Extended Forecast Discussion NWS Weather Prediction Center College Park MD 253 PM EDT Fri Nov 1 2024 Valid 12Z Mon Nov 04 2024 - 12Z Fri Nov 08 2024 ...Heavy rain and severe weather expected across portions of the central U.S. into early next week... ...Overview... The central U.S. will be the most prominent focus for active weather during portions of next week. In particular, continuing into Monday expect an area of heavy rainfall over parts of the Plains/Midwest as southern Plains upper dynamics eject northeastward while the leading wavy front interacts with abundant moisture contributed by strong Gulf inflow. Then most guidance shows shortwave energy arriving from the Pacific and dropping into the Rockies/Four Corners region, possibly closing off an upper low after midweek. This feature may produce additional rainfall over the southern Plains and vicinity late in the week but likely with lighter totals than in the preceding event. An area of rain and higher elevation snow should pass through the Pacific Northwest and move south through the Rockies with the progression/evolution of the energy aloft. This pattern will favor very warm temperatures (likely including some daily records) over the East and below normal readings over most of the Interior West and Rockies. Meanwhile, the National Hurricane Center is monitoring possible Caribbean tropical development that may emerge into the Gulf, with typically broad guidance spread for track and timing. ...Guidance/Predictability Assessment... Updated deterministic and ensemble guidance remains in fairly good agreement and consistent run-to-run on the large-scale pattern through most of the medium range period (Monday Nov 4 - Friday Nov 8). The week begins with a highly amplified pattern with strong ridging over the eastern half of the country and a deep trough over the West, continuing a period of more meridional flow along a wavy frontal system over the central part of the county aiding in moisture return/storm development. The guidance then indicates lead energy will lift north along the western side of the ridge over the Midwest promoting a slow but steady progression of the frontal system eastward, while additional northern/southern stream energy reinforce mean troughing over the western U.S. by mid-week. Both deterministic and ensemble guidance from the 06Z GFS/GEFS and 00Z ECMWF/ECENS then indicate the southern stream energy leads to a closed low over the Southwest. Guidance differences grow between the 06Z GFS/00Z ECMWF on the timing of lead northern stream energy coming out of the West along the northern tier of the country. This is reflected with a frontal system at the surface that follows a slower, more southerly track in the ECMWF and the GFS further north. In addition, the GFS is faster in bringing the closed low in the West over the Plains, leading to more northerly/aggressive lee cyclogenesis ahead of the weekend while the ECMWF remains to the southwest, favoring precipitation chances over the southern Rockies/Plains. The ECMWF fits within the envelope of the relatively similar GEFS/ECens means, and the 12Z GFS came in more similar to the ECMWF. The other area of interest is with respect to potential tropical development in the southwestern Caribbean that may eventually track northward into the Gulf of Mexico. The National Hurricane Center is currently giving this system a 70% chance of development in the next 7 days with a favored path that brings an area of low pressure into the western Gulf of Mexico, which aligns with the 00Z ECMWF/06Z GFS and ECens/GEFS means. The latest CMC was much more aggressive with the potential system, bringing an area of low pressure northeastward towards the eastern Gulf of Mexico. Despite the generally good agreement on the overall pattern, specifics on frontal placement early in the period over the center of the country still lead to much higher uncertainty with the QPF forecast. The GFS/ECMWF/CMC all agree on the potential for a continuation of heavy rain Monday shifting eastward from the Plains towards the Mississippi Valley, but with disagreement on the location of a potentially narrow pre-frontal axis. This may linger longer over the same region into Tuesday as well, with the ECMWF favoring slower frontal progression and additional rainfall. Additional uncertainty arises around the potential for heavy rainfall in South Florida and then at least some rainfall along the Gulf Coast with increasing moisture from the potential tropical system moving into the Gulf, as well as with additional precipitation in the Southwest/Plains later next week depending on the progression of the closed low eastward. The updated WPC forecast used a composite of the deterministic guidance for the first part of the period given good agreement. A contribution of the ECens/GEFS means was used for the latter half of the period, with the CMC removed due to the disagreement in handling the potential tropical system, and a heavy favor towards the ECMCF over the GFS given the clustering of the ECMWF with the ECens/GEFS solutions. Leaned a bit heavier on the means earlier in the period for the QPF forecast given spatial differences in the deterministic guidance, with the expectation amounts may increase as certainty in the frontal progression grows. ...Weather/Hazards Highlights... Strong deep-layer flow from the Gulf of Mexico ahead of a wavy front pushing through the central U.S. will contribute abundant moisture to a heavy rain event expected to continue from the weekend into the first part of next week. The Storm Prediction Center is also monitoring severe weather potential over parts of the central/southern Plains. Currently the relatively higher potential for repeat and training convection with locally high rainfall rates extends from around eastern Oklahoma/western Arkansas northeastward into northern Illinois, corresponding to the Slight Risk area in the Day 4 Excessive Rainfall Outlook. This region will initially see dry ground conditions but some rainfall is likely in the shorter term leading into Day 4, particularly from southeastern Oklahoma/northwestern Arkansas into central Missouri. A Moderate Risk may eventually be needed here with more confidence in the location of the heaviest rainfall totals and how much overlap there is with rainfall on day 3. The coverage of significant rainfall will likely decrease by Day 5/Tuesday as the upper dynamics and primary frontal wave continue northeastward. However lingering moisture/instability may promote some additional rainfall, with considerable spread among guidance for amounts and low confidence in more significant heavier rainfall totals. The Day 5 ERO depicts a Marginal Risk area from the Lower Ohio Valley southwestward as a reasonable starting point given the forecast uncertainty. Incoming Pacific dynamics will bring an episode of terrain- enhanced rain and heavy, higher elevation snow to the Pacific Northwest early in the week with precipitation then dropping south through the Rockies, plus with some rainfall developing over the Plains. Specifics for Rockies/Plains totals will be sensitive to the details of flow aloft. The southern extent of cooler air with this system may also lead to some wintry precipitation outside of mountain areas into the central/southern High Plains, but again with low confidence in coverage and any accumulations. A broad area of well above normal temperatures will drift across the eastern half of the country, with gradual progression of a couple fronts eventually leading to some moderation by late week. Warmest anomalies should be for morning lows, with some areas 20F or more above normal for one or more days, but daily records will be possible for both highs and warm lows during early-mid week. In contrast, a majority of the Interior West and Rockies should see chilly temperatures for most of next week with readings tending to be 5-15F below 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