Extended Forecast Discussion NWS Weather Prediction Center College Park MD 254 PM EDT Fri Jul 14 2023 Valid 12Z Mon Jul 17 2023 - 12Z Fri Jul 21 2023 ...Heat wave persists across the southern tier next week, with rising heat indices in the East as the week progresses... ...Overview... A persistent and anomalously strong ridge will continue to cause excessive to record-breaking heat across much of the southern tier of the country through next week. The Desert Southwest can expect unrelenting heat with highs in the 110s likely to break records, while areas of the Southeast are likely to see heat indices climb upwards to dangerous levels again Tuesday-Friday. Farther north, a more progressive pattern is expected, with shortwaves and troughs aloft promoting surface fronts and showers/storms across parts of the central and eastern lower 48. ...Guidance Evaluation/Predictability Assessment... Model guidance continues to be locked in on showing an upper high of 594+ dm over the Four Corners states as the period begins Monday, with ridging expanding east across the Southeast on Wednesday-Thursday as upper troughing lifts out of the east-central CONUS. Models also are well clustered for the lead time with a shortwave and associated surface low moving east across southern Canada near the U.S. border through much of the week, though with some timing differences with this and the front by Thursday and beyond. The eastern Pacific starts to show some placement differences with an upper low, especially with the new 12Z GFS coming in with a southern position compared to other models. But overall model agreement is rather good. The early part of the WPC model blend consisted of a blend favoring the 00/06Z GFS and 00Z ECMWF with smaller proportions of the CMC and UKMET, gradually transitioning the blend to about half ensemble means with time. Changes to the NBM in terms of QPF included increasing coverage of light amounts in the Intermountain West, and trying to reduce a double axis of enhanced rain totals in the Ohio Valley on Tuesday-Wednesday that seemed to be from model differences clustering with two axes rather than legitimate. ...Weather/Hazards Highlights... The best prospects for moderate to heavy rainfall next week are expected in a general corridor from the eastern Dakotas/Minnesota southeastward across the Midwest/Ohio Valley and central Appalachians. The combination of a quasi-stationary front lingering across the Ohio Valley and a couple of surface lows passing across the Great Lakes in tandem with shortwave perturbations in northwest flow aloft will support the development of multiple mesoscale convective systems that should overall be fairly progressive in nature, but there could also be some instances of training convection that could lead to heavier rainfall totals. For now, both the Day 4 and Day 5 Excessive Rainfall Outlooks feature Marginal Risk areas for the regions mentioned. Additionally, a Marginal Risk remains in place for parts of the Northeast for possibly heavy showers and storms that develop ahead of a cold front on Tuesday, and it is possible parts of this region could eventually need a Slight Risk in future updates, especially if the main axis sets up over areas that have been hammered by recent downpours. Meanwhile, this update adds Marginal Risks to parts of the Florida Peninsula for Monday and Tuesday since there could be high rain rates with convection in a moist (2+ standard deviations above average precipitable water values) and unstable environment. There is some chance for monsoonal moisture to come into the Southwest by Monday, which WPC has been carrying a Marginal Risk for, and scattered showers could spread farther northeast on Tuesday and increase in coverage/amounts in the High Plains into mid-late next week. Additional rounds of convection are possible for the Ohio Valley into the East for the latter half of next week as well. The main headline will continue to be the widespread oppressive heat and humidity that will affect the south-central to southeastern U.S. and also extreme heat for the Desert Southwest through the end of the week (and likely beyond per latest CPC forecasts). Numerous daily record high temperatures are likely, perhaps challenging a few all-time records in a few instances across the Southwest. Areas of the Great Basin may be more fortunate to see decreasing coverage of highs over 100F after Monday, but may warm up again into late week. Farther east, oppressive humidity with dewpoints in the mid 70s to low 80s will lead to dangerous heat indices of 105-115+ for eastern Texas into the Lower Mississippi Valley on Monday and extending east into the Deep South/Florida and portions of the Carolinas as the week progresses. See Key Messages issued by WPC for additional information and graphics related to this heat wave. Tate/Hamrick 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