Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 PM 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 east-central U.S. upper trough already starting to weaken as of Tuesday should continue to retreat thereafter while an Atlantic upper ridge builds over Florida and the Southeast late in the week. This eastern U.S. evolution 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 a short-range eastern Pacific upper low (part of a Rex block configuration including an Alaska/northwestern Canada ridge) should eject east across southern Canada and push fronts across the northern U.S. Trailing energy aloft will likely reinforce the mean trough near the Pacific Northwest by Thursday, with an embedded low (weaker than the one crossing British Columbia on Tuesday) possibly ejecting by next Saturday. Some diurnally favored showers and thunderstorms will be possible over and near the Four Corners states while fronts/upper shortwaves may produce some episodes of convection over the eastern half of the country. Coverage of heavy rainfall threats should be less than average for the middle of summer, though the forecast pattern may favor some increase of rainfall coverage and intensity from the Midwest/northern half of the Mississippi Valley eastward toward the end of the week. ...Guidance Evaluation/Predictability Assessment... As has been the case for a while, the most contentious part of the mass field forecast continues to involve exactly how the initial British Columbia upper low progresses eastward across southern Canada and possibly shears out at some point, with some possible influence from flow coming around the Alaska/northern Canada upper ridging. Recent cycles of guidance have settled into a routine of oscillating in each 12-hourly cycle, with 00Z runs tending to be slower than the 12Z ones. The new 12Z cycle generally maintains this tendency, while also tending to shear out the upper low earlier. The end result is a somewhat faster eastward progression of the front, anchored by a weaker and faster surface low, but with portions of the front possibly not reaching quite as far south by late in the period. For the reloading Pacific Northwest upper trough, most guidance clusters fairly well into day 6 Friday, though the 12Z GFS starts to eject the embedded upper low a little more quickly and the new 12Z CMC leans a bit to the southwest side of the spread (but closer to consensus than the 00Z run's day 7 Saturday upper low). By the end of the period the 00Z ECMWF mean and 00Z GFS were closest to the guidance average, faster than the GEFS mean but slower than the 00Z ECMWF and 06Z/12Z GFS. The new 12Z ECMWF has cast new doubt on this trough's forecast though, with a noticeably faster trend for much of the period. Even with the large scale pattern agreement over the eastern half of the country early-mid period, the models/ensembles continue to show ample differences in terms of QPF as the northern U.S. pattern east of the Rockies transitions from a typically uncertain northwest flow regime toward a zonal alignment. GFS/ECMWF/CMC runs and their ensemble probabilities continue to disagree over where the best emphasis for merely measurable or heavier rainfall may occur. A prominent example is over the Appalachians/Mid-Atlantic around midweek with the GFS runs on the heavy/expansive side versus most other guidance (though oddly with the most relative support from CMC ensembles). Additional QPF differences arise once southern Canada low pressure and trailing fronts become more dominant features, with timing/upper flow amplitude details affecting QPF placement and amounts. The WPC QPF continues to show somewhat broader coverage of light amounts in the north-central U.S. and eventually the eastern 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 00Z/06Z operational models early and then increased 06Z GEFS/00Z ECens total weight to 50 percent by the end of the period. Oscillating trends with the northern tier system led to somewhat slower frontal timing, but tempered by keeping progression a little ahead of what the literal blend would have suggested by the latter half of the period. ...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. However the combination of forecast moisture/instability along with model/ensemble signals from the GFS/ECMWF and GEFS/ECens/CMCens favor maintaining the Marginal Risk in the Excessive Rainfall Outlook for Day 4/Tuesday from the Mid-Mississippi Valley (where possible Tuesday morning activity may linger from Day 3 period) through parts of the Great Lakes and Northeast. Portions 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. EROs continue to depict no 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. Meanwhile a low pressure system tracking through southern Canada could help focus rain/storms in the Upper Midwest by Wednesday-Wednesday night. Latest models are not particularly agreeable with rainfall forecasts, but there seems to be better consensus toward an associated area of enhanced moisture that supports a localized heavy rainfall threat and maintaining the 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. Training/repeat activity could eventually become more of a factor for a time if flattening flow aloft allows some of the front to take on a more east-west orientation. 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 fills. The cold front forecast to progress from the northern Plains around midweek into the Northeast by Saturday should erode the northern tier heat area from west to east, keeping the event fairly short-lived over the northern third of the country. The heat may persist longer to the south. Details of convection and residual cloud cover will add an extra element of uncertainty to the forecast of daytime highs over some areas. Meanwhile the only notable area of potentially below normal temperatures will be the Pacific Northwest given the rounds of troughing aloft in the vicinity. Rausch/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