Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 PM EDT Sun Aug 13 2023 Valid 12Z Wed Aug 16 2023 - 12Z Sun Aug 20 2023 ...Heat wave to prevail across the south-central U.S. through the period... ...Hazardous heat across the West to wane by late week... ...Overview... The upper flow pattern should be fairly amplified across the northern tier and into parts of the East this week as an initial shortwave exiting the Northeast on Wednesday gives way to another vigorous system moving through the Upper Midwest, Great Lakes, and the Northeast later this week. Meanwhile, hazardous heat will continue across the West into Wednesday or Thursday before an amplified trough coming in by next weekend should quickly bring temperatures back to normal or below normal. Heat over the southern Plains and central Gulf Coast looks to continue and expand northward into the central states through the period. ...Guidance Evaluation/Predictability Assessment... As the medium range period begins Wednesday, model guidance is in good agreement regarding initial energy/troughing exiting the East and strong ridging from the southern Plains to the Northwest, as well as with a lingering broad closed low off the California coast. Then by Thursday-Friday, fortunately model guidance has come into notably better agreement with the timing and strength of the next shortwave that looks to track across the Midwest to Northeast, after several GFS runs in a row were significantly flatter and faster with the system. Starting with its 00Z run and even more so in the 06Z and 12Z runs, the GFS started to tend toward the non-NCEP camp with a more amplified/energetic slower shortwave. There are still some timing differences but at least the spread is more reasonable/typical for the day 4-5 lead time. The ECMWF/CMC/UKMET sped up just a bit with its track as well, adding to the better agreement among recent model cycles but leading to a faster nudge from continuity. Thus the WPC forecast was able to do a multi-model deterministic blend early in the forecast period. Model guidance is generally agreeable with an upper low in the eastern Pacific meandering gradually closer to and perhaps over the West Coast by next weekend. Its potential to join with the northern stream flow later in the period will depend on shortwave details of that flow...which are rather uncertain into the Northwest and eastward near the U.S./Canada border. One such issue is the 00Z ECMWF and now also the 12Z ECMWF (though to a bit lesser extent) developing a strong trough across south-central Canada and the north-central U.S. late this week that puts it out of phase with other guidance and affects temperatures considerably. As the forecast period progressed, lessened and removed the ECMWF from the blend while reducing deterministic model contributions overall in favor of the pretty agreeable GEFS and EC ensemble means. ...Weather/Hazards Highlights... The lower 48 looks fairly quiet for August in terms of precipitation through at least mid to late week, though with some areas of concern. Ample moisture with precipitable water anomalies likely over the 90th percentile should pool along and ahead of a lingering frontal boundary over the Southeast, which could result in pockets of heavy to excessive rainfall along the Southeast Coast and over the Florida Peninsula. As such, a Marginal Risks are in place for this region on both the Day 4/Wednesday and Day 5/Thursday Excessive Rainfall Outlooks. Florida looks to remain wet into next weekend too. To the north, showers and storms should accompany the shortwave and surface frontal system through the Great Lakes into the Northeast Wednesday-Friday, with some remaining uncertainty regarding rainfall amounts. At this point it seems any heavy rain will move pretty progressively to preclude any excessive rainfall risk, but the risk could be nonzero across the northeastern U.S. Thursday-Thursday night especially if prefrontal convection forms and is slower moving than the rain accompanying the front. Across the West, monsoonal moisture could cause scattered showers and storms across the Four Corners states through Wednesday-Thursday, and rainfall is likely to expand in coverage across the West by late week into next weekend as an amplified trough shifts eastward a bit toward/into the West Coast and draws in Pacific moisture that is perhaps enhanced by eastern Pacific tropical systems. The big story during the medium range period though will continue to be oppressive heat across the southern Plains to central Gulf Coast. There may be some slight moderation of the temperatures/dewpoints around Wednesday, but heat indices look to rise again Thursday and beyond across the south-central CONUS. Numerous daily record highs and warm overnight lows are likely, and the combination of heat and humidity will also result in extremely dangerous heat indices in excess of 110F in many locations. The heat wave across the West should continue into at least Wednesday with temperatures around 10-20 degrees above average across the Northwest and around 5-15 degrees above average in the central valley of California into the Desert Southwest. This equates to highs in the 90s/100s and even localized 110s as well as warm lows, both of which could set temperature records. See Key Messages issued by WPC for additional information and graphics related to these two heat waves. Western areas' temperatures could moderate somewhat by Thursday but more significantly on Friday and beyond as upper troughing takes hold. This will also push an upper ridge east, serving to warm up the central U.S. by late this week. Daytime highs in the 90s to near 100 look to build into the central Plains to Midwest, though with some uncertainty in how far north the above normal temperatures reach. Tate/Santorelli 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