Extended Forecast Discussion NWS Weather Prediction Center College Park MD 306 PM EDT Mon Aug 28 2023 Valid 12Z Thu Aug 31 2023 - 12Z Mon Sep 04 2023 ...Tropical Cyclone Idalia will be pulling offshore Thursday with perhaps some lingering rain and winds in the Carolinas... ...Overview... Tropical Cyclone Idalia is forecast to track near the Carolinas early in the medium range period, then gradually pull away from the coast later this week. Some lingering rain and wind on the backside of the system could affect the Carolinas on Thursday. Scattered storms may also persist into the weekend along the Gulf Coast as a frontal boundary stalls across the region. Farther west, the pattern looks to amplify by late week as an upper low forms near the West Coast, leading to some rainfall for the Pacific Northwest and northern California, while farther south the next surge of monsoonal moisture appears to move up across the Desert Southwest into the Four Corners states around Thursday-Friday. An upper ridge east of the low will help increase temperatures to well above normal across the central U.S. late week into early next week. ...Guidance Evaluation/Predictability Assessment... Model solutions for Idalia remain highly uncertain, especially with the timing of the system. The ECMWF continues to be the fastest solution, while the GFS is considerably slower. For this forecast package, a deterministic blend of the GFS/ECMWF/CMC/UKMET were used to create a forecast that is in line with the NHC forecast track. The timing and track of Idalia will depend on interaction with a frontal system to the west and troughing aloft potentially helping accelerate it northeastward. In terms of the upper level pattern, the model guidance is in relatively good agreement throughout the medium range period. An upper low will slide down the West Coast while a ridge strengthens over the Central U.S. and troughing develops over the Northeast. The CMC has come into better agreement with other guidance on the pattern, but the deterministic solution is still a bit west of the consensus. For this reason, the CMC was weighted slightly less than the GFS/ECMWF/UKMET in the WPC model blend. The blend is purely deterministic through the first half of the period, and the GEFS and EC ensemble means were added during the second half to smooth out subtle differences in the deterministic solutions as they go out in time. ...Weather/Hazards Highlights... The primary wind and flooding concerns from Tropical Storm Idalia have now shifted into the short range period, but the Carolinas in particular could still see heavy rainfall and high winds Thursday. There has been a faster trend in the storm track over the past day, with more of the rain likely falling in the short range. Minor changes were made to the Day 4 ERO with only a slight expansion and northward shift with the Slight Risk area over eastern North Carolina. The forecast rainfall for Thursday and Thursday night has a significant amount of uncertainty, which will ultimately depend on the track and speed. Small shifts in the forecast could have a sizable affect on the heavy rainfall potential. 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 Thursday-Friday, especially if high rain rates occur in sensitive urban areas. Some light to moderate rain is possible over the Pacific Northwest and northern California with an amplifying upper trough/embedded closed low expected to settle along the West Coast through late week. As the trough shifts inland, coverage of precipitation could increase eastward into the Intermountain West early next week. Farther south, convection appears likely to increase by Thursday and Friday across Arizona and Utah in particular with a surge of monsoonal moisture from mean southerly flow. Plenty of instability in place and above average moisture levels into the Desert Southwest support Marginal Risks in the ERO both Thursday and Friday with this activity. Cooler than average high temperatures are forecast for the Northwest and shifting/expanding into California to the Great Basin given the troughing aloft. Below normal temperatures are forecast across the East as well for Thursday-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 north into the north-central 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. Dolan/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