Extended Forecast Discussion NWS Weather Prediction Center College Park MD 258 AM EDT Fri Jul 14 2023 Valid 12Z Mon Jul 17 2023 - 12Z Fri Jul 21 2023 ***Dangerous heatwave to persistent from the Desert Southwest/Intermountain West to the Gulf Coast states all week*** ...Synoptic Overview... The main thing that will make weather headlines next week 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 basically no abatement through Friday 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 with above average forecast confidence. There are some minor differences with the UKMET concerning a slightly less amplified trough over the Midwest and Ohio Valley early in the 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 Wednesday. Even by Friday, 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 30% by Friday. 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... The best prospects for 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 MCS events 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 will feature Marginal Risk areas for the regions mentioned, and also a portion of eastern Arizona on Day 4 where a greater concentration of monsoonal moisture could lead to a few isolated instances of flooding. There is also a Marginal Risk area planned for portions of the Northeast U.S. for 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. Going beyond to the Day 6 and 7 time period, there will likely be in increase in convection across portions of the western High Plains and also across the central Appalachians, potentially reaching the Northeast once again by Friday. The main headline will continue to be the widespread oppressive heat and humidity that will affect much of the south-central 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. Farther east, downright oppressive humidity with dewpoints in the mid 70s to low 80s will lead to dangerous heat indices of 105-115+ for a large portion of the Gulf Coast region from eastern Texas into the Deep South and Florida. Again, some daily temperature records are possible. See key messages issued by WPC for additional information and graphics related to this heat wave. Above normal temperatures may also expand northward into parts of the central Plains and Midwest early next week. 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