Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 PM EDT Sat Jul 15 2023 Valid 12Z Tue Jul 18 2023 - 12Z Sat Jul 22 2023 ...Dangerous heat wave persists across the southern tier next week, with hazardous heat in the Southwest through the period and oppressive heat indices in the Southeast through the end of the workweek... ...Overview... A persistent and anomalously strong upper 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 as the ridge builds eastward, before possibly retracting by next Saturday and allowing for more typical summer heat in those eastern areas. 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 and into Texas as the period begins Tuesday, with the 00Z EC ensemble members even indicating an 80 percent chance of a 600dm ridge in the southern High Plains on Wednesday as ridging expands eastward temporarily into the Southeast especially Wednesday-Thursday. Models then agree fairly well that troughing should amplify a bit into the Southeast late week and serve to retract the ridge/high farther west, focusing in the Four Corners region again and perhaps expanding north across the Intermountain West. North of the persistent ridge, most model guidance is 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. However, the 00Z CMC did not have as consolidated a shortwave as other models and split, so it was not preferred by late week. Models also agree in showing an upper low in the eastern Pacific drifting gradually southeast with time. This feature's position is pretty well clustered in the models/ensemble means, though the 00Z GFS was displaced somewhat southeast of other guidance. Overall model agreement is rather good and thus confidence in the pattern is above average. The forecast blend consisted of a blend favoring the 00Z ECMWF and 06Z GFS with smaller proportions of the CMC and UKMET, including some ensemble means by days 5-7 though was able to keep a majority deterministic model blend. Preferred the ECens mean over the GEFS means as the former showed some slight stream separation near the Great Lakes region to indicated a shortwave, more like most deterministic models. ...Weather/Hazards Highlights... For Tuesday-Wednesday, convection is likely along the base of a trough from the Dakotas/Midwest southeastward across the Midwest/Ohio Valley. Mesoscale convective systems are possible within this northwesterly to westerly flow in conjunction with low pressure/frontal systems at the surface. On Tuesday night, global models are already directly indicating rain rates of 2+ inches of rainfall within 3 hours over parts of the Ohio Valley, and localized rain rates will likely be higher than that. Adjusted the Slight Risk in the Excessive Rainfall Outlook southward a bit per model trends. Similarly, the Marginal Risk on Wednesday was adjusted south in the Ohio Valley while one in the Upper Midwest was maintained. Meanwhile in the Northeast, increased moisture ahead of a front could lead to some locally heavy rainfall on Tuesday. Generally amounts should stay about half an inch to an inch, but given that antecedent conditions in the region are already saturated due to recent rainfall, more rainfall could cause additional flooding/flash flooding issues. Additional rounds of rainfall are also likely in the East into the latter part of next week. Over the Florida Peninsula, Marginal Risks are in place on Tuesday and Wednesday since there could be high rain rates with convection in a moist (2+ standard deviations above average precipitable water values) and unstable environment. Urban areas would be of most concern for any flash flooding possibilities. Some monsoonal moisture could start to come into the Southwest next week. Rain totals should remain light to moderate through Tuesday-Wednesday but may increase in amounts over the central High Plains by Thursday-Friday with a front in the vicinity. Widespread oppressive heat unfortunately looks to continue into next week across the southern tier of the country. The Desert Southwest will continue to see the core of the hottest absolute temperatures, with highs well into the 110s and lows perhaps only dropping into the 80s or even 90s in some locations. The southern Plains can expect highs nearing or exceeding 100F that should be unrelenting through next week. Numerous daily records are likely to be set. Farther east, the concern will be for oppressive humidity with dewpoints in the mid 70s to low 80s to combine with the heat and produce heat indices of 105-115+ for eastern Texas into the Lower Mississippi Valley, the Deep South, Florida, and the Carolinas. Far southern Texas could approach heat indices of 120. These are likely to peak Tuesday-Friday, but as the upper ridge retracts back west by next weekend, these areas could moderate closer to typical summer heat. See Key Messages issued by WPC for additional information and graphics related to this heat wave. Interior areas of the Northwest are likely to see around average temperatures as the period begins Tuesday but warm to above average for the latter half of the week, while the north-central to northeastern U.S. should see near to slightly below average conditions next week Tate 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