should gradually become lighter and more scattered over the course of next week. 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 Arizona, with the Desert Southwest seeing highs reaching the 110s once again. The Florida Peninsula could also see a few record highs early in the week before upper heights decline a bit. Record warm lows will be possible across a broader portion of the southern tier. 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. Some above average temperatures will also briefly spread up the West Coast following a bit of upper-ridge amplification, with highs in the central California valleys climbing as high as 105F or so next Monday. The northern half of the Rockies into parts of the northern-central Plains will be on the cool side next week, with Rockies/High Plains locations seeing the best potential for highs 10-15F below normal next Monday-Tuesday. Kebede/Rausch Hazards: - Heavy rain across portions of the Lower Great Lakes and Interior Northeast, Mon, Aug 7. - Severe weather across portions of the Lower Great Lakes, Interior Northeast, and Appalachians, Mon, Aug 7. - Flooding possible across portions of the Middle Mississippi Valley and the Ohio Valley. - Hazardous heat across portions of the Southwest, the Southern Plains, the Lower Mississippi Valley, and the Southeast, Mon-Fri, Aug 7-Aug 11. - Hazardous heat across portions of California, Mon, Aug 7. - Hazardous heat across portions of the Southern Plains and Lower Mississippi Valley, Wed-Fri, Aug 9-Aug 11. - Hazardous heat across portions of the Southeast, Mon-Tue, Aug 7-Aug 8. 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