Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM EDT Sat Jul 01 2023 Valid 12Z Tue Jul 04 2023 - 12Z Sat Jul 08 2023 ...Major heatwave from Texas through the mid-lower Mississippi Valley to the Southeast to ease next week... ...Overview... The latest models and ensemble means maintain good continuity and reasonable agreement for the expected pattern from Tuesday/Independence Day through next Saturday. A weak upper trough with possible embedded low will lift through the Northeast U.S. and eastern Canada into midweek, pushing along an associated wavy frontal system that may produce some lingering rainfall early in the period. Farther west will be the primary focus of the forecast, a wavy cold front that will push slowly but steadily south and east from the north-central Plains into the southern/eastern states as an upper trough extending from Canada into the lower 48 progresses eastward. This front will likely be responsible for multiple days of showers and thunderstorms from the central into eastern U.S. Meanwhile eastern Pacific ridging should support upper trough development near the West Coast during the latter half of next week, which in turn may help to strengthen Interior West/Rockies ridging a bit. ...Guidance Evaluation/Predictability Assessment... The current distribution of guidance continues to support a forecast approach of using a composite of operational models for about the first half of the period (12Z/18Z runs in the case of this latest update) and then adding in some GEFS/ECMWF means by days 6-7 Friday-Saturday given the typically increasing spread of some model details. Majority clustering has been fairly consistent over the past day for the weak trough crossing the Northeast (with embedded low most likely tracking just west/north of Maine) even though there are finer-scale details that temper predictability. The new 00Z UKMET does not compare well to other guidance though. Consensus also looks good for the upstream trough crossing Canada and the northern half or so of the lower 48, reaching east-central North America by next Saturday. The aforementioned approach led to only minor adjustments from continuity for the upper trough and leading wavy front. Toward the end of next week the GFS has been differing from the majority for flow details over/near the Northwest, as its recent runs have been tending to push Pacific ridging more into western Canada and splitting the energy that should form the West Coast upper trough. This leads to the GFS scenario having a farther west/southwest upper low off the West Coast while more trough energy progresses farther eastward across the northern tier states and southern Canada (especially the 12Z/30 and new 00Z runs). The 00Z GEFS mean is much closer to the majority for the West Coast trough. ...Weather/Hazards Highlights... The wavy cold front progressing from the northern Plains and Upper Midwest into the eastern/southern U.S. over the course of next week will likely produce multiple episodes of showers and thunderstorms, with some locally heavy. The Days 4-5 Excessive Rainfall Outlooks covering the Tuesday through Wednesday night time frame will depict a fairly broad Marginal Risk area from parts of the Mississippi Valley into the High Plains, following the southeastward progression of convection along the cold front. Low level upslope flow over the High Plains behind the front could also enhance rainfall. Portions of the northern-central Plains may be relatively more sensitive to additional rainfall given above-average rainfall observed over the past couple weeks. Currently there is not enough agreement for combined location/amounts of heaviest QPF to favor any embedded Slight Risk areas but one or more could arise at some point depending on shorter-term guidance clustering and/or overlap with wet ground conditions. Beyond midweek expect the area of showers/storms to continue moving southward and eastward, eventually reaching into the South and East Coast regions. Persistence of the trailing end of the front over the central High Plains through the rest of the week may help to promote development of additional convective systems over the central Plains. Elsewhere, the weaker wavy front crossing the East early-mid week should also produce some areas of locally heavy rainfall given sufficient moisture and instability. Currently the best signal for some of this activity to linger into the Day 4 time frame (Tuesday-Tuesday night) is over northern New England where a Marginal Risk area is planned in light of recent and forecast rainfall leading into that outlook. Farther south, the Day 4 outlook will reflect latest guidance suggestion of a westward shift of the Marginal Risk area over the Mississippi/Tennessee Valley region for convection that could develop along a weak front. There are some indications that moisture and convective potential could increase along this front farther east over the Carolinas and vicinity by day 5 (Wednesday-Wednesday night). The signal does not appear to be quite coherent enough for a risk area yet but is worth monitoring. Areas closer to the Gulf Coast will likely see typical diurnally enhanced showers/thunderstorms of varying intensity each day. Excessive heat across the southern tier should become less widespread next week as upper ridging returns closer to climatological strength and latitude. With time, the best potential for maximum heat index values to reach 105-110F or so should become more confined to the far Southeast/Florida and southern Texas. Over the West, the greatest anomalies will be over the Pacific Northwest with highs up to 15-20F above normal into midweek followed by gradual moderation as upper troughing sets up along the West Coast. Some daily record highs may be possible Tuesday-Wednesday. Persistent upper ridging will keep hot conditions over the southern Rockies/High Plains, with some locations seeing highs 5-10F above normal on one or more days. In contrast, to the north of the Plains cold front expect unseasonably cool highs of 10-20F or so below normal to drop into the northern and central High Plains Tuesday-Thursday, with gradual moderation heading into the weekend. The leading warm sector will bring above normal temperatures from the Midwest into Northeast during Tuesday-Thursday, followed by a return to near or slightly below normal readings after frontal passage. 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, experimental 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