Extended Forecast Discussion NWS Weather Prediction Center College Park MD 258 AM EDT Sun Jul 16 2023 Valid 12Z Wed Jul 19 2023 - 12Z Sun Jul 23 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... ...General Pattern Overview... The main story that will make weather headlines will be the continuation of the heat wave from the Intermountain West to the Deep South states in association with an anomalous upper ridge anchored over the southern Rockies and Desert Southwest. The ridge axis will initially be elongated eastward across interior portions of the Gulf Coast region through Friday before retracting back to the west as a trough builds from the Great Lakes to the East Coast. Meanwhile, shortwave disturbances in northwest flow aloft around the northern periphery of the ridge will likely support the development of multiple mesoscale convective systems that will bring episodes of heavy rainfall and strong storms from the Upper Midwest to the Northeast/Mid-Atlantic states. ...Guidance Evaluation/Predictability Assessment... The 00Z model guidance suite continues to have an excellent overall depiction of the synoptic scale pattern for the Wednesday through Sunday time period with above average forecast confidence. There are some minor differences with the UKMET concerning surface low evolution off the East Coast by Thursday, but certainly within the margin of error, so a deterministic model blend works well as a starting point in the forecast process through Friday, with slightly more weighting towards the ECMWF/GFS solutions. Even by next weekend, the models still are very close overall with the ridge centered over the Four Corners region, and an amplifying trough across the eastern half of the U.S. with above average confidence continuing and inclusion of the ensemble means capped at just 35% by Sunday. The 00Z ECMWF is a little stronger with the eastern trough and hinting at a weak closed low over the Great Lakes region, and the CMC is a little stronger with a surface low near New England by Sunday, but both of these models are still reasonably close for a day 7 forecast. In terms of QPF, the baseline was about 60% NBM, and 20% each of the GFS and ECMWF. Values were raised by about 30 percent over the Florida Peninsula to better account for sea breeze convection. ...Weather/Hazards Highlights... Multiple rounds of showers and thunderstorms, some of which may be associated with mesoscale convective complexes, are expected to cross the Ohio Valley on Wednesday around the northeastern periphery of the big upper ridge situated over Texas. The QPF signal in the 00Z model guidance suite has shifted a little bit to the south over the past few runs across portions of Kentucky and northern Tennessee, and therefore a Slight Risk area is planned for the new Day 4 excessive rainfall outlook from central Kentucky to northeastern Tennessee. The potential exists for some scattered 1-2 inch rainfall totals here, which may fall in a relatively short period of time. The existing Marginal Risk has been trimmed back some across South Florida to account for convection favored near the southwestern coast on Day 4. Going into the Day 5 period Thursday, a Slight Risk area is planned for the western High Plains of Colorado and northeast New Mexico where slow moving storms are likely to congeal into a large MCS that may bring enhanced rainfall rates to portions of that region. 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 into next weekend as well. Numerous daily record high temperatures are likely, perhaps challenging a few monthly records in a few instances across the Southwest. Farther east, oppressive humidity with dewpoints in the mid 70s to near 80 degrees will lead to dangerous heat indices of 105-115+ for eastern Texas into the Lower Mississippi Valley on Wednesday and Thursday, and 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 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