Extended Forecast Discussion NWS Weather Prediction Center College Park MD 156 PM 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... Latest 00z/06z guidance from this morning continues to support a general deterministic model blend through day 5, with addition of the GEFS/ECENS means by days 6-7 given the typical increase in spread of the details. On the large scale though, models do show good agreement on the overall pattern through the entire period. The 00z UKMET is a little different with the troughing through the Northeast, but the rest of the guidance does show good consensus on that feature. Elsewhere, the 06z (and to a lesser extent the new 12z) GFS is sharper with the shortwave upstream moving through the northern U.S. resulting in a little stronger and farther south surface low pressure near the Great Lakes late week. Out West, the latest cycle of models shows the same differences as noted in the previous forecast -- the GFS tending to split energy near the Northwest coast. This presents some issues farther north into Western Canada and near Alaska as well, so trended away from the GFS for the latter half of the period. Only minor adjustments were needed from the overnight WPC medium range package. ...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, over an area which may be relatively more sensitive given above normal rainfall the past couple of weeks. Currently though, 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 latest Day 4 outlook reflects latest guidance suggestion of an eastward expansion of the Marginal Risk along a weakening front from the Mississippi/Tennessee Valley to the North Carolina coast with some potential lasting into Wednesday as well (still too uncertain for even a marginal risk though). 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 low 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. Santorelli/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