Extended Forecast Discussion NWS Weather Prediction Center College Park MD 315 PM 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... Updated 00Z/06Z guidance is in good agreement during the early part of the medium-range period (Tuesday Nov 5 - Saturday Nov 9), depicting a lead southern stream wave lifting northward ahead of a broad area of high pressure over the East Coast while mean troughing remains over the west in an amplified pattern. This will lead to a weakening wavy frontal boundary through the Midwest/Mississippi Valley as the lead energy lifts northward ahead of additional energies from the West. Guidance differs on the timing of northern stream shortwaves and accompanying frontal systems in the mid- to late period along the northern tier of the country, though with little apparent impacts on sensible weather. Attention turns west as a shortwave helps reinforce mean troughing, with all the deterministic guidance and GEFS/ECens means depicting the development of a closed low over the Southwest. Eastward progression of the low is fastest in the UKMET followed by the GFS. The ECMWF is the slowest with the CMC most similar. The ECens mean and GEFS mean generally follow their respective deterministic counterparts. The EC AI suite of models show various solutions within the range of guidance, indicating increasing uncertainty during this timeframe. The 12Z GFS came in ahead of the ECMWF/CMC similar to the 06Z run. Eyes will also be on the tropics as the National Hurricane Center monitors a system in the southwestern Caribbean, currently indicating an 80% chance of tropical system formation in the next 7 days. The GFS/ECMWF as well as the ensemble means indicate an area of low pressure lifting northward towards the eastern Gulf of Mexico before taking various paths further north or west, with the GFS further to the north and into the central Gulf and the ECMWF to the west. The GFS path is further east compared to prior runs. The CMC has been an outlier with a much faster, eastern path through the Gulf and into the southeastern U.S. The updated NHC/WPC coordinated forecast brings an area of low pressure into the Gulf just to the west of the GFS path, showing a more eastward trend than the prior forecast but still incorporating the potential for more westward movement as indicated by the ECMWF. The updated WPC forecast uses a composite blend of the deterministic guidance with good overall agreement in the early period. The UKMET is removed in the mid-period given its much faster progression of the southwestern closed low, and the CMC is removed given the more aggressive northeastward track of the Caribbean low pressure system. The ECMWF is emphasized over the GFS for the end of the period favoring more detail of the western position of the closed low given some agreement from the CMC in this area, with a contribution from both the GEFS/ECens means to help account for the increasing uncertainty. The QPF forecast remains more difficult, with notably high uncertainty for potential rainfall over Florida and the southeastern U.S. as moisture increases ahead of the approaching low, including high spatial disparity with the ensemble means. Favored the NBM which indicates higher rainfall over central/south Florida, though with amounts moderated a bit for now given the uncertainty. Later in the forecast toward the end of the week, higher rainfall amounts are indicated with more moderate spatial differences late Thursday/early Friday as the closed low over the Southwest begins to emerge over the Plains. Spatial differences increase once again later Friday/early Saturday with the GFS favoring a faster northeastward progression towards the Midwest while the ECMWF favors areas further west over the Plains. The deterministic guidance was used to increase totals given the signal for heavier precipitation through early Friday before switching to higher contributions from the means given the spatial disparity. ...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. Guidance trends have indicated more confidence that some snow could also occur over portions 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. 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