Extended Forecast Discussion NWS Weather Prediction Center College Park MD 256 AM EDT Sun Jun 30 2024 Valid 12Z Wed Jul 3 2024 - 12Z Sun Jul 7 2024 ...Major heatwave expected for the lower elevations of California and the Desert Southwest next week, and remaining hot and humid for the south-central states... ...Pattern Overview... Multiple shortwave passages across the northern tier of the nation will fuel scattered to numerous showers and storms for the middle to end of the week, particularly across the Midwest and portions of the Central Plains where the heaviest rainfall is likely. Heat and humidity will continue to make weather headlines across much of the southern tier states as a strong ridge of high pressure aloft governs the overall weather pattern, and heat also expands across much of California and the lower elevations of Arizona and southern Nevada as the upper ridge builds across the West by the end of the forecast period. There will also be copious moisture in place across much of the Southern Rockies through mid-week that will fuel scattered storms, followed by drier weather. ...Guidance/Predictability Assessment... The 00Z models are in very good agreement for the beginning of the forecast period Wednesday, with just some minor mesoscale differences with the shortwaves crossing southern Canada. Therefore, a general model compromise works as a starting point in the forecast process. The same thinking holds true going into the 4th of July Holiday, even though the GEFS mean is a little more suppressed with the West Coast upper ridge. Similar to the past couple of days, the greatest model differences going into the end of the week are centered across the Western U.S., and with the northward extent of the upper ridge, and also with a trough dropping south near the Northern Rockies. Model spread is less for the eastern half of the U.S., and the use of the ensemble means accounted for about half of the forecast blend by next Sunday. ...Weather/Hazards Highlights... There will likely be a few mesoscale convective complexes that cross the Central Plains and Midwest on Wednesday in conjunction with a quasi-stationary frontal boundary. Deep moisture advecting northward from the western Gulf will intercept this boundary and fuel the development of showers and strong thunderstorms, some of which could train over the same general areas and lead to some instances of flooding. With the potential for some 1-2 inch per hour rainfall rates, a Slight Risk area remains valid going into the new Day 4 ERO Wednesday across portions of eastern Kansas and into central Missouri. The overall QPF signal in the guidance for the Day 5 period on the 4th is looking more diffuse with scattered higher QPF maxima, so a broad Marginal Risk area is planned from northern Oklahoma to the Ohio Valley, with potential Slight Risk areas in future updates as the details become more evident. There is also another Marginal Risk area planned for the Upper Midwest where a separate round of convection is possible. Monsoonal flow into the Southwest states will likely continue the threat for showers and storms going into Wednesday. There is a Marginal risk in place across parts of Arizona and New Mexico for the Day 4 ERO, with saturated soils in many of these areas owing to heavier rainfall in the short range forecast period. The good news is the deeper moisture should be south of the Mexico border for the 4th of July, and therefore no Marginal Risk area is planned for the new Day 5 outlook. It will continue to remain hot and humid across the Southern U.S. for the middle of the week and into next weekend, with the hottest conditions centered across the Southern Plains where triple digit high temperatures are likely. Some of this heat likely reaches the Mid-Atlantic region after a brief cool-down by the end of the week with highs returning to the 90s amid increasing humidity levels. There will likely be some abatement to the heat going into next weekend, but still feeling summer-like. It will also be getting very hot for the inland lower elevations of California and into the Arizona Deserts with widespread 105-115 degree readings likely, and locally higher for the normally warmer locations. Hamrick 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 (QPF), excessive rainfall outlook (ERO), winter weather outlook (WWO) probabilities, heat indices, and Key Messages can be accessed from: 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