Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Mon Jul 10 2023 Valid 12Z Thu Jul 13 2023 - 12Z Mon Jul 17 2023 ...Hazardous to extreme heat will build across much of the Southern U.S. and into the West Coast states through this week and beyond... ...Heavy rain concerns likely to focus over parts of the Midwest and Mid-Mississippi Valley... ...Overview... The upper level pattern over the CONUS late this week and into the weekend may trend more amplified with time as troughing is maintained across the Eastern U.S. and and upper level ridging builds into the West. The Eastern trough will be anchored by a deep upper low near Hudson Bay with multiple waves of energy rotating through the base driving rounds of showers and storms across the Midwest and into the Northeast. Hazardous to extreme heat will be ongoing as the period begins Thursday across much of the Southern tier of the U.S. and back into the Southwest and California through the weekend and well into next week, as per the latest information from the Climate Prediction Center. There is increasing confidence this could be a long duration and possibly intense heat wave for portions of the West and Southern U.S.. ...Guidance Evaluation/Predictability Assessment... The latest guidance continues to capture the overall synoptic evolution reasonably well, with plenty of uncertainties in the details, especially smaller scale shortwaves around the southern periphery of the Hudson Bay low which has some significant implications for frontal structure and sensible weather including potential flash flood risk areas across parts the Mid-Mississippi Valley and Midwest. There's some disagreement too with placement and eventual evolution of the upper low. 12z (Jul 9) ECMWF is displaced well west of the better consensus and by day 7/Monday, tries to split the core energy while the GFS/CMC and ensemble means support one main feature sliding east. The new 00z run for today (which came in after the WPC forecast) seems much more reasonable and in line with current guidance. The WPC forecast for tonight used a blend of the deterministic guidance through day 5, with increasing contributions from the ensemble means on days 6 and 7 to mitigate the smaller scale differences and uncertainties. ...Weather/Hazards Highlights... A frontal boundary settling from the central Plains into the Midwest/Ohio Valley should provide the best focus for daily rounds of showers and storms Thursday and Friday. 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 both the Days 4 and 5 Excessive Rainfall Outlooks. It's likely though this will shift and need to be adjusted in future updates. 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 added to the Day 5 ERO for eastern portions of New England. By next weekend, rainfall should drift south with the front more into parts of the south-central Plains with lingering showers across the Northeast. Hazardous heat is expected to expand across the southern tier through at least next weekend (and likely beyond per latest CPC forecasts) as the upper ridge strengthens over the South and into parts of the West. 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, air temperatures should be more near normal, but the real story will be oppressive humidity leading to heat indices of 105-115 for a large portion the South from eastern Texas into the Southeast and Florida. Again, some daily temperature records are possible. Temperatures into the 90s to near 100 are possible Thursday farther north into the south-central Plains but should cool off by the weekend as upper level troughing moves across the region. 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