Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 PM EDT Sun Sep 29 2024 Valid 12Z Wed Oct 02 2024 - 12Z Sun Oct 06 2024 ...Overview... Latest models continue to show progressive mean flow across the northern lower 48 from midweek through next weekend, supporting a series of fronts during the period. The front expected to reach the Northwest around Friday may produce organized light to locally moderate rain, but otherwise any precipitation along the fronts should be fairly light and scattered. Farther south, persistent upper ridging over and near the Southwest will keep that part of the country unseasonably hot with some of the heat reaching the Plains. There is a persistent signal for increasing rainfall along the Gulf Coast as a weakness develops between the Southwest ridge and another ridge building over Florida and parts of the Atlantic/Caribbean. In addition some models continue to show potential for tropical development from the northwestern Caribbean into the Gulf of Mexico, further enhancing rainfall if a system nears the Gulf Coast. However there is still a lot of uncertainty over the combined details of flow aloft and possible tropical development. ...Guidance/Predictability Assessment... The period begins mid-next week with a southern stream shortwave/associated coastal low moving away over the open Atlantic from the northeastern U.S., a northern stream upper-trough and accompanying cold front to the west moving through the northeastern U.S., and generally more zonal flow with western extent as the northern stream remains in place to the north along the U.S./Canadian border. The GFS is a bit stronger with the coastal low, but with the separation between the low and the upper- trough/front to the west, precipitation chances will be trending down by mid-week. The northern stream remains progressive to the west, with embedded shortwave energy in the flow that not surprisingly has some timing differences amongst the guidance. However, prior frontal passages should lead to dry conditions with limited concern over potential precipitation chances that might have a greater disparity in expected location given the guidance differences. The pattern remains a bit uncertain by late next week/weekend with differences in the deterministic guidance with respect to an upper-trough over the northeastern Pacific approaching southwestern Canada and traversing eastward in vicinity of the northern border. The GFS has trended closer to the ECMWF with respect to the path of the upper-trough, depicting a lower- amplitude wave remaining along the northern border rather than digging southward into the western U.S., though still depicting the potential for some energy to split from the northern stream over northern California. However, the 00Z ECMWF trended weaker with the upper-trough compared to the past few runs, and also brings it eventually more southward than the other guidance with eastward extent, which keeps the degree of uncertainty in the forecast. The differences are also apparent in the surface reflection, with a stronger surface low taking a more northerly path in the GFS compared to the ECMWF. One positive change is that both the CMC and CMC ensemble mean have generally come in phase with the other guidance despite some timing differences with the progression of the upper-trough. At the surface, the CMC is more similar to the GFS with a stronger surface low taking a more northerly track in Canada compared to the ECMWF. The ECens mean was most similar to the cluster of deterministic solutions, offering a base solution with the deterministic guidance providing more specific details. Similar to the activity to the east, even with the noted differences, precipitation chances look to remain rather limited regardless of the eventual system path/timing given the more northerly track amongst the guidance overall. Portions of the Pacific Northwest may get some light to moderate precipitation. However, there is an effect on forecast temperatures, with an upward trend over the northwest and north-central U.S. less progression of cooler air to the north southward. Most attention will likely focus on the potential for tropical development in the Caribbean/Gulf of Mexico, with the National Hurricane Center indicating a 50% chance of development over the next 7 days. Both the deterministic and ensemble guidance indicate a disturbance in the Gulf but to varying degrees with respect to timing, path, and strength. The GFS in particular has been more aggressive in the prior few runs with a stronger, faster system following a more western track. For now, the WPC/NHC coordinated forecast brings an area of low pressure northward from the western Caribbean and into the eastern Gulf of Mexico by next weekend, eventually approaching the Gulf Coast of Florida. This is a compromise solution between the 00Z ECMWF/CMC and 06Z GFS as well as the ensemble members that tend to cluster around this solution. The 12Z GFS did revert to a more aggressive faster and western solution. Trended wetter with the QPF forecast in vicinity of the Gulf Coast and over Florida given the consistent expectation for at least increasing tropical moisture in the vicinity. The NBM had trended significantly upward, especially over Florida, compared to the prior forecast. A contribution from the ensemble means was used to temper amounts a bit and increase coverage given the uncertainty tied to tropical development, but would expect a need to steadily increase totals as confidence increases. Opted for a composite blend for the updated WPC forecast early in the period as the deterministic guidance (00Z ECMWF/CMC/UKMET and 06Z GFS) is in relatively good agreement. An increasingly greater contribution from the ECens mean was included during the mid- to late forecast period as this solution offered the best compromise between increasing differences in the 00Z CMC/06Z GFS and 00Z ECMWF in the late period, with the deterministic guidance used to provide a bit greater detail. ...Weather/Hazards Highlights... Regardless of how much tropical development occurs over the Gulf of Mexico, the forecast pattern evolution should support an increase of moisture/rainfall along the Gulf Coast with time. By Day 5/Thursday some solutions are suggesting that parts of the central Gulf Coast could start to see some locally heavy activity. However flash flood guidance values are quite high over the region and guidance is very dispersed regarding if/where heavy rainfall may occur, so the Day 5 ERO depicts no risk area for now. From late week into the weekend the best potential for significant rainfall will be along the eastern half of the Gulf Coast, including Florida, but with considerable uncertainty in the specifics as reflected by guidance spread for possible tropical development and timing/track. Expect lighter rainfall elsewhere along portions of the fronts supported by northern stream dynamics. Such rainfall areas include the Northeast on Wednesday. Some organized light to locally moderate rain will be possible over the Northwest with a front arriving around Thursday-Friday, followed by increasing rain chances over the east-central U.S. as the front reaches that part of the country by next Sunday. The combination of persistent upper ridging over or near the Southwest and warm flow ahead of a couple fronts as they approach the Plains will produce well above average temperatures from the southern half or so of the West into the central U.S. through the end of the week. Expect highs 10-15F above average with locally higher anomalies. Record highs should be most numerous from the Southwest/California into the central High Plains on Wednesday, followed by a gradual decrease to perhaps a few locations by Saturday. Next weekend's cold front should finally begin to suppress the very warm to hot weather by Sunday, though perhaps not quite yet for the Southwest. The Northwest should see more moderate readings on either side of normal. Temperatures over the East will vary with frontal progression but likely average out to near or moderately above normal. Putnam/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