Extended Forecast Discussion NWS Weather Prediction Center College Park MD 519 PM 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... Guidance is well clustered around a single evolution of the synoptic pattern during the medium range period. The models suggest that the bubble high in the Southwest will expand eastward into the Gulf Coast, while shortwave energy swings through the Midwest and Northeast beneath an upper trough anchored over James Bay. A non-UKMET blend was used throughout the period, in part, because it struggled with depicting the high over the Southwest on day 3. Therefore, a general model blend consisting of the 00z EC/CMC and 06z GFS were used through day 5. The deterministic models were in better agreement than usual. The 00z ECE and 06z GEFS were introduced on day 6 and continued through the end of the period with the inclusion of the 00z CMCE on day 7 and the removal of the 06z GFS. Some uncertainty remains with respect to the timing and strength of shortwave energy propagating through the Midwest during the medium range, which is reflected in an obscure QPF. ...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. Kebede/Santorelli Hazards: - Heavy rain across portions of the Northeast, Sun, Jul 16. - Flooding possible across portions of the Northern Plains. - Flooding occurring or imminent across portions of the Northeast. - Flooding likely across portions of the Northeast. - Hazardous heat across portions of the Central Plains, the Central Rockies, the Lower Mississippi Valley, the Central Great Basin, the Tennessee Valley, the Southern Rockies, the Middle Mississippi Valley, the southern Mid-Atlantic, California, the Southeast, the Southern Appalachians, the Southern Plains, the Southern Ohio Valley, and the Southwest, Sun-Thu, Jul 16-Jul 20. Additional 3-7 Day Hazard information can be found on the WPC medium range hazards outlook chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php 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