Extended Forecast Discussion NWS Weather Prediction Center College Park MD 258 AM 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, especially central/western areas, 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... Recent guidance has been exhibiting decent majority clustering for the character of northern stream flow in principle, but with enough stray solutions from one model or another plus wide ensemble spread to temper confidence. A positive trend over the past day has been the adjustment of the CMC toward what has been the most common dynamical/machine learning (ML) model theme which by Friday would have a trough over the eastern Pacific, ridge building into western North America, and a broad trough to the east. 12Z/18Z GFS runs had strayed from this pattern due to sending out some leading Pacific shortwave energy, ultimately resulting in a very out-of- phase solution for downstream flow aloft and surface frontal positions by next weekend. The new 00Z GFS has snapped back closer to consensus. UKMET runs still differ from the majority though. There are still differences for trough amplitude/timing as it comes into the Northwest Saturday (including the 00Z ECMWF trending noticeably flatter), and as it amplifies somewhat into the central U.S. by next Sunday, continuing to favor a blend approach. Most guidance expects a trailing upper ridge to reach the West Coast at that time. The forecast continues to be challenging across the southern tier and into the tropics. GFS/CMC runs remain the most aggressive in their development of a Caribbean wave and tracking the system at least as far as the northeastern Gulf of Mexico. The 12Z ECMWF had reflected a similar idea but in slower form (reaching only the central Gulf by Sunday), while the new 00Z run takes a lot longer to show any definition at the surface. ECMWF AIFS guidance has indicated a northeastward nudge versus the 12Z ECMWF run, while remaining dynamical and ML guidance shows a wide array of possibilities. Meanwhile there are still important detail differences for the potential Plains/Mississippi Valley weakness aloft that would support a wetter pattern along the Gulf Coast (most likely central/eastern portions) regardless of the degree of tropical development. Considerations based on 12Z/18Z guidance led to starting with an operational model composite on Wednesday but then quickly replacing the GFS with the GEFS mean due to developing differences for northern stream flow. The blend removed the UKMET late in its run due to its northern stream details, followed by the CMC for its farther east Gulf system. This ultimately led to next weekend's forecast consisting of some 12Z ECMWF and the rest 18Z GEFS/12Z ECMWF means. The blend was adjusted to reflect yesterday's WPC/NHC coordinated track for the possible Gulf system through Saturday, best represented by a slightly slower version of the 18Z GFS. Northeastward extrapolation into Sunday continued to be a little south of the 18Z GFS. ...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, 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 and perhaps a little light/scattered rain with a Great Lakes front by Friday. Some organized light to locally moderate rain will be possible over the Northwest with a front arriving around 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. 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