Extended Forecast Discussion NWS Weather Prediction Center College Park MD 258 AM EDT Wed Oct 2 2024 Valid 12Z Sat Oct 05 2024 - 12Z Wed Oct 09 2024 ...Anomalous heat will persist across the Southwest into the weekend... ...Overview... Guidance continues to show the expected pattern transition from progressive northern tier flow extending into the weekend toward an amplifying eastern U.S./Atlantic Coast trough early next week (with possible New England or vicinity upper low) while a mean ridge covering much of the West extends into the Plains, producing an extended period of unseasonably warm to hot weather. Ahead of the dynamics that will ultimately establish the eastern upper trough, a front crossing the Great Lakes and Northeast should produce some organized rain and possible surface low development could enhance amounts for parts of New England. Meanwhile, details remain uncertain within the general Gulf of Mexico evolution favoring low pressure and heavy rainfall potential. Most guidance suggests heaviest rainfall will initially stay over the Gulf but then eventually some of it may spread eastward into Florida. There may be multiple surface features to focus rainfall, including one lifting from the northwestern Caribbean into the Gulf and another that may start out over the western Gulf, with some potential for these to combine by the first half of next week. ...Guidance/Predictability Assessment... Aside from typical smaller-scale details, the primary question mark with the northern stream flow is how upper troughing will evolve once it reaches eastern North America during the early to middle part of next week. Although not unanimous, there has been a majority cluster suggesting that an upper low will close off somewhere over eastern Canada or New England. This is certainly plausible with the overall pattern looking more blocky as an upper ridge strengthens over/near Greenland. CMC runs have been leaning to the western side of the spread for the overall trough/upper low, while other model runs have been varying for shortwave details that determine the timing and path of the upper low. Machine learning (ML) models likewise show such spread and with a tilt toward a New England/Canadian Maritimes upper low on average. The forecast over and near the Gulf of Mexico continues to have low predictability due to a combination of smaller-scale features. One wave may track into the central/east-central Gulf from the northwestern Caribbean while another feature may evolve over the western Gulf, with the surface/QPF details also potentially affected by energy aloft near the western Gulf Coast. Per yesterday's coordination with NHC, these features may phase at some point next week as they approach and interact. Guidance shows the system(s) could linger and meander over the Gulf even through the first half of next week, though perhaps beginning to drift eastward with time as guidance shows some progression of a positively tilted upper trough. This appears to be a messy evolution as fronts and moisture gradients are also evident nearby. Continue to monitor outlooks from the National Hurricane Center for the latest information on features of interest. The updated forecast started with a 12Z/18Z operational model composite early in the period and then transitioned toward a blend of the 18Z GFS/12Z ECMWF and their means late, as the CMC increasingly strayed from the majority cluster over the East. Operational model weight also tilted more toward the ECMWF by the end of the forecast due to the low-confidence eastern Gulf development tracking into Florida in the 18Z GFS. ...Weather/Hazards Highlights... The rainfall forecast for areas along the Gulf Coast during the weekend continues to be in the proverbial holding pattern, with a relative majority of guidance keeping most of the significant totals over the Gulf of Mexico, especially on Saturday. The guidance messages become more mixed by Sunday. In varying ways, the GFS/UKMET/CMC and machine learning guidance show potential for some heavy rainfall to reach western Florida but other models and ensemble probabilities suggest that it will take additional time for this activity to reach Florida. At the very least, reasonable consensus of the GFS/ECMWF and their means would have moisture anomalies increasing over the state as an east-west front becomes aligned across the peninsula so some increase in rainfall on Sunday relative to Saturday seems reasonable. The question is to what degree. The Days 4-5 Excessive Rainfall Outlooks for Saturday- Sunday maintain no risk areas for now, but with an eye to parts of the Florida Gulf Coast region for possible introduction of an area on Day 5 if there is further clustering/continuity in future cycles. The pattern continues to look favorable for enhanced rainfall over Florida during the first half of next week with a general area of low pressure expected to drift into the eastern Gulf and a potentially wavy front still draped across the peninsula. Elsewhere, the frontal system reaching the Great Lakes and Northeast by Sunday-Monday and possible low development near New England and the Canadian Maritimes thereafter may generate rainfall of varying intensity. There is still a fairly wide range of possible QPF totals from the eastern Great Lakes into New England depending on exactly how this system evolves. A mean frontal zone with one or more embedded waves should reach near the northern Pacific Northwest by early-mid week, producing some periods of mostly light rain. The strong and persistent upper ridge covering the West and extending into the Plains will support a lengthy period of well above normal temperatures over many parts of the western/central U.S. Highs 10-20F or so above normal will be common and numerous daily records appear likely from the southern half of the West into parts of the High Plains, especially during the weekend but also lingering into early next week to some extent. Best potential for some highs 20-25F above normal will be over the central Plains and Midwest on Saturday and northern Plains by midweek. Above normal temperatures over most of the East during the weekend should decline to near or somewhat below normal as upper troughing sets up near the East Coast. Rausch 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 (QPF), excessive rainfall outlook (ERO), winter weather outlook (WWO) probabilities, heat indices, and Key Messages can be accessed from: 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