Extended Forecast Discussion NWS Weather Prediction Center College Park MD 306 AM EDT Mon Aug 28 2023 Valid 12Z Thu Aug 31 2023 - 12Z Mon Sep 04 2023 ...Tropical Storm Idalia will be pulling offshore on Thursday with perhaps some lingering rain and winds in the Carolinas... ...Overview... At the start of the medium range period Thursday, Idalia is forecast to be in the western Atlantic and pulling eastward after tracking across Florida to the Carolinas midweek. Some lingering rain and wind could affect the Carolinas through Thursday on the backside of the tropical system. Scattered storms may also persist into the weekend from the central Gulf Coast into Florida with a meandering frontal system. 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 guidance for Idalia has been waffling over the past few days, with some wiggles in the track but especially differences with the timing of the system. On the whole the National Hurricane Center's forecast and model outputs have been trending faster with the tropical cyclone by the Thursday timeframe. The 12Z ECMWF was particularly fast though, so favored other guidance like the GFS there to better comply with the NHC 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. The midlatitude synoptic-scale flow is in reasonably good agreement with the bulk of guidance into late week, with a pattern featuring troughing that is quick to exit the Northeast, a shortwave/small closed low coming across the north-central U.S./south-central Canada, and potent energy dropping south and forming a closed upper low near/atop the West Coast. The main outlier in the 12/18Z model cycle was the 12Z CMC, which was slower with the latter two features even beginning Thursday and never caught up to the positions of other guidance, and the 12Z CMC mean was east with the West Coast trough. The newer 00Z deterministic CMC fortunately seems to be in better alignment with consensus with these features, though ends up seeming slow/west as the period progresses. By early next week, the timing of the upper low shifting eastward and potentially opening up into a trough becomes somewhat questionable among the 12/18Z guidance suite as well as the newer 00Z suite--the latter generally had a faster trend compared to the previous cycle, but this remains uncertain. Additionally, while upper ridging in the central U.S. is quite agreeable among models, the north-central U.S. may become a trouble spot as additional troughing makes its way into south-central Canada and models vary as to how far south it may come and potentially suppress the northern side of the ridge. Thus the WPC forecast blend favored a deterministic blend favoring the GFS and ECMWF early in the period, but shifted toward including and increasing GEFS and EC ensemble means to about half by the end of the period given increasing spread. ...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. Thus the ERO risk areas were shrunk once again for Thursday/Day 4, as eastern North Carolina seems to be the only area with enough agreement to see heavy rain perhaps remaining into Thursday for a Slight Risk, while a Marginal Risk covers areas that could see heavy rain or possibly no rain at all dependent on Idalia's eventual track and timing. Additionally, fronts 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 conditions, at least in terms of 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 temperatures would be around 10-20 degrees above average. 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