Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 PM EDT Wed Sep 18 2024 Valid 12Z Sat Sep 21 2024 - 12Z Wed Sep 25 2024 ...Heavy rain with possible flooding is forecast for the central Plains to Midwest this weekend... ...Overview... Upper troughing centered over the Four Corners states as the medium range period beginning Saturday will gradually push into the central U.S. into next week while reloading. This trough and frontal systems at the surface will lead to cooler temperatures behind them, but provide support for likely heavy rain across central portions of the U.S. particularly this weekend. In the East, upper troughing will linger into the weekend, which along with weak surface frontal systems over the western Atlantic may lead to showers across parts of the Eastern Seaboard, along with onshore flow for some possible coastal flooding. This trough looks to push away early next week in favor of ridging. A stalling front may focus rain over South Florida at times for perhaps nonzero flooding concerns over the urban corridor. Meanwhile NHC and WPC will continue to monitor possible tropical development in the Caribbean that may slowly move northward next week. ...Guidance/Predictability Assessment... Overall pattern over the CONUS at the start of the forecast period features a lingering trough over the East Coast, a northern stream trough over the Upper Midwest, a southern stream ridge over the south-central U.S., and a southern stream upper-low over the Four Corners region. The 00Z/06Z GFS and 00Z ECMWF, echoing prior forecasts, show the streams remaining separate as both systems lift to the northeast, with the Four Corners system entering the Plains and bringing a potential threat of heavy rainfall to the central Plains. The 00Z CMC continues to differ from the other guidance, showing the northern and southern stream troughs phasing much quicker compared to the other guidance. Differences amongst the remaining guidance increase in the mid- to late period as a secondary shortwave incoming from the west amplifies into the Southwest, with the energy eventually potentially becoming cutoff by Tuesday. This has become a much more notable trend in the ECMWF guidance as compared to the GFS, with a hint in the 00Z ECens mean. The EC AI model suite does tend to favor this more than not, though with the lack of consensus and support from the means it is difficult to resolve at this timeframe. The overall pattern otherwise for the CONUS is fairly agreeable on a larger-scale, with the central U.S. upper low/trough lifting northeast in much of the guidance as the streams phase, with large- scale ridging over the western U.S. similarly shifting eastward in its wake as upstream energy reaches the Pacific Northwest. The central U.S. trough in the latest 06Z GFS does not lift into the Midwest as quickly as the ECMWF and the ensemble means while also showing a closed high over the Southwest embedded within the broader longwave ridging compared to the weak cutoff low in the other solutions. One last but important feature of interest is the potential for tropical development in the western Caribbean mid-next week. There remains a large amount of uncertainty in the development of a system during this time frame, with the 00Z GFS/CMC and some GEFS and ECens ensemble members faster in both developing a system and bringing it northward into the Gulf, while the 00Z ECMWF does not. Currently anticipate that an area of low pressure may move into the Gulf of Mexico mid-next week, but there remains a large amount of uncertainty and the NHC and WPC will continue to monitor. It is noted that the lingering disagreement on the evolution of the central U.S. trough will likely impact how fast this potential system lifts northward. The updated WPC forecast begins with a blend of the 00Z ECMWF, a mix of the 00Z/06Z GFS, and 00Z UKMET. Similar to the prior forecast, the 00Z CMC is not included given how quickly the solution diverges over the central U.S. as the two streams phase. The differences in the guidance as to if/where there is cutoff energy over the Southwest, or a closed high as the 06Z GFS shows, is harder to resolve given the overlap, so favored a large contribution from the means (and did not include the 06Z GFS) for the end of the period. ...Weather/Hazards Highlights... By Saturday, dynamical support for thunderstorms with heavy rain will increase as the upper low approaches from the Four Corners. Models therefore show a considerable uptick in rainfall amounts on the cusp of the instability gradient, with 3-5 inches locally. For the Day 4/Saturday ERO, continue to depict a broad Marginal Risk stretching from Iowa west through the central Plains and into eastern Colorado. The heavy rain is forecast to shift slightly northeastward on Sunday, prompting a Marginal Risk from northeastern Colorado east through southern Nebraska, northeastern Kansas, and stretching into the Mid/Upper Mississippi Valley for the Day 5 ERO. On both days, 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 given the amounts currently depicted in the deterministic guidance. Antecedent conditions are generally dry, but heavy rain rates could pose flash flooding issues regardless. Moderate to heavy rain is expected to gradually shift across the Midwest Monday and the east-central U.S. Tuesday. While uncertain, rain chances may get renewed by Tuesday in the south- central U.S. as well. 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 some signal for moderate to heavy precipitation in the Washington coastal ranges/Cascades toward the northern Rockies at times. In the East, troughing remaining overhead and weak Atlantic fronts/surface lows could promote showers over parts of the Eastern Seaboard this weekend into early next week. These showers look to remain light, but persistent onshore wind flow may create a coastal flooding hazard along the Mid-Atlantic coast before an offshore low moves away. 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. This is all ahead of potential tropical development farther south in the Caribbean that may slowly make its way north into the middle of next week. Moderate to heavy rainfall may reach portions of the central to eastern Gulf Coast by mid-next week and beyond, but there remains a lot of uncertainty on the exact location and timing tied to the evolution of the potential tropical system. Cooler than normal temperatures particularly in terms of highs will be in place for the Rockies westward on Saturday and focusing in the northern/central High Plains on Sunday underneath the upper trough. Meanwhile farther east, above normal temperatures by 10-15 degrees are forecast for the south-central Plains northeast to the Ohio Valley and Great Lakes region Saturday as a ridge reminds in place between the central U.S. and East Coast upper troughing. The above average temperatures will decrease in scope early next week, focusing mainly over the Ohio Valley and then gradually moderating there too. The Eastern Seaboard could see below average highs into early next week. Temperatures may be near normal in most places by the middle of next week, but pending upper level pattern uncertainties. Putnam/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