Extended Forecast Discussion NWS Weather Prediction Center College Park MD 216 AM EDT Thu Jul 13 2023 Valid 12Z Sun Jul 16 2023 - 12Z Thu Jul 20 2023 ...Hazardous to extreme heat is forecast for the entire southern tier of the U.S. through early next week and beyond... ...Additional heavy rain threats for the Northeast on Sunday... ...Overview... Broad troughing will be maintained across the Eastern U.S. during the medium range period as an upper low initially near Hudson Bay drops south and east skirting the Upper Great Lakes before it lifts into southeast Canada, and multiple shortwaves rotate through the base of this trough across the Midwest and Northeast. This should tend to focus showers and storms across these regions, with again potential for heavy rainfall in an already soaked Northeast. Initial ridging out west will shift east into the northern Plains as an upper low/shortwave moves into the Northwest early next week. Upper ridging may build briefly again behind it mid next week ahead of a deep low dropping out of the Gulf of Alaska. Meanwhile, an anomalously strong upper ridge will be maintained over the southern tier of the U.S. bringing hazardous to dangerous heat from the Southwest/California eastward to the southern Plains, Southeast, and Florida. ...Guidance Evaluation/Predictability Assessment... Overall, the latest guidance remains in excellent agreement on the overall synoptic pattern through the period, though continues to offer typical differences in the smaller scale 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 Midwest and into the Northeast. Timing of this system becomes a concern late period as the CMC was quite a bit faster lifting it across Southeast Canada. Timing also continues to improve with the next low/trough into the Northwest and western Canada, except the 12z (July 12) UKMET which was significantly different in the better consensus for both this low and the next in the Gulf of Alaska. Otherwise, even late in the period, this is very good agreement the upper high over the Southwest may slowly shift back eastward more centered over Texas and New Mexico. Tonight's blend for the WPC forecast used a mainly composite blend of the deterministic solutions through the first half of the period, increasing the ensemble means by the second half along with the ECMWF and GFS. This maintains good continuity with the previous WPC forecast as well. ...Weather/Hazards Highlights... A front moving into the Midwest and Ohio Valley and eventually stalling early next week will be the focus for showers and storms through much of the period. Locally heavy rain is likely in spots, but still enough uncertainty to preclude anything more than a broad marginal risk on the days 4 and 5 EROs. A surface low lifting through the Northeast will bring yet another round of heavy rainfall as anomalous precipitable water values stream northward ahead of a cold front. Slight risk holds on the day 4 ERO across much of New England, where some locations are extremely susceptible to flooding due to recent significant heavy rainfall. It is possible a moderate risk upgrade may be needed as we get closer in time, but right now the models keep the heaviest axis just east of the more susceptible areas of New Hampshire and Vermont. Elsewhere, showers and storms linger near the Gulf Coast and Florida through early next week, with monsoonal moisture returning to the Southwest. Did opt to place a marginal risk in the area on the day 5 ERO where the better signal for rainfall, locally heavy, was located. This threat should continue into mid next week as well. Hazardous to potentially dangerous heat will impact much of the Southwest/California eastward into the Southern states (and likely beyond per latest CPC forecasts). Numerous daily high temperatures, including the all-time high temperature in Las Vegas, NV, and warm overnight low records are threatened this weekend across this region. Farther east, the main story will be oppressive humidity leading to heat indices of 105-115 (locally higher) for a large portion of 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. Much above normal temperatures across the Northwest on Sunday should moderate and shift east as heights lower early next week over this region. Above normal temperatures may expand northward into parts of the central Plains and Mississippi Valley and points east early next week. 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