Extended Forecast Discussion NWS Weather Prediction Center College Park MD 321 AM EDT Sat Nov 2 2024 Valid 12Z Tue Nov 05 2024 - 12Z Sat Nov 09 2024 ...Overview... The majority of guidance shows strong upper ridging over parts of the southeastern U.S. and western Atlantic while strongly digging western U.S. trough energy likely closes off a low that drifts over the Four Corners before emerging into the Plains by late week or early weekend. The upper low should produce an area of potentially significant snowfall over parts of the Rockies around midweek and then spread rain over the Plains later in the week. To the south of the upper ridge, guidance continues to show possible Caribbean into Gulf of Mexico tropical development (being monitored by the National Hurricane Center) with a lot of guidance spread for where the surface system itself may track, as well as for the path of moisture to its north/east. A wavy front pushing into the Mississippi Valley by Tuesday may produce some enhanced rainfall over that region. Northern stream flow will see moderate progression, with a ridge building into western Canada and mean troughing setting up farther east. A Pacific trough may approach the Northwest by next Saturday. ...Guidance/Predictability Assessment... This forecast update incorporated an average among the 12Z/18Z GFS and 12Z ECMWF/UKMET for the first half of the period, followed by some inclusion of the 18Z GEFS/12Z ECens means along with continued input of the 12Z ECMWF and eventual transition of GFS input to only the 12Z run. The largest scale consideration this cycle was to discount the 12Z CMC/CMCens which were notably faster and more open with the early- period western upper trough, plus quite progressive to lift the possible Caribbean/Gulf system into the East. While signals are somewhat more mixed for the tropical feature, other dynamical guidance plus all of the 12Z machine learning (ML) models favor some variation of the Four Corners upper low. However the ML models over the past couple days have been up to a few degrees of longitude eastward with the upper low position. The new 00Z CMC has improved considerably in terms of adjusting toward the consensus for the Four Corners low. By Saturday the 18Z/00Z GFS runs stray farther north/northeast of most other guidance including the ML models, favoring the aforementioned switch to just the 12Z run for that component of the blend. Upstream, there are typical detail/timing differences for the Pacific trough approaching the Northwest by Saturday but decent clustering overall for a day 7 forecast. ML models show similar ideas to the dynamical guidance. Meanwhile, models are struggling to resolve the combination of possible Caribbean/Gulf of Mexico development as well as details of upper ridging whose evolution will determine how much moisture/shortwave energy could reach the Southeast/Florida. The 12Z ECMWF showed how these two could be separate considerations, and latest guidance still shows a lot of spread. ECMWF AIFS runs continue to lean on the northern/northeastern side of the ML spread over the Gulf, while a couple ML models suggest a stronger drying influence of upper ridging along and just inland from the Southeast coast (closest to the UKMET) versus some other guidance. The manual forecast stays very close to continuity in reflecting yesterday's WPC/NHC coordinated depiction of the possible tropical system and extrapolation into the new day 7. ...Weather/Hazards Highlights... A slow wavy front should approach/reach the Mississippi Valley and western Gulf Coast by Tuesday-Tuesday night, with continued potential for some training of moderate to heavy rainfall but likely to a less pronounced degree than expected just to the west in the short range. The Day 4 Excessive Rainfall Outlook maintains a Marginal Risk area from the Lower Ohio Valley southwestward, now extending to the northern half of the Texas coast. There has been some improvement for guidance clustering regarding where the relative maximum of rainfall should be within this region (along and just east of an area from eastern Arkansas into far southern Illinois). However ground conditions are generally neutral to dry and the GFS/ECMWF do not suggest too much instability reaching that far north. Thus for now will maintain just the Marginal Risk area while monitoring for signals of any heavier bands that could favor an upgrade. Especially by Wednesday, guidance diverges considerably regarding the details of anomalous moisture/heavier rainfall that may overspread parts of the Southeast/Florida to the north of the possible Caribbean into Gulf tropical development. This will depend on finer scale details of upper ridging whose western side will determine how much moisture/shortwave energy may lift northward. The most common theme is for significant precipitable water anomalies to reach portions of the Florida peninsula at the very least, so the Day 5 ERO starts out with a Marginal Risk area over the southern two-thirds of Florida. Other parts of the Southeast may ultimately be included depending on guidance trends. Guidance spread for what happens with this moisture and the possible Gulf feature after Thursday is still very wide, keeping confidence low. Upper trough dynamics likely closing off an upper low around the Four Corners should push an area of mountain snow southward through the Rockies, with highest totals likely to be over southern Colorado/northern New Mexico based on the current forecast path of the upper low. Some snow could extend into parts of the High Plains. Expect rainfall over the Plains to increase in coverage and intensity after midweek as the Four Corners upper low begins to drift eastward. There is still uncertainty for the details, but gradually increasing confidence in some areas of heavier rainfall. A large portion of the eastern U.S. will see well above normal temperatures, especially from Tuesday into Thursday morning when some areas will see plus 20F or greater anomalies (more for morning lows versus daytime highs) along with potential for daily records for highs/warm lows. Frontal progression will temper the most extreme anomalies but persistent Southeast upper ridging should keep the southern two-thirds of the East above normal through Friday-Saturday. In contrast, the upper trough/low likely affecting the West/Rockies for multiple days should produce chilly temperatures for most of next week with highs of 10-20F below normal most common Wednesday-Friday. 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