Extended Forecast Discussion NWS Weather Prediction Center College Park MD 307 AM EDT Sun Jul 23 2023 Valid 12Z Wed Jul 26 2023 - 12Z Sun Jul 30 2023 ...Dangerous heat will continue across the Southwest while hot temperatures are likely across the Midwest as well... ...Overview... The main hallmark of the upper-level pattern continues to be the persistent upper ridge/high anchored atop the southern Rockies, keeping extremely hot temperatures across the Desert Southwest. An Atlantic upper ridge may also build west across Florida and the Southeast by late week. The flow pattern farther north will be relatively low amplitude but includes some ridging stretching into the northern half of the Plains and Mississippi Valley, promoting much above normal temperatures there and east toward the Ohio Valley/Great Lakes and the Mid-Atlantic/Northeast especially Wednesday-Friday. Energy tracking across southern Canada should push fronts into the northern tier of the U.S. and provide a focus for rounds of convection, but with low confidence in the details. Diurnally driven showers and thunderstorms will be possible over and near the Four Corners states through late week and the weekend with monsoonal moisture incoming under the ridge. Meanwhile, mean upper troughing is likely near the Pacific Northwest, producing the only area of persistent below normal temperatures across the lower 48. ...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 early week 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 been following a cadence of slower 00Z runs and faster 12Z runs, so again tried to keep a middle ground for timing of the associated fronts/lows. The 12Z ECMWF seemed to be an outlier though in its handling of this upper low and upstream energy behind, tending to combine them and leading to a much faster second upper low compared to other guidance that lingers it near the Pacific Northwest. The newer 00Z ECMWF leans away from this and seems to show better consensus with the other guidance. But the newer 00Z GFS is stronger with the first low as it presses east, digging troughing more deeply into the east-central U.S. and is likely too fast/south with its front and QPF. So some of these differences are far from resolved. Even with the large scale pattern agreement over the eastern half of the country early-mid period, these above differences in timing/upper flow amplitude details and their associated surface front variations are causing ample QPF differences 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. The WPC QPF continues to show somewhat broader coverage of light amounts in the north-central U.S. and 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 the 12/18Z operational models but as the 12Z ECMWF seemed to be a relative outlier, gradually lessened its proportion and removed it from the blend with time, while increasing the GEFS and EC ensemble mean proportions to about half by the end of the period. ...Weather/Hazards Highlights... Showers and thunderstorms look likely across parts of the central and eastern U.S. through the latter part of the workweek, but unfortunately most foci for organized convection are still quite uncertain. For Wednesday/Wednesday night and thus the Day 4 ERO, low pressure and fronts tracking through southern Canada approaching the Midwest may provide one focus though, so a Marginal Risk is in place for this activity. Areas farther south across the Mid-Mississippi and Ohio Valleys into the Appalachians could also convection with heavy rainfall amounts given the instability in place, but EROs continue to depict no excessive rainfall areas at this time due to the uncertainty in the placement of storms. For Day 5/Thursday, likewise there is considerable uncertainty with placement of heavy rainfall within the general area of the northern two-thirds of the Plains and Mississippi Valley into the Appalachians and Mid-Atlantic/Northeast ahead of frontal boundaries. For now, have no ERO risks in place given the ample model spread, but if model guidance starts to converge on an area of focus rather than showing QPF states apart between models, risk areas may need to be added. By later in the week the main cold front and low pressure system pushing southeast look 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 this 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 should see some scattered showers and thunderstorms as well, and a Marginal Risk of flash flooding is in place for South Florida for Thursday as precipitable water values will be well above an already high average and convection could produce heavy rainfall rates, with urban areas particularly sensitive. 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 could continue to be hazardous through the week, but with perhaps some possible easing by next weekend. Highs around 100F are forecast to stretch into much of Texas and farther north into parts of the Plains and Mississippi Valley. The largest anomalies for highs of 10-20F above normal will be located across the northern Plains to Upper Midwest/Great Lakes for Wednesday-Thursday, while plus 10-15F anomalies are likely across the Ohio Valley and Mid-Atlantic/Northeast peaking Thursday-Friday. A handful of record highs may be possible in the south-central U.S. into Wednesday and over Florida underneath ridging, but the heat in the East may tie/set more widespread records for highs and warm minimum temperatures. 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. Tate/Rausch 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