Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Tue Aug 29 2023 Valid 12Z Fri Sep 01 2023 - 12Z Tue Sep 05 2023 ...Heat looks to increase across the Plains to Midwest late this week into next week... ...Overview... As Idalia should be tracking away from the U.S. as the medium range period begins Friday, focus will shift to an amplifying upper-level pattern composed mainly of an upper trough (with an embedded closed low late week) over the West Coast, while upper ridging dominates east of it across the central CONUS. These features should shift gradually eastward into early next week. This pattern will allow for well above average temperatures to build across the Plains and Upper Midwest. Meanwhile rain chances are possible across much of the West underneath and east of the upper trough within moist mean southerly flow. Additionally, scattered storms could persist late week into early next week along the Gulf Coast and Florida as a frontal boundary stalls across the region. ...Guidance Evaluation/Predictability Assessment... Model guidance shows reasonably good agreement with the pattern described above, especially early in the period/late this week. Some spread in the trough axis/low along the West Coast is mainly with the 12Z CMC on the western side of the envelope. The 00Z model suite shows even better agreement in its position. By early next week the trough should be shifting eastward but the details become more uncertain within the trough, especially as secondary rounds of energy come into it from upstream. For now the WPC forecast favored a deterministic blend early with more weighting on the ECMWF and GFS, with gradually increasing proportions of the GEFS and EC ensemble means with time to smooth out differences of shortwaves within the flow, which can hopefully be better resolved with time. Something we are monitoring, though certainly not biting on at this point, is the potential for at least a portion of Idalia's energy to drift south in the western Atlantic to the east of Florida and then actually track back westward into the Atlantic Coast of Florida. The 12/18/00Z GFS runs all showed some semblance of this, with the 12 and 18Z runs very quick to make this loop. Interestingly the 00Z CMC indicates this potential as well, though is slower to do so, but is not too far off from the 00Z GFS, both showing the system approaching the Florida Atlantic Coast by around next Tuesday. Some GEFS and CMC ensemble members are showing a similar track while essentially no 12Z EC ensemble members are. The National Hurricane Center track takes Idalia east away from the U.S. late week and then decelerating with time, but in the eastward direction. There is a fair amount of uncertainty in its track since it ends up in an area of weak flow with little to steer it. So this will continue to be monitored. ...Weather/Hazards Highlights... Rain is likely across the Northwest with an amplifying upper trough/embedded closed low expected to settle along the West Coast late week. The ample forcing for ascent that this will provide as well as some instability in place could allow for some localized flooding issues in interior parts of the Northwest, in portions of Oregon, northern California, and Nevada. Marginal Risks of excessive rainfall are in place for this threat for Friday and Saturday, with burn scars and urban areas particularly vulnerable. Farther east, convection is likely on Friday across Arizona and Utah in particular and northward into the northern Great Basin/Rockies with a surge of monsoonal moisture from mean southerly flow east of the trough. Plenty of instability in place and above average moisture levels into the Desert Southwest support Marginal Risks in the ERO both Friday and Saturday with this activity, likely peaking on Friday. As the trough shifts inland, coverage of precipitation could focus across the Intermountain West early next week and eventually into the northern Plains. Additionally, a stalled front across the Gulf Coast will provide a focus for moisture to pool, and scattered thunderstorms are likely from the central Gulf Coast into Florida through much of the week and the weekend. There may be a nonzero chance of flash flooding with this activity that looks to peak Friday, especially if high rain rates occur in sensitive urban areas. Cooler than average high temperatures are forecast for the Northwest and shifting/expanding into California to the Great Basin given the troughing aloft. Slightly below normal temperatures are forecast across the East as well for on Friday before warming to near normal by the weekend. But in between, a strengthening upper ridge will allow temperatures to rise considerably in the central U.S. by late week and through the holiday weekend. Highs well into the 90s will expand all the way into the northern Plains and east across the Mississippi Valley, while highs exceeding 100 are likely for the southern half of the Plains. These forecast high temperatures are around 10-20 degrees above average, perhaps locally higher in the Upper Midwest. Some record warm maximum and minimum temperatures could be set there as well. As the upper ridge shifts east with time, temperatures in the East look to warm to about 10-15 degrees above normal by Monday-Tuesday. 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