Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Sat Jul 22 2023 Valid 12Z Tue Jul 25 2023 - 12Z Sat Jul 29 2023 ...Dangerous heat will continue across the Southwest and build farther into the north-central and northeastern U.S. 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 over the east-central U.S. early in the week should gradually deamplify and eventually allow for an Atlantic upper ridge to build over Florida and the Southeast late in the week. This should raise temperatures to well above normal in the Ohio Valley/Great Lakes and Northeast/Mid-Atlantic for the latter part of the workweek before a cold front moderates temperatures there by next Saturday. Meanwhile 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 additional troughing likely to remain in place near the Pacific Northwest mid-late week. 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... With the model/ensemble guidance agreeing fairly well for the Rockies/Plains upper high and weakening eastern U.S. trough, the main model differences continue to involve the eventual timing/movement of the upper low centered atop British Columbia at the start of the period Tuesday, and additional energy coming in behind to produce another round of troughing near the Pacific Northwest for the latter part of the week. With the first feature, the 12/18Z model cycle seemed to be generally in better agreement than in previous days with the timing of its ejection eastward. The main exception was the 12Z UKMET that was slower and merged it with upstream energy. However, the incoming 00Z model cycle has trended slower overall with the upper and surface lows compared to the previous cycle, affecting associated frontal positions as well. Then with the energy upstream, model guidance has been consistent with forming another trough and possibly a closed low centered near the Pacific Northwest into late week. The newer 00Z model cycle maintains this but is overall a little weaker and faster with its track eastward/inland by Friday-Saturday, so there are still some details to figure out. 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 (likely trending more zonal with time) in the north-central U.S. Exact frontal positions and even smaller-scale boundaries can influence the QPF amounts and placement considerably. Even in the ensemble means there are considerable differences in any QPF focus--for example, the 12Z EC ensemble mean had no QPF from around the Tennessee Valley to the Central Appalachians in between areas of light QPF, while in the GEFS mean QPF is maximized there (though still light, generally a tenth to quarter inch). The WPC QPF continues to show somewhat broader 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 started with a blend of the 12Z ECMWF, 18Z GFS, and 12Z CMC deterministic models, incorporating the GEFS and EC ensemble means in the blend by days 5-7. ...Weather/Hazards Highlights... At the beginning of the period Tuesday, moist inflow into the mean but eroding upper trough atop the north-central to northeastern U.S., along with fronts at the surface, will lead to the possibility of convection from the northwest flow pattern in the Mid-Mississippi Valley eastward into the Ohio Valley and into the Northeast ahead of the trough axis. Model guidance is still quite variable with the placement of storms and rainfall amounts, so maintained the Marginal Risk in the Excessive Rainfall Outlook for Day 4/Tuesday with the expansion westward into the Mid-Mississippi Valley for possible Tuesday morning activity lingering from Day 3 as well as perhaps another round of storms later Tuesday in that region. Parts of the Northeast remain susceptible to flooding with any additional rainfall due to very wet antecedent conditions, so forecast rainfall amounts will be closely monitored for that area in case the risk level may need to be upgraded. By Day 5/Wednesday there is even more spread in any signal for focused rainfall across the East. Did not introduce any excessive rainfall areas at this time due to the uncertainty in the placement of storms, but convection that does form could contain heavy rainfall amounts given the instability in place. A low pressure system tracking through southern Canada could help focus rain/storms in the Upper Midwest on Wednesday with some predictability, so plan to introduce a Marginal Risk there. By later in the week the cold front associated with this low pressure system looks to provide a clearer focus for convection generally across the Midwest to Great Lakes region and the Northeast. Meanwhile over the 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. Florida and parts of the Gulf Coast may see some scattered showers and thunderstorms as well. 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 above 110F during peak afternoon heating as high dewpoints combine with the hot temperatures, especially on Tuesday before some minor moderating of the temperatures. A few daily record highs will be possible mainly over and near the southern High Plains into midweek, with some daily record warm lows also possible over some of the above regions. 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 (by 5-15F) 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. The heat may be relatively short-lived in portions of those regions though as a cold front looks to push south through those areas next weekend. 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