Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM 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 next Tuesday... ...Overview... A strong upper ridge should persist over the U.S.-Mexico border, perhaps weakening slightly around midweek and then rebuilding by Friday-Saturday. This ridge will maintain 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. Guidance still shows weakening of an upper ridge that initially bridges across the Florida Peninsula through Tuesday or so, to the south of an eastern North America trough whose embedded shortwaves (from Pacific through northern tier U.S. flow along with some potential 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. Farther west, a northeastern Pacific upper trough and weakening frontal system could bring some rainfall to the Pacific Northwest around the middle of next week. An upper low/trough expected to develop near the California coast by Friday-Saturday may include some energy aloft initially associated with an East Pacific tropical system forecast to develop soon, but with fairly modest effects on sensible weather. ...Guidance Evaluation/Predictability Assessment... Latest guidance continues the theme of reasonable agreement for the overall mean pattern but very low predictability for embedded details due to their relatively small scale and in some cases dependence on convective specifics. Resolving the Great Lakes into New England system Tuesday-Wednesday is merely one example of this type of pattern's sensitivity. 24-36 hours ago there was a firm majority suggesting that the primary surface low would track north-northeast into Canada with the trailing front, possibly with a triple point wave, crossing the Northeast in fairly progressive fashion. However the 12Z guidance with continuation of new 00Z runs so far show the main low tracking from the Great Lakes through or near New England somewhat more slowly, due to somewhat less interaction of southern Canada energy with the northern tier U.S. shortwave. 18Z/00Z GFS runs from yesterday were the first to offer this idea. Behind this system, additional complexities await as a southern Canada upper low and shortwave energy emerging from the northern Rockies should support a potentially wavy northern tier into Midwest/Great Lakes front (eventually reaching the East Coast). Most guidance has been depicting this system for the past day or so but specifics will likely take a while to get resolved. More shortwave energy tracking into North America from the Pacific should produce yet another wave/frontal system but 12Z/18Z guidance strayed out of phase. The average of models/means tilts slightly toward a wave/front reaching the Upper Midwest/Great Lakes by day 7 Saturday versus the high pressure forecast by the 12Z/18Z GFS. The new 00Z GFS has adjusted much closer to the 12Z ECMWF/ECens/CMC scenario. Shortwave details are also quite uncertain for the upper trough/low forecast to evolve near California. There is general agreement that this feature should incorporate upper level energy associated with a soon-to-develop East Pacific tropical system but there is a lot of spread for timing its arrival. Note that this upper energy will become detached from the surface circulation that should weaken farther to the south. A blend of 12Z/18Z operational models reflected the consensus shift for the Northeast system and the best intermediate scenario for other features early in the period. The increasing detail spread later in the forecast favored a steady increase of 18Z GEFS and 12Z ECens/CMCens input, reaching 60 percent total weight by day 7 Saturday. ...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 has introduced a Slight Risk area from upstate New York eastward/northeastward, 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 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. With guidance thus far not clustering well enough to resolve any meaningful Slight Risk areas, plan to depict a Marginal Risk from the Midwest/Mississippi Valley into the Southeast and wait for improved clarity/focus for one or more embedded Slight Risk areas in future runs. Parts of the northern Plains may be worth monitoring given a wavy front over the area and a diffuse signal for enhanced rainfall, but there is not yet enough consensus for a risk area at this time. 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. 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 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