Extended Forecast Discussion NWS Weather Prediction Center College Park MD 336 PM EDT Mon Sep 30 2024 Valid 12Z Thu Oct 03 2024 - 12Z Mon Oct 07 2024 ...Overview... The majority of guidance through the new 12 UTC cycle still suggests a moderately progressive pattern from the eastern Pacific through the northern lower 48 and southern Canada, eventually transitioning toward a larger scale western ridge/eastern trough by next Monday. For the most part, fronts pushed along by northern tier dynamics should bring only some areas of light rain at times. Meanwhile a persistent upper ridge over the Southwest/Four Corners, at times connecting with ridging to the north, will keep the southern half of the West unseasonably warm/hot with some of the heat reaching the Plains. Meanwhile the guidance seems to be no closer to resolving significant differences within a general evolution favoring low pressure/heavy rainfall potential over the Gulf of Mexico and potentially extending into some areas along the Gulf Coast. 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... Latest guidance still shows that the most significant forecast problem in the northern stream has been the evolution of trough energy reaching the northeastern Pacific by early Friday. Over recent days there has been a decent cluster in principle among the dynamical ensemble means, more than half of the models, and the majority of machine learning (ML) models plus ECMWF AIFS ensemble mean, toward inland progression of the shortwave by Saturday and then some trough amplification into the East by early next week as an upper ridge builds into western North America. However the ensemble spaghetti plots have been (and still are) quite messy after Friday, while different models on different days have differed from this majority cluster. In the 00Z cycle, it is the GFS that strays by holding the Pacific shortwave back--which some earlier CMC/UKMET/ICON runs had done, but now those latter models have joined the more consistent ECMWF cluster. The new 12Z GFS has partially trended back toward this majority. The forecast continues to be very uncertain over and near the Gulf of Mexico. There have been some indications that one feature may track from the northwestern Caribbean into the central/eastern Gulf and possibly affect Florida. However, latest guidance seems to have at least delayed approach. There may be a separate feature over the western Gulf, which from coordinated continuity is currently depicted as a trough near the western Gulf Coast. Continue to monitor outlooks from the National Hurricane Center for the latest information on features of interest. ...Weather/Hazards Highlights... It generally remains the case that guidance differences over the Gulf of Mexico and Gulf Coast region continue to result in low confidence for resolving where heavy rain may affect coastal and/or inland areas during the period. During the Thursday-Friday period covered by the Days 4-5 Excessive Rainfall Outlooks, ML models showed some support for some recent GFS and ECMWF runs with some potential focus from southeastern Louisiana into the western Florida Panhandle, with a little eastward expansion with time. However other models/ensemble probabilities are less enthusiastic over this region, while also showing some enhanced rainfall potential along parts of the Texas coast. Flash flood guidance values are high throughout the Gulf Coast region, though Florida would be more sensitive from recent wetness. Currently the Days 4-5 EROs do not depict any risk areas given the wide dispersion of guidance, while awaiting a more coherent signal for rainfall focus. Recent indications have been for increasing rainfall to reach into Florida by the weekend, though latest runs hint at some delay in the moisture's arrival. In addition, there has been somewhat of a more suppressed trend for the northern periphery of rain that has been forecast to extend into the Southeast. Elsewhere, anticipate lighter rainfall along portions of the fronts supported by northern stream dynamics. One front may produce areas of light rain over the Great Lakes/Northeast late this week. Another front reaching the Pacific Northwest around Friday may produce some light rain, with trends over the past couple days leaning toward lower amounts. Rainfall may expand along this front when it reaches into the East by next Sunday/Monday. Latest guidance continues the recent trend toward more persistent upper ridging over the West. As a result, there may be decent coverage of highs 10-15F or so above average extending all the way into early next week with numerous daily record highs, albeit becoming a little more confined to the Desert Southwest toward Monday. The heat will extend into the Plains at times, with Saturday likely to feature the highest temperatures over that region (plus 10-20F anomalies), including some daily records over the central High Plains. Temperatures over the East will vary with frontal progression, with highs averaging out close to normal and morning lows leaning more above normal. Rausch/Schichtel 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