Extended Forecast Discussion NWS Weather Prediction Center College Park MD 258 AM EDT Tue Oct 1 2024 Valid 12Z Fri Oct 04 2024 - 12Z Tue Oct 08 2024 ...Overview... For the Friday-Tuesday period, latest models and ensembles continue to show a pattern transition from progressive northern tier flow toward an amplifying eastern U.S./Atlantic Coast trough while a mean ridge covering much of the West extends into the Plains, producing an extended period of unseasonably warm to hot weather. Separate fronts affecting the Northeast and Northwest late this week should generate mainly light rain, but the front and potential embedded surface system reaching near the East Coast next week could support more organized rainfall over that region. Farther south, not much has changed regarding the overall forecast evolution favoring low pressure and heavy rainfall potential over the Gulf of Mexico with some pockets of enhanced moisture possibly extending into some areas along the Gulf Coast, but with poor agreement/consistency for details. There may be multiple surface features to focus rainfall, including one lifting from the northwestern Caribbean into the Gulf and another that may be over the western Gulf. ...Guidance/Predictability Assessment... Guidance agreement for northern stream flow late this week into the weekend has been incrementally improving, with fairly strong clustering among most dynamical model/ensemble solutions over the past day but with a lingering stray solution or two. For example, the 18Z GFS was a bit on the slow side with the upper shortwave progressing from the East Pacific into the Upper Midwest, while by Sunday the the new 00Z CMC closes off an embedded low in a way not depicted in other guidance. Trough details become more of a question mark toward the East Coast by next Monday-Tuesday. ECMWF runs have consistently closing off an upper low around New England and the previously open GFS now shows a closed low in the 00Z run. Machine learning (ML) models and the ECMWF AIFS mean keep the trough more open like the ensemble means, though the 00Z GEFS/12Z ECens means eventually imply an upper low over the Canadian Maritimes after Tuesday. In spite of the more open upper trough, the ML models do offer some signal for a surface wave near New England, just in less pronounced form than the solutions with a closed upper low. Also worth keeping an eye on, by Tuesday some ML models show more potential for a shortwave from Hudson Bay south/southwestward versus what most dynamical guidance shows. 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 over Texas and nearby areas. As for the latter, the 00Z GFS is a notably strong solution with its upper feature over Texas. Minus the 12Z CMC which was very fast with its Caribbean/eastern Gulf system (as some earlier runs were), recent trends in guidance overall have been to hold low pressure over the Gulf for a longer time before it tracks eastward toward Florida. Some new 00Z solutions are still over the central Gulf by next Tuesday. The updated forecast reflects WPC/NHC coordinated continuity over the western Gulf with a trough, and close to continuity with the spot low over the central/eastern Gulf. Continue to monitor outlooks from the National Hurricane Center for the latest information on features of interest. For the most part, an operational model blend sufficed for the early part of the forecast. The 18Z GFS component was split with the GEFS mean due to the former being slow with the northern tier Pacific shortwave. By mid-late period, the 12Z CMC was removed due to its nonconforming Gulf system while the blend transitioned toward a blend of the 18Z GFS/12Z ECMWF and their ensemble means due to decreasing confidence in detail specifics. ...Weather/Hazards Highlights... Very little has changed over the past day in terms of the low confidence for where areas of heavy rain could extend into some coastal/inland areas near the Gulf Coast during the period. For the Friday-Saturday time frame covered by the Days 4-5 Excessive Rainfall Outlooks, there are minority model solutions showing pockets of enhanced rainfall along parts of the central/east-central Gulf Coast and/or the Texas coast. However while separate ensemble systems differ over where the best heavy rain potential will be over the Gulf, they do not extend their better probabilities to land. The combination of poor guidance clustering/continuity and high flash flood guidance values along the Gulf Coast region continue to favor not depicting any risk areas on the Days 4-5 EROs. Recent guidance has been suggesting that some heavy rain may eventually spread across Florida by early next week, perhaps aided by a front approaching from the north. From Friday through the weekend, expect lighter rainfall along portions of the fronts supported by northern stream dynamics. One front may produce areas of light rain over the Northeast late this week while a front reaching the Pacific Northwest around Friday may produce some light rain as well. A frontal system and possible developing embedded low may produce more organized rainfall toward the East Coast by next Monday-Tuesday. Coverage and magnitude will depend on system specifics that have lower confidence at this time. An approaching Pacific front may bring light rain to the northern Pacific Northwest early next week. 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 may 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 Friday-Sunday. Eastern U.S. temperatures will vary in the near to above normal range until the East Coast trough drops some highs moderately below normal next Tuesday. 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