Extended Forecast Discussion NWS Weather Prediction Center College Park MD 305 AM EDT Sat Jul 08 2023 Valid 12Z Tue Jul 11 2023 - 12Z Sat Jul 15 2023 ...Heat expected to rebuild from the Southwest to the Gulf Coast next week... ...Heavy rain concerns likely to focus over parts of the Midwest and Mid-Mississippi Valley... ...Overview... The upper level pattern over the CONUS next week remains rather unchanged as a deep upper low remains anchored near Hudson Bay through the period, and upper ridging expands across the Southwest and Southern U.S.. Multiple waves of energy will rotate around the low through the Midwest and Northeast and drive rounds of showers and storms that could result in areas of heavy rainfall and flooding. Underneath the ridge, hazardous heat will build over the Southwest and Texas and expand eastward to the Gulf Coast region through next week. Hazardous heat will likely persist into the week two time frame as well per the latest forecast from the Climate Prediction Center. ...Guidance Evaluation/Predictability Assessment... Models show very good agreement on the overall large scale pattern across the CONUS through next week. As is typical for these time frames, there remains plenty of uncertainties in the details regarding a couple of shortwaves moving through mean troughing from the Midwest into the Northeast, which has implications for surface features and sensible weather. This is related to continued uncertainties in the placement of the parent upper low near Hudson Bay later next week. Models also show an upper low meandering near the Gulf of Alaska into next weekend. The ECMWF is more bullish in energy breaking off from this and moving into the Pacific Northwest around Friday into the Northern Rockies on Saturday. The ECMWF seems out of place with this idea and was not preferred in the later periods of the WPC blend tonight. Across the South, the upper high initially over the Four Corners/west Texas region should expand across much of the southern half of the CONUS by next weekend. There is some key differences remaining to be resolved in the details, particularly in the periphery area across the nations midsection impacting temperatures across the central Plains into the mid-Mississippi Valley. Blending in ensemble mean guidance later in the period seemed to mitigate these differences for now. ...Weather/Hazards Highlights... A front settling from the central Plains into the Midwest should provide the best focus for daily rounds of showers and storms through much of the medium range period, with areas of heavy rainfall increasingly likely in some areas. For the Day 4 Excessive Rainfall Outlook, there was enough model agreement for the addition of a slight risk area from eastern South Dakota into far northwest Iowa given high moisture and instability and that some of this region has seen above normal rainfall over the past week or two. A marginal risk surrounds this area into the Upper Midwest as well as back into the northern High Plains. By Day 5/Wednesday, the heavy rainfall axis should shift east with the boundary from the Mid-Mississippi Valley into the Upper Great Lakes. It seems likely there will be heavy to excessive rainfall somewhere within this region, but there is still a considerable amount of uncertainty on where. For this reason, the day 5 ERO at this point only depicts a broad marginal risk, but future upgrades to a slight risk are possible as models come into better focus. Elsewhere, showers and storms will continue across parts of the Southeast and central Gulf Coast into Wednesday near a lingering frontal boundary. Weak low pressure lifting along the Northeast coast will also continue to bring wrap around showers across parts of the Northeast. Given recent heavy rains and additional rain expected in the short range period, a marginal risk remains in place in the day 4 ERO. Additional rainfall looks to impact this region and south into the Mid-Atlantic later in the week as well. Hazardous heat is expected to return and expand across the southern tier through at least next week (and likely extending beyond per latest CPC forecasts) as the upper ridge strengthens over the South. The greatest high temperature anomalies (of 10-15F above average) are forecast under the center of the upper high over West Texas and New Mexico, with a broadening area of above normal temperatures elsewhere. Increasing humidity will result in oppressive heat indices possibly exceeding 110F from the eastern half of Texas to the Gulf Coast, and hazardously high heat indices could expand to the Florida peninsula as well late next week. Humidity will be naturally lower back into the Southwest, though air temperatures could still reach 110-115F in some places. Little relief is expected in these areas overnight with anomalously warm nighttime lows in the forecast. Above normal temperatures will also expand into the West Coast states late next week. Elsewhere, a cold front dropping south from Canada should bring temperatures down to 5-10F below average over parts of the northern and central Plains, while the Southeast and up the East Coast should stay near normal for this time of the year. Santorelli 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