Extended Forecast Discussion NWS Weather Prediction Center College Park MD 246 AM EDT Sun Jul 09 2023 Valid 12Z Wed Jul 12 2023 - 12Z Sun Jul 16 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 next week remains rather unchanged as a deep upper low remains anchored near Hudson Bay, and upper ridging expands across the southern tier states and eventually up the West Coast. Multiple waves of energy will rotate around the southern periphery of the low and drive rounds of showers and storms that could result in areas of heavy rainfall and flooding across the Midwest and Northeast. Underneath the ridge, hazardous heat will build over the Southwest and Texas and expand eastward to the Gulf Coast and Florida, as well as back into California through next week. Per the latest information from the Climate Prediction Center, this could be the start of a long duration and possibly intense heat wave for portions of the West and Southern U.S.. ...Guidance Evaluation/Predictability Assessment... Guidance continues to maintain good agreement on the large scale evolution of the synoptic pattern during the period, but struggles with the details, which have implications on surface features and sensible weather. The 12z UKMET seemed out of line with significantly stronger troughing moving through the Northeast on Wednesday, resulting in a well defined surface low off the Northeast coast and was not included in tonight's blend. Elsewhere, the ECMWF is still a bit stronger with shortwave energy through the Northwest later this week. Otherwise, models agree that through next weekend and beyond, an anomalously strong upper ridge will persist over the West Coast. The WPC forecasts for tonight utilized a blend of the latest ECMWF, GFS, and CMC models for the first half of the period, with some introduction of the ensemble means days 6 and 7 to mitigate the smaller scale differences. ...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 later this week. Heavy rainfall is looking increasingly likely in some places, particularly across portions of the middle Mississippi Valley into the Midwest where a slight risk has been added to both the Day 4 and Day 5 Excessive Rainfall Outlooks given significant moisture and instability available to fuel convective storms. The northern portion of this front should drift into the Northeast on Friday fueling another round of showers and storms. There was still enough uncertainty to preclude a slight risk across the region on day 5, but given how susceptible portions of especially Vermont/New Hampshire are (and significant rainfall expected in the short range period), it's possible a targeted slight risk may be needed in future updates. By next weekend, rainfall should drift south with the front more into parts of the south-central Plains and continue across the Northeast located on the eastern edge of the upper level trough. Hazardous heat is expected to return and 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 up the West Coast. 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 are 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 Wednesday and Thursday farther north into the south-central Plains but cool off later in the week 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. 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