Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM EDT Fri Jul 21 2023 Valid 12Z Mon Jul 24 2023 - 12Z Fri Jul 28 2023 ...Dangerous heat will continue across the Southwest and build farther north and east into the Plains/Midwest as next week progresses... ...Overview... A strong and persistent upper ridge/high anchored over the southern Rockies will keep hotter than usual temperatures across the Southwest northeastward into the Plains and Midwest under the ridge axis. Downstream, an upper trough is forecast atop the east-central U.S. early in the week, which should gradually deamplify and allow for hotter than normal temperatures to spread into the Ohio Valley/Great Lakes and Northeast/Mid-Atlantic by late next week. Farther west, an eastern Pacific upper low (part of a Rex block configuration including an Alaska/northwestern Canada ridge) may eject east across southern Canada and drop fronts into the northwestern to north-central U.S., with potential for troughing to build into that region once again. Some diurnally favored showers and thunderstorms will be possible over the Four Corners states while fronts/upper shortwaves may produce some episodes of convection over the eastern half of the country, but overall the coverage of heavy rainfall threats should be less than average for the middle of summer. ...Guidance Evaluation/Predictability Assessment... Model guidance is in pretty good agreement with the synoptic scale pattern described above, especially the persistent upper high and the troughing across the East gradually filling with time. The main model differences are with the eventual timing/movement of the upper low in the eastern Pacific at the start of the period Monday. GFS runs have been consistently on the faster side with the upper and associated surface low track eastward by Wednesday/day 5 or so, while CMC runs are on the slower side. The 12Z ECMWF was in between and was the most agreeable with the ensemble means, so the WPC forecast preferred its depiction. The newer 00Z GFS and ECMWF are fortunately showing better consistency with the timing, though the 00Z CMC is still slow and makes the pattern in south-central and southeastern Canada out of phase. While models agree fairly well on the larger scale, they continue to show ample differences in terms of QPF especially in the uncertain northwest flow pattern in the north-central U.S. Overall the WPC QPF broadened the coverage of light amounts in the north-central U.S. compared to the NBM using a multi-model and ensemble blend to provide some depiction of the rainfall potential, but these rainfall forecasts could change notably with time if/as models start to converge. For the mass fields the WPC forecast used a blend heavy on the 12Z ECMWF along with some 12/18Z GFS and 12Z CMC and UKMET early in the period, gradually adding in and increasing the GEFS/EC ensemble mean proportion to almost half with time amid increasing model differences. ...Weather/Hazards Highlights... Showers and storms are likely in the East early in the workweek with moist inflow ahead of the upper trough axis. With scattered heavy amounts possible, a Marginal Risk is delineated for Monday's Excessive Rainfall Outlook for generally the Appalachians region into the Carolinas and Florida. On Tuesday the heaviest rain totals look to focus across the northeastern U.S. ahead of an upper shortwave and cold front. A Marginal Risk is in place for the northern Mid-Atlantic into the Interior Northeast for this activity. If rain amounts more agreeably trend upward, higher excessive rainfall risk levels are possible there considering the very wet antecedent conditions. Farther south, a lingering front across the Gulf Coast could spark some convection particularly on Monday-Tuesday before the front dissipates. Uncertainty whether the rain will focus on- or offshore precludes a Marginal Risk farther west of Florida at this time (southeastern Louisiana may be the most likely place for heavy rain to end up onshore). Meanwhile into the northern half of the Plains/Mississippi Valley into the Midwest/Great Lakes region, general northwest flow and a couple of frontal systems could produce showers and thunderstorms, perhaps with locally heavy rainfall. At this point model guidance shows too much variation in location of enhanced amounts to depict any risk areas in the EROs. Shorter term guidance may eventually provide enough focus to define regions with relatively greater heavy rainfall potential. These areas could continue to see convection into the latter half of next week as well. Then farther west, the Four Corners states are likely to see diurnally driven convection each day next week as monsoonal moisture comes in underneath the southern Rockies upper ridge. The EROs depict a Marginal Risk area on both days. The relentless upper high centered over the southern Rockies will keep temperatures in the Desert Southwest around 5-10F above average for highs in the 110s. Though these anomalies are a little less extreme than in the shorter term, the heat will continue to be hazardous into next week. Highs over 100F are forecast to stretch into much of Texas as well, and eastern Texas near the Gulf Coast can expect heat indices well over 110F during peak afternoon heating as high dewpoints combine with the hot temperatures. Upper ridging extending to the north will also allow for the Plains and Midwest to warm to generally 10-15F above normal through much of next week, reaching the 100F mark at some locations. Warmer than normal temperatures are likely to expand east into the Ohio Valley, Great Lakes, and Mid-Atlantic/Northeast regions by the latter part of next week after the upper troughing erodes. Meanwhile the only notable area of potentially below normal temperatures will be the Pacific Northwest given the rounds of troughing aloft in the vicinity. Tate 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