Extended Forecast Discussion...amended NWS Weather Prediction Center College Park MD 358 AM EDT Mon Jul 17 2023 Valid 12Z Thu Jul 20 2023 - 12Z Mon Jul 24 2023 ***Dangerous heat wave persists across the southern tier, with hazardous heat in the Southwest even into next week and oppressive heat indices in the Southeast through the end of the workweek*** ...General Overview... The main story that will make weather headlines will be the continuation of the heat wave from the Intermountain West to the central Gulf Coast 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 Thursday before retracting back to the west, and building northward to include more of the northwestern High Plains, and 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. The UKMET is a little slower to erode the eastern extent of the upper ridge axis across the Gulf Coast region through Friday, but otherwise in fairly good agreement elsewhere. Going into next Monday, the models show more significant differences across southwestern Canada where the GFS has a considerably stronger upper low that impinges some upon the northern Rockies, but is close to the consensus across most of the continental U.S. The CMC is also a little to the east of the model consensus with the trough axis across the eastern U.S. by next Monday, but otherwise in good agreement. A multi-deterministic model blend works well through the end of the week, and then some ensemble means were included for the weekend, with up to 40% ensemble means by next Monday. 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... A surface low is forecast to develop and intensify across the Great Lakes during the day on Thursday, with a trailing cold front extending south across the central Appalachians and a moist conveyor ahead of it. Widespread showers and thunderstorms are expected to develop ahead of this front from the central Appalachians to the interior Northeast through early Friday morning. Now that the model signal is more concentrated with the 00Z guidance, a Slight Risk area is in effect from southwest Pennsylvania to the southern Adirondacks for the Day 4 period, and this is also owing to wet antecedent conditions from multiple rounds of recent heavy rainfall in this region. The potential now exists for scattered one inch rainfall totals, and isolated 2+ inch totals where storms persist the longest. Farther west, some monsoonal moisture looks to advect northward into portions of the southern Rockies, and as it meets with a frontal system in the central High Plains, this will become a focus for convection as well. Thursday into Thursday night looks to be the main day for potential flash flooding as convection is likely to congeal overnight into a large MCS with potentially high rain rates, and this is where the Day 4 Slight Risk extends from eastern Colorado to central Kansas for this potential. Additional showers and storms are possible into Friday and beyond in this region as well. Widespread oppressive heat unfortunately looks to continue throughout the week across the southern tier of the country. The Desert Southwest will continue to see the core of the hottest overall temperatures, with highs well into the 110s for the lower elevations, and lows only in the 80s to lower 90s providing practically no relief from the persistent heat, with more daily record highs likely. Farther east, the concern will be for oppressive humidity with dewpoints in the mid 70s to near 80 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 parts of the Carolinas. Far southern Texas could approach heat indices of 120 degrees. These are likely to peak Thursday and Friday, but as the upper ridge retracts back west by next weekend, these areas should moderate closer to typical summer heat. Interior areas of the Northwest and the northwestern High Plains are likely to see temperatures warming to above average for the latter half of the week into next weekend. See Key Messages issued by WPC for additional information and graphics related to this heat wave. Downstream, the northern Plains and Midwest are likely to be pleasant temperature wise, with highs in particular forecast to be below average, while the northeastern U.S. can expect near normal temperatures. 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