Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 PM EDT Sun Aug 27 2023 Valid 12Z Wed Aug 30 2023 - 12Z Sun Sep 03 2023 ...Tropical Storm Idalia is forecast to impact portions of Florida northeastward into Georgia and the Carolinas Wednesday-Thursday with heavy rain and high winds... ...Overview... This morning the National Hurricane Center upgraded Tropical Depression Ten to Tropical Storm Idalia. The NHC forecast from the 15Z advisory shows Idalia moving northward through the eastern Gulf of Mexico during the short range period and strengthening, reaching near the Florida Big Bend by early Wednesday as a hurricane. The system will likely take a northeasterly track after that, spreading heavy rainfall from Florida into Georgia and the Carolinas, and then continue eastward over the Atlantic away from North Carolina. Recent model spread for this system's exact track offers potential for notable forecast changes but at least the newest model runs are starting to cluster a little better for the track. The upper pattern farther west should become more amplified by late week and next weekend as amplifying West Coast trough energy closes off an upper low and Southwest U.S. into central Plains ridging rebuilds more over the central U.S./Rockies. A system initially ejecting from the Northwest may produce lingering heavy rain near the Canadian border into Wednesday, while the upper low developing near the West Coast may then lead to some rainfall to its east. The next surge of monsoonal moisture appears to move up across the Desert Southwest into the Four Corners states around Thursday-Friday. Initially cooler weather is likely across much of the country at least compared to last week, but temperatures will likely rebound to above normal in the central U.S. especially by late week into next weekend. ...Guidance Evaluation/Predictability Assessment... Through late this week the guidance agrees fairly well for most aspects of the large scale pattern evolution and synoptic features. The 12Z CMC is slow to eject initial energy out of the Northwest due to greater emphasis on a trailing feature versus a more consolidated eastern upper low in most other solutions. Non-CMC runs exhibit typical timing spread for this feature that should open up as it continues across southern Canada. Regarding the amplifying upper trough/embedded low along the West Coast, latest deeper trends in the ensemble means offer improved confidence in the operational model consensus while the latest GEFS/ECMWF means seem to favor leaning away from solutions that are on the eastern part of the spread by the end of the period (12Z CMC and to some degree 06Z GFS). By next weekend there is greater spread across Canada where flow is forecast to become increasingly progressive on average, but latest GFS runs hold onto more amplified ridging from the northern Plains into Canada. GEFS mean runs hint at some ridging but are flatter over Canada. New 12Z models arriving so far are finally improving the track clustering for Idalia for the time being, closer to the official National Hurricane Center forecast. Now the greater spread appears to be for timing versus track. Much faster trend of the new 12Z ECMWF versus the prior run highlights this uncertainty. Looking back at recent runs for track, the GFS had been on the western side of the envelope after landfall, CMC runs have been fast (00Z UKMET quite fast as well), and the ECMWF has been generally favored as it has been persistent--though with a gradual westward trend over the past few runs for better odds for a system onshore near the western Atlantic coast rather than offshore. It appears that these differences are related in part to disagreements on exactly how an upper low/trough over the Gulf and Gulf Coast states and a separate upper trough lifting away from the Northeast/Mid-Atlantic may interact with the tropical system. The WPC forecast started with a deterministic model blend early in the forecast period. The forecast gradually increased the proportion of the ensemble means to 40 percent by late in the period with lingering input from the 00Z ECMWF/00Z-06Z GFS/00Z CMC (in order of more to less weight). ...Weather/Hazards Highlights... Tropical Storm Idalia is forecast to move into Florida around the Big Bend area by Wednesday, with the forecast track of the system northeastward across the Atlantic coastlines of Georgia and the Carolinas. Slight Risks of excessive rainfall are in place for these areas for the Days 4-5 EROs on Wednesday and Thursday. The forward speed of the system and how far inland it tracks will determine the flash flood potential over the affected land areas. But regardless, the heavy tropical rainfall with high rain rates could cause flash flooding even if the system is moving quickly. A cold front located west of the tropical cyclone could help elongate the footprint of the heavy rainfall. Wednesday (Day 4 in terms of ERO timing) may see the heaviest rain totals from the tropical cyclone. But with continued spread in model guidance as to the exact track, plus the currently forecast heaviest totals over eastern Georgia and southern South Carolina not really overlapping heavy rain forecasts in the short range, held off on any Moderate Risk at this point. But the ERO risk level on Wednesday is likely to be more enhanced than some Slight Risks. The positions of the risk areas are also subject to change as forecasts for the tropical system are refined. In addition, tropical downpours and squalls from the outer rainbands of the system may pose some flash flooding threat farther south into Florida for example. Additionally, fronts across the Gulf Coast will provide a focus for moisture to pool, and scattered thunderstorms are likely there through much of the week into next weekend. Planned updates for the Days 4-5 EROs are fairly modest, mainly accounting for latest guidance trends trimming the potential western extent of rainfall (especially by day 5) while including a little more of the western coast of the northern Florida Peninsula in the Slight Risk area on Day 4 to account for the potential of locally heavy rainfall in southwesterly flow, as some guidance is signaling. In the Northwest, heavy rain could continue into Wednesday across parts of the northern Rockies close to the Canadian border as the strong upper low overhead provides good forcing for rainfall while some instability may linger. Some light to moderate rain is possible over northern California and the Pacific Northwest with an amplifying upper trough/embedded closed low expected to settle along the West Coast by late week through the weekend. Farther south, after a relative lull in the monsoon around midweek, increasing convection appears likely by late week into Arizona and the surrounding states. Plenty of instability in place and above average moisture levels at least into the Desert Southwest support a Marginal Risk in the ERO for Day 5/Thursday with this activity. Most of the current excessive heat concerns look to abate by Wednesday. The western Gulf Coast may see highs a few degrees above average, but heat indices will be considerably lower after a front or two come through. The Desert Southwest could see highs in the low 110s for one more day on Wednesday before a moderating trend later in the week. Meanwhile the Northwest and the East should generally be cooler than average Wednesday-Friday, warming a bit to around normal in the East by next weekend, while below average highs drift south a bit from the Northwest into California and the central Great Basin. However, the central U.S. is likely to see heat building gradually through late this week and into next weekend. Highs well into the 90s will expand into the north-central Plains and Midwest, while highs exceeding 100 are likely for the southern half of the Plains. These highs would be around 10-15 degrees above average and perhaps locally higher over the central Plains. Rausch/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