Extended Forecast Discussion NWS Weather Prediction Center College Park MD 334 PM 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... The extended forecast will feature an amplified trough over the western states and an upper ridge over the central U.S. that will gradually shift eastward through the week. This pattern will support well above seasonal normal temperatures across the Plains, Upper Midwest and then reaching the East. A vast area of showers and thunderstorms across the West and Southwest could lead to areas of locally excessive rainfall and localized flooding concerns. The rain is expected to lift into the north-central U.S. in advance of cold front. 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... The latest guidance has a decent handle on the overall pattern showing the main upper trough with its axis over the West Coast states and broad ridging farther east. There are some subtle differences with the upper stream energy entering the West Coast and then western trough even on day three. The interaction of this upstream energy with the preexisting low should combine with the trough at some point and help push it eastward. The specifics of this feature vary within the model guidance and generally are within the normal noise at this time. With time, the guidance suggest this feature becomes absorbed into the trough while the ridge shifts into the eastern U.S. The details of this flow do have impacts for positions of surface fronts moving across the West and beyond. The trough is depicted to deamplify a bit as it translates downstream by the middle of next week while the ridge moves over 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. This feature will continue to be monitored. The WPC forecast favored a multi-model approach that used the 00Z ECWMF/CMC/UKMET/GEFS Mean/EC Ensemble Mean and 06Z GFS. The use of the means were slowing introduced and increased as the forecast period progressed. This approach was similar to continuity from the previous forecast period. ...Weather/Hazards Highlights... Much of the West will have rainy conditions into Sunday thanks to the amplified upper trough in place of the West Coast. Strong mean southerly flow to the east of the trough axis 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 while lingering over parts of the Intermountain West and central Utah. A Marginal Risk is place for Monday across the Northern Rockies/High Plains and into central Utah 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. Temperatures 15 to 25 degrees above seasonal average are expected across the north-central states 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. Campbell/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 Hazards: - Heavy rain across portions of the Central Plains, the Central Great Basin, the Northern Plains, the Northern Rockies, the Central Rockies, and the Northern Great Basin, Sun-Mon, Sep 3-Sep 4. - Flooding possible across portions of the Southeast and the Mid-Atlantic. - Flooding occurring or imminent across portions of the Southeast and the Great Lakes. - Hazardous heat across portions of the Lower Mississippi Valley and the Southern Plains, Sun-Thu, Sep 3-Sep 7. - Hazardous heat across portions of the Central Plains, the Middle Mississippi Valley, the Great Lakes, the Upper Mississippi Valley, and the Northern Plains, Sun-Mon, Sep 3-Sep 4. - Hazardous heat across portions of the Mid-Atlantic, the Northeast, and the Central Appalachians, Mon-Wed, Sep 4-Sep 6. 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