Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM 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 rising heat indices in the East as the week progresses*** ...Overview... The main thing that will make weather headlines Tuesday through Saturday will be the continued persistence of an anomalous upper ridge that will be anchored in place across the Desert Southwest and extending eastward across Texas and the central Gulf Coast, with little to no abatement through the end of the week with unrelenting heat and humidity likely setting some record highs. This in turn will favor a series of downstream upper shortwaves and troughs from the Upper Midwest to the East Coast that will sustain a series of mesoscale convective complexes within northwest flow aloft, especially from Minnesota to the Ohio Valley and central Appalachians, and showers and storms for the Northeast ahead of a cold front on Tuesday. ...Guidance Evaluation/Predictability Assessment... The 00Z model guidance suite has an excellent overall depiction of the synoptic scale pattern for next week and into Saturday with above average forecast confidence. There are some minor differences with the UKMET concerning a slightly faster surface low/wave over the Ohio Valley early-mid week, but certainly within the margin of error, so a deterministic model blend works well as a starting point in the forecast process through early Thursday. Even by Friday/Saturday, the models still are very close overall with the ridge centered over eastern AZ/western NM, and a well developed trough crossing the Great Lakes, with above average confidence continuing and inclusion of the ensemble means capped at just 35% by Saturday. In terms of QPF, values were raised above the NBM to more closely match the QPF pattern portrayed by the ECMWF for the Rockies and Desert Southwest to account for a gradual increase in monsoonal moisture across this region. Rainfall totals were also increased by about 30% for the Florida Peninsula owing to a low bias noted in the NBM. Elsewhere, the baseline for QPF was about 60% NBM, and 20% each of the GFS and ECMWF. ...Weather/Hazards Highlights... Multiple rounds of showers and thunderstorms, some of which may be associated with mesoscale convective complexes, are expected to cross the Midwest and the Ohio Valley on Thursday around the northeastern periphery of the big upper ridge situated over Texas. The QPF signal in the 00Z model guidance suite has increased over the past few runs and also agrees better in the placement of the main QPF axis, and therefore a Slight Risk area is planned for the new Day 4 excessive rainfall outlook from central Illinois to central Kentucky. The potential exists for some scattered 2+ inch rainfall totals here during this time, which may fall in a relatively short period of time. For the Northeast U.S., showers and storms ahead of an approaching cold front are expected to result in areal rainfall averages of 0.5 to 1 inch on Tuesday. Even though this type of rainfall is normally not all that noteworthy, the antecedent conditions owing to potentially excessive rainfall over the weekend will tend to make the northern Appalachians more vulnerable to additional rain, so a Slight Risk area was added for this region as well on Day 4 to account for this. The existing Marginal Risk remains for South Florida with anomalous moisture in place. Going into the Day 5 period Wednesday, portions of the Ohio Valley and into the central Appalachians may continue to have lingering showers and storms, and some of these could also be locally heavy, so a Marginal Risk is planned here. The same holds true for northern parts of the Upper Midwest where an organized convective complex may develop, with a Marginal Risk area planned. Another complex of heavy showers and storms may form over portions of the western High Plains by midweek, so that will continue to be monitored. 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 probably into next weekend as well. 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. Some 120 heat indices are within the realm of possibility for the normally more humid locations across southern Texas. See Key Messages issued by WPC for additional information and graphics related to this heat wave. Hamrick 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