Extended Forecast Discussion NWS Weather Prediction Center College Park MD 256 PM 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 over the east-central U.S. early in the week 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. Toward the end of next week an Atlantic upper ridge potentially building over to Florida and the Southeast Coast could further aid the trend to higher heights over the East. 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. Energy surrounding/trailing the upper low may reinforce the mean trough near the Pacific Northwest coast 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 in the eastern Pacific at the start of the period Monday and influence on surface fronts over the northern U.S. Ongoing shuffling among consecutive runs of various guidance keeps confidence lower than desired for the optimal solution. Recent GFS runs had been on the faster side with the upper low and associated surface low/front but the 06Z/12Z runs have adjusted somewhat slower. The 00Z ECMWF is faster than its 00Z run from 24 hours ago but slower than the intervening 12Z run. Meanwhile the CMC had tended to be on the slower side but the new 12Z run has jumped significantly faster. Guidance behavior seems to favor a model/ensemble average approach until clustering improves/stabilizes. Behind this system the models/ensembles show better than average agreement for the trough and possible embedded upper low near the Pacific Northwest coast into late week, with an intermediate solution looking good at this time. Farther east there are still some smaller scale differences within the eastern Canada/northeastern U.S. upper trough with low predictability effects on the front forecast to reach the Northeast by midweek. 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. 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 00Z/06Z operational model blend early and then transitioned to about half models and half ensemble means by late in the period. The blend split the GFS component between the 00Z/06Z runs due to varied preferences of one or the other depending on the region, while minimizing CMC influence along the U.S.-Canadian border by mid-late period. Latest consensus among 00Z/06Z guidance ultimately slowed progression of the northern tier system by mid-late week and maintained definition of the Northeast front a little longer. Guidance behavior suggests additional adjustments one way or the other are likely. ...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 maintained for Monday's (Day 4) Excessive Rainfall Outlook for generally the Appalachians region into the Carolinas and Florida. Also during the Day 4 time frame the latest guidance is starting to show some loose clustering with the idea of convective potential over parts of the Midwest, south of a front dropping into the Upper Midwest/Great Lakes. Thus the plan is to introduce a Marginal Risk area covering most of Iowa and parts of Illinois/Missouri. On Tuesday the eastward progress of the Great Lakes front and supporting upper shortwave should promote heaviest rain totals across the northeastern U.S. A Marginal Risk remains in place for the northern Mid-Atlantic into the Interior Northeast for this activity. Very wet antecedent conditions will make this area sensitive to any additional heavy rainfall. 12Z guidance thus far has not increased the overall signal for heavy activity versus the prior cycle, but this area will require monitoring for any possible higher trends in guidance that would favor an upgrade in risk level. 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). Beyond Tuesday, expect additional episodes of convection from about the northern half of the Plains eastward, with one or more frontal boundaries helping to focus this activity that could be locally heavy. In general there may be a tendency toward higher rainfall totals around the end of next week with upper flow across the northern tier becoming more zonal and better potential for fronts to contain embedded waves. 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. The Pacific Northwest should see some rainfall early in the week as the upper low over the eastern Pacific comes ashore into British Columbia. 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. A few daily record highs will be possible mainly over and near the southern High Plains early-mid week, 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 5-10F) 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. 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