Extended Forecast Discussion NWS Weather Prediction Center College Park MD 311 PM EDT Sun Jun 30 2024 Valid 12Z Wed Jul 03 2024 - 12Z Sun Jul 07 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 to southeastern states... ...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/Middle Mississippi Valley 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 midweek that will fuel scattered storms, with a brief break in the moist pattern before it likely returns this weekend. ...Guidance/Predictability Assessment... Recent model guidance remains in good agreement with the mid- to large scale features with just some minor mesoscale differences with the shortwaves crossing southern Canada through around Day 5/Friday. The greatest differences in the northern stream arise with a shortwave diving from southwestern Canada into south-central Canada and/or the northwestern U.S. over the weekend. The 00Z GFS seemed most aggressive in pushing troughing from this shortwave into the western U.S. compared to the consensus of other models. The incoming 12Z model suite still shows some spread with this feature. Also of concern for model diagnostics is the track of Hurricane Beryl as it is forecast to reach the Bay of Campeche or southwestern Gulf of Mexico this weekend. Deterministic and AI/ML models and ensemble members show spread in its position. The largest clustering shows Beryl being steered into northern Mexico. But there is some guidance that curves the system farther north into the U.S., including the 00Z CMC that seemed to be a fast and northeastern outlier. Beryl's track will continue to be monitored over the next several days, and see NHC's website for official forecasts. The updated forecast was based on a blend of the deterministic models favoring the 06Z GFS and 00Z ECMWF early in the period, with inclusion and gradual increase of the GEFS and EC ensemble means to about half by Day 7. ...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 Day 4 ERO Wednesday across eastern Kansas into Missouri and western Illinois. Marginal Risk areas stretch east into the Ohio Valley/Lower Great Lakes where scattered convection with potentially heavy rain rates is likely to develop ahead of a cold front. A couple of rounds of thunderstorms farther west in Kansas and vicinity prompted including that region in a Marginal Risk area as well, in the biggest change to the previously issued outlook. By Day 5/Independence Day, the overall QPF signal in the guidance is looking more diffuse with scattered higher QPF maxima, so a broad Marginal Risk still seems to be the best way to go for now, with potential Slight Risk areas in future updates as the details become more evident. Farther north, additional mesoscale convective systems are likely to form and track through north-central parts of the country. A Marginal Risk is in place for parts of the Dakotas on Day 4 and into the Upper Midwest Day 5. Good support for lift should be present there with a shortwave, but with less anomalous moisture as farther south. A cold front slowly pressing east will eventually spread thunderstorms to the Eastern Seaboard into the weekend, while more rounds of storms are possible in the central U.S. as well. 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 into the central High Plains 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, so no Day 5 ERO risk is needed there. This break may be brief, however, as tropical moisture may advect into the region again by the weekend. 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 on Wednesday- Thursday where triple digit high temperatures are likely. The heat will abate somewhat there into late week and the weekend (but still feeling summer-like), while the Southeast to Mid-Atlantic should see temperatures and heat indices rising. 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. Above normal temperatures by 10-20 degrees should stretch into Oregon and Washington late week into next weekend as well. Tate/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