Extended Forecast Discussion NWS Weather Prediction Center College Park MD 301 AM EDT Tue Jul 11 2023 Valid 12Z Fri Jul 14 2023 - 12Z Tue Jul 18 2023 ...Hazardous to extreme heat will build across much of the Southern U.S. and into the Southwest through early next week and beyond... ...Heavy rain concerns likely to focus from the central Plains eastward to the Northeast... ...Overview... The upper level pattern over the CONUS late this week and into the weekend may trend slightly more amplified with time as troughing is maintained across the Eastern U.S., and upper level ridging builds into the West. The Eastern trough will be anchored by a deep upper low meandering near Hudson Bay with multiple waves of energy rotating through the base driving rounds of showers and storms across the nations midsection and into the Northeast. A potentially dangerous heat wave will be tamping up as the period begins Friday across much of the Southern tier of the U.S. and back into the Southwest lasting through the weekend and even well into next week, as per the latest information from the Climate Prediction Center. There is increasing confidence this could be a long duration heat wave for much of the South and Southwest. ...Guidance Evaluation/Predictability Assessment... The latest guidance remains in reasonable agreement regarding the overall synoptic pattern across the CONUS during the medium range, though continues to offer typical differences in the details. Some uncertainties surrounding impulses rounding through the Midwest, which have significant implications for surface features and sensible weather, including potential heavy rainfall/flooding, especially across parts of the mid-Mississippi Valley and Midwest, and into the Northeast. Upper ridging over the Northwest early in the period should shift east with time as a closed upper low moves into British Columbia early next week. The 12z (July 10) CMC was an outlier bringing that closed low farther south into the Pacific Northwest and was excluded from the late period blend for tonight. Otherwise, even late in the period, there is relatively excellent agreement on an anomalously strong upper high building and holding strong over the Southwest U.S. and the upper low near Hudson bay dropping slowly southward and grazing the Upper Great Lakes next Tuesday. The blend for the WPC forecast tonight used the deterministic models through day 5, with increasing contributions from the ensemble means late period with the ECMWF and GFS. ...Weather/Hazards Highlights... A frontal boundary dropping slowly southward through the Midwest this weekend will be the focus for daily rounds of showers and storms. Given available moisture and instability, heavy rainfall seems a good bet for portions of the Mid-Mississippi Valley into the Midwest, but exact location remains rather uncertain still. The best overlap in the current available guidance points to the southern Missouri to Kentucky region, where a slight risk was placed on the Days 4 Excessive Rainfall Outlook. The northern portion of this front will move into the Northeast with rounds of additional rainfall, and given how susceptible this region is, a slight risk was maintained on the day 4 ERO for portions of New England. By next weekend, rainfall should drift south with the front more into parts of the south-central Plains to the mid-South and the Northeast. A broad marginal risk was drawn on the day 5 ERO along the entire boundary. It is plausible embedded slight risks will be needed closer in time, but there is still a lot of uncertainty in exactly where to preclude anything at this time. Showers should linger early next week across parts of the Gulf Coast into the Southeast and Mid-Atlantic region. Hazardous heat is expected to expand across the southern tier through at least this coming weekend (and likely beyond per latest CPC forecasts) as the upper ridge strengthens over the Southwest and into much of the South. Much above normal temperatures will expand from the southern High Plains into the Southwest and California, with potential for numerous record daily daytime highs and warm overnight lows possible. Farther east, the main story will be oppressive humidity leading to heat indices of 105-115 (locally higher) for a large portion the South from eastern Texas into the Southeast and Florida. Again, some daily temperature records are possible. See key messages issued by WPC for additional information and graphics related to this heat wave. Elsewhere, a cold front dropping south from Canada should bring temperatures down to 5-15F below average over parts of the northern and central Plains through Sunday with some moderation early next week as upper ridging tries to build in. 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