Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM 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... Most guidance reflects the general theme of the large scale mean pattern, but most details involve shortwaves that are small enough in scale to have low predictability several days out in time. At least for now the model/mean consensus shows two primary features of interest. The first is a system forecast to track northeast from the Great Lakes, with the second being a front that drops into the northern tier around Tuesday-Wednesday and then continues into the Great Lakes/central Plains. Regarding the initial Great Lakes system, operational models suggest it will be unusually strong for August. In fact the deeper side of the envelope (12Z ECMWF/00Z CMC) would challenge this month's sea level pressure records for some stations in Michigan. The 18Z GFS strayed to the slow/south side of the spread, so GFS input for the forecast came from the 12Z GFS that compared better to other models/means. The new 00Z GFS is getting closer but may still be somewhat south. On the other hand the 00Z CMC trends slow after Monday in contrast to the fast/north 00Z UKMET. Shortwave detail issues then affect specifics of a potential frontal wave over or near New England by Tuesday-Wednesday. The trailing front during mid-late week should have dynamic support from energy emerging from the Northwest U.S. as well as an upper low/trough rotating through southern Canada. This potential combination tempers confidence but at the moment there is decent clustering among most models and ensemble means. The main discrepancy comes from the 12Z ECMWF which holds its southern Canada upper low well west of other guidance, thus causing the surface system to lag consensus. The prior 00Z/03 ECMWF was closer to the majority in principle. Guidance comparisons favored starting with a 12Z operational model blend for the first part of the period and then steadily increasing 18Z GEFS/12Z ECens mean input, reaching 60 percent total by day 7 Friday. This downplayed seemingly unfavorable traits of the 18Z GFS early in the period and 12Z ECMWF during the latter half of the period. ...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. 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