Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM EDT Thu Aug 31 2023 Valid 12Z Sun Sep 03 2023 - 12Z Thu Sep 07 2023 ...Heat forecast to increase across the Plains, Midwest, and Mid-Atlantic/Northeast next week... ...Wet pattern across the West into early next week, with flooding possible... ...Overview... An amplified upper-level pattern will be in place to begin next week, composed of an upper trough over the West Coast states and an upper ridge over the central CONUS. These features should shift gradually eastward next week. This pattern will allow for well above average temperatures to build across the Plains and Upper Midwest and stretching into the East. Meanwhile, widespread across parts of the West this holiday weekend could cause flooding concerns. Rain chances will shift into the north-central U.S. ahead of a cold front as the week progresses. Scattered showers and storms could persist across the Gulf Coast region into Florida especially early in the week as a shortwave and frontal boundary linger there. ...Guidance Evaluation/Predictability Assessment... Model guidance starts in reasonably good agreement across the CONUS in showing the main upper trough with its axis over the West Coast states and broad ridging farther east. However, even by Day 3/Sunday, differences in timing show up with a shortwave coming through the northeastern Pacific toward the trough. The interaction of this upstream energy with the preexisting low should combine with the trough at some point and help push it eastward, with variations among 12/18Z model guidance. Did not think there were any models that could be considered outliers at this point with the shortwave, just uncertainties that arise from the feature coming from the typically uncertain northern Pacific. The newer 00Z guidance indicates the shortwave may get absorbed into the trough sooner rather than later. The details of this flow do have impacts for positions of surface fronts moving across the West and beyond. In any case, models show that the trough should deamplify a bit as it moves downstream through midweek, while ridging moves into the East. Another trough will approach the Northwest by Wednesday-Thursday, while the ridging in the East erodes--helped possibly by energy that was once Idalia. Idalia's track in the western Atlantic does become uncertain as many models curve the storm back to the west eventually by around midweek, even approaching (but not hitting) the Northeast or Atlantic Canada. So will continue to monitor. Given the somewhat agreeable large-scale flow but the detail differences, the WPC forecast blend utilized a multi-model deterministic blend of the 18Z GFS and the 12Z ECMWF/CMC/UKMET for the early part of the forecast period. As is typical, introduced the agreeable EC and GEFS ensemble means into the blend and gradually increased them with time to smooth out smaller-scale differences from model to model. But was able to keep a majority of deterministic guidance through Day 7, which can be good for maintaining strength of systems. ...Weather/Hazards Highlights... Rainy conditions are likely across much of the West into Sunday with the amplified upper trough settled over the West Coast. East of the trough, strong mean southerly flow will carry monsoonal moisture with precipitable water values well above normal and instability into the interior West. On Sunday, the heaviest rainfall is forecast for northern Utah into eastern Idaho and western Wyoming, and a Slight Risk remains in place in the ERO for these areas with a wider Marginal Risk across parts of the Great Basin and Rockies. Burn scars, slot canyons, and urban areas will be particularly vulnerable to flooding. As the trough continues to shift inland, precipitation should expand/push into the northern Plains by Monday. A Marginal Risk is planned from the northern Rockies to northern High Plains for Monday as moisture levels remain above normal, but not quite to the same extent as on Sunday. Showers and storms also may be moving faster than on Sunday. So a Slight Risk is not planned into Monday for now, but an upgrade is not out of the question in future forecast updates if better model agreement in rainfall positions and for heavier totals occurs. Rain chances are expected to spread east into the Midwest by Tuesday-Wednesday as the upper trough and cold front move eastward. Farther south, a shortwave is forecast to hover over the Lower Mississippi Valley, which in combination with a frontal boundary will provide support for scattered showers and storms. Rainfall amounts do not look to be as heavy as in the short range period though. Similarly, some showers are possible across especially the southern part of the Florida Peninsula. Heat is forecast to build across parts of the central and eastern U.S. next week underneath upper ridging. The northern Plains and the Upper Midwest are likely to see temperatures 15-25 degrees above average on Sunday and Monday, and record warm maximum and minimum temperatures could be widespread. Highs will be well into the 90s if not possibly even hitting the 100F mark there, with lows potentially staying in the mid-70s for limited overnight relief. The north-central U.S. should see some relief with cooler temperatures pushing west to east by Tuesday-Thursday as an upper trough and surface cold front move through. But this will push above normal temperatures into the Ohio Valley and Great Lakes region and the Mid-Atlantic and Northeast, with anomalies around 10-15 degrees. Farther south, temperatures generally 10-15 degrees above average (around 100F for highs) will extend summer for the southern half of the Plains through next week. The western U.S. on the other hand looks to be on the cool side especially for high temperatures particularly early in the week, with some moderation by midweek. 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