Extended Forecast Discussion NWS Weather Prediction Center College Park MD 303 AM EDT Tue Sep 17 2024 Valid 12Z Fri Sep 20 2024 - 12Z Tue Sep 24 2024 ...Heavy rain with possible flooding is forecast for the north- central Plains to Upper Midwest this weekend... ...Overview... Upper troughing over the West late week will pivot eastward and provide support for likely heavy rain across portions of the central U.S. that looks to peak over the weekend. The trough pushing east along with frontal boundaries should gradually cool the central U.S. closer to normal after a warm period through late week. Meanwhile in the East, upper troughing will linger for late week into the weekend, which along with weak surface frontal systems over the western Atlantic may lead to showers across parts of the Eastern Seaboard. A stalling front may focus rain over South Florida at times for perhaps nonzero flooding concerns over the urban corridor. ...Guidance/Predictability Assessment... Model guidance begins the period Friday in good agreement with the overall pattern, indicating upper troughing over the West with a possible embedded upper low over California/Nevada, ridging over much of the central U.S. though limited in its northward extent due to northern stream energy, and East Coast troughing. Generally this pattern will shift eastward over the weekend, and an average of the 18Z GFS and 00Z CMC/ECMWF worked through the weekend while the 12Z UKMET seemed too slow in the West. Model spread increases considerably regarding the evolution of the western trough/low and how it interacts with northern stream shortwaves, in the 12/18Z model cycle and the newer 00Z model cycle as well as the AI models. The 00Z ECMWF is generally stronger with a shortwave developing a compact closed low moving through south-central Canada Sunday compared to its previous run, while GFS runs have had this feature but with timing differences. Meanwhile the new 00Z CMC deviated considerably from its previous run and from other guidance in phasing the northern and southern stream troughing energies over the weekend to create one upper low in the north-central U.S. Sunday, which seems like a low probability solution since other guidance does any phasing of the streams later. The upstream pattern is also rather uncertain into early next week as guidance varies on ridging or troughing coming into the Northwest. The 18Z GFS seemed like an outlier at first blush compared to other deterministic models as it dug energy more deeply into the northern High Plains by Tuesday, but it had support from the ensembles and the newer 00Z models did trend this way somewhat. Some guidance, especially recent Canadian and GFS model runs, offer increased tropical potential up from the Caribbean into early next week. Uncertain position and digging of upper troughing by then to the Southeast would affect any lower latitude interactions, so WPC and NHC will continue to monitor. The WPC forecast was based on a blend of the 18Z GFS and 12Z ECMWF/CMC early, transitioning to favor more ensemble means by the later period given increasing spread. ...Weather/Hazards Highlights... Low pressure/frontal systems over the central U.S. will help produce scattered rain and thunderstorms there on Friday. There is low confidence in any area of focus for heavy rain, so continued to leave the Day 4/Friday ERO blank pending better model agreement. However, by Saturday, dynamical support for thunderstorms with heavy rain will increase as the upper low within the trough approaches. Models therefore show a considerable uptick in rainfall amounts on the cusp of the instability gradient, with 3-5 inches locally. For the Day 5/Saturday ERO, have outlined a Marginal Risk centered over Nebraska/Kansas and vicinity. There may be a need for higher risk levels within the Marginal in the future if models align more agreeably on an heavy rain axis (and that axis is not over the Nebraska Sandhills that are not prone to flash flooding). Moderate to heavy rain is expected gradually shift into the Midwest Sunday-Monday. Upstream, the questions with the pattern aloft create uncertainty with the QPF in the Northwest into the weekend and early next week, but there is a growing signal to focus a deeper influx of moisture and system energy into the region monitor. In the East, troughing remaining overhead and weak Atlantic fronts/surface lows could promote showers over parts of the Eastern Seaboard late this week into early next week. These showers look to remain light. The Florida Peninsula could also see some focused convection due to a frontal boundary in a moist environment. This front stalling into the weekend and beyond could increase rain totals, which may create flooding problems especially for the typically sensitive urban corridor of Southeast Florida. Initial troughing aloft in the West will lead to continued below normal temperatures there through late week, though moderating compared to the short range to only 5-10 below normal. The West Coast, especially California, looks to warm to a bit above normal by early next week pending the upper level pattern uncertainties. Cooler than average temperatures will migrate into the north- central Plains under the trough early next week. Warmer than normal temperatures, including 90s to near 100F in the southern Plains, will last into late week and then gradually moderate. The scope of above normal temperatures will decrease into the weekend and mainly focus across the Upper Great Lakes to the Ohio Valley. The Eastern Seaboard looks to be around normal on average through the period, with parts of the Mid-Atlantic cooler than usual for highs. Tate 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