Extended Forecast Discussion NWS Weather Prediction Center College Park MD 420 PM EDT Tue Aug 29 2023 Valid 12Z Fri Sep 01 2023 - 12Z Tue Sep 05 2023 ...Heat forecast to increase across the Plains and Midwest this weekend into early next week... ...Overview... Idalia is forecast to be tracking away from the U.S. as the medium range period begins Friday, and focus will shift to an amplifying upper-level pattern composed of an upper trough (with an embedded closed low late week) over the West Coast and upper ridge over 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 and rain chances for much of the West and parts of the Gulf Coast and Florida. ...Guidance Evaluation/Predictability Assessment... Model guidance seems to have a good handle on the upper level pattern with good agreement through the weekend. Differences in the upper level pattern are most pronounced early next week when another closed low is forecast to move into the Pacific Northwest. The GFS brings this low onshore the fastest and quickly combines the energy into the trough that was over the West coast at the beginning of the period. The ECMWF brings the low onshore slower, keeping it from being swept up into the original trough. These differences will have considerable impacts on the timing of a surface frontal system that will move across the Northwest early next week. For the afternoon forecast, a deterministic blend with nearly equal weights of the CMC/UKMET/ECMWF/GFS was used for Friday and Saturday. Sunday onward, ensemble means were blended with the deterministic CMC/GFS/ECMWF (ECMWF weighted slightly heavier) to smooth out smaller scale differences. 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. Since yesterday, the deterministic GFS runs have all been showing some semblance of this. Interestingly, the 00Z CMC indicated this potential as well, but the 12Z CMC backed off of it a little bit. A good amount of 12Z GEFS ensemble member are showing a similar track, but only a couple members of the 06Z EC ensemble are showing something similar. The National Hurricane Center track takes Idalia east-southeast away from the U.S. late week, decelerating with time. A lack of steering flow between the Carolinas and Bermuda is contributing to a fair amount of uncertainty in the track forecast beyond day 5, and developments in the forecast guidance will be monitored closely in the coming days. ...Weather/Hazards Highlights... Rainy conditions are likely across much of the West through this weekend with the amplifying upper trough/embedded closed low settled over the West Coast. There will be ample forcing and instability in place to allow for some localized flooding concerns in the interior Northwest and Great Basin in parts of Oregon, northern California, and Nevada. Marginal Risks of Excessive Rainfall are in place for this threat Friday and Saturday, and burn scars and urban areas will be particularly vulnerable. Marginal Risks areas are also in place for parts of the Southwest and Intermountain West on Friday and Saturday as a surge of monsoonal moisture works its way north into the region. Model guidance is showing PWATs well above normal Friday and Saturday near the Nevada/Arizona/Utah border, which will likely be the area to receive the most rainfall and have the highest risk of flash flooding. As the trough shifts inland, coverage of precipitation could focus across the Intermountain West early next week and eventually into the northern Plains. Showers and thunderstorms will also impact portions of the Gulf Coast and Florida as a stalled frontal boundary lingers across the region. Moisture will pool along the front, producing scattered precipitation chances through the weekend. There may be a nonzero chance of flash flooding with this activity, mainly on Friday, especially if high rain rates occur in sensitive urban areas. Precipitation chances along the Gulf Coast will decrease early next week. 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 on Friday before warming to near normal this weekend. In the Central U.S., the strengthening upper ridge will allow temperatures to rise considerably in 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 Mid-Atlantic and Northeast look to warm to about 10-15 degrees above normal by Monday-Tuesday. 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 Hazards: - Heavy rain across portions of the Central Great Basin, the Northern Great Basin, and the Southwest, Fri, Sep 1. - Flooding possible across portions of the Southeast and the Mid-Atlantic. - Flooding occurring or imminent across portions of the Great Lakes. - Hazardous heat across portions of the Lower Mississippi Valley and the Southern Plains, Mon-Tue, Sep 4-Sep 5. - Hazardous heat across portions of the Central Plains, the Great Lakes, the Upper Mississippi Valley, and the Northern Plains, Sat-Mon, Sep 2-Sep 4. - High winds across portions of coastal North Carolina, Fri, Sep 1. - Heavy rain across portions of mainland Alaska, Fri-Mon, Sep 1-Sep 4. - Heavy rain across portions of mainland Alaska, Fri-Sun, Sep 1-Sep 3. - High winds across portions of southwest mainland Alaska and the Alaska Panhandle, Fri-Sat, Sep 1-Sep 2. 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