Extended Forecast Discussion NWS Weather Prediction Center College Park MD 258 PM EDT Thu Sep 19 2024 Valid 12Z Sun Sep 22 2024 - 12Z Thu Sep 26 2024 ...Heavy rain with possible flooding is forecast for the central Plains to Midwest and Ohio Valley into early next week... ...Overview... Northern and southern stream upper troughing over the central U.S. early next week are forecast to combine and gradually shift east into the Great Lakes to Mississippi Valley as the week progresses. This pattern aloft and surface frontal systems will promote showers and thunderstorms, including potentially heavy rain across the central Plains, Midwest, Great Lakes region, and Ohio Valley. Farther south, 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... Models are pretty consistent with a split flow pattern over the lower 48 at the beginning of the period, eventually phasing into a deep mid-upper-level trough over eastern North America toward the middle-to-end of next week. Meanwhile, an upper ridge develops over the West and propagates into the Central U.S. by mid-week. The main area of uncertainty pertains to a potential tropical system that is forecast to propagate into the Gulf of Mexico sometime during the middle-to- latter half of next week. The operational GFS has trended slower and farther south with next week's Caribbean/GoM tropical system. A mixture of the 00z ECMWF/UKMET/CMC and 06z GFS were used in the days 4 and 5 blends with the EC and GFS favored. The 00z ECE and 06z GEFS were introduced to the blend on day 5 with the operational Canadian removed due to a seemingly overamplification of a mid- level vort over the eastern Pacific and its more bullish/progressive solution with respect to the tropical disturbance in the Caribbean/Gulf of Mexico. The Canadian ensemble is introduced to the blend on day 6 and continued through day 7 to mitigate for potential over-suppression of the tropical by the EC (especially) and GFS suites. ...Weather/Hazards Highlights... The southern stream upper low over the central High Plains early Sunday and pivoting northeast will provide ample dynamical support for thunderstorms with heavy rain on the cusp of the instability gradient. On Sunday, models generally show support for 2 to locally 4 inches of rain across parts of the central Plains into the Mid- Mississippi Valley. Continue to depict a broad Marginal Risk for those areas, and there may be a need for higher risk levels within the Marginal in the future if models align more agreeably on a heavy rain axis. Antecedent conditions are generally dry, but heavy rain rates of 1 to 2 inches per hour could pose flash flooding issues regardless. Chances for moderate to heavy rain should shift east into the Great Lakes and Ohio Valley on Monday as the shortwave moves east. Rainfall totals look to decrease as the shortwave weakens, but with some upper level support and instability, a Marginal Risk still seems warranted for Day 5/Monday, though perhaps lower end than on Day 4/Sunday. Rain is forecast to focus along the eventually stalling front in the east- central U.S. Tuesday-Thursday. Rain chances also are likely to get renewed in the south-central U.S. by Tuesday with another front and energy aloft. Farther west, most areas should stay dry under mean ridging. The Washington coastal ranges/Cascades could be an exception at times. Then in South Florida, showers and storms may focus near a frontal boundary lingering in a moist environment. These do not look to be too heavy next week until around midweek, when there may be some surge of tropical moisture northward ahead of any potential tropical development farther south in the Caribbean that may slowly make its way north into the Gulf during next week. There is still considerable uncertainty in the track and timing of any possible tropical system, so continue to check updated forecasts. Cooler than normal temperatures particularly in terms of highs will peak across the central Plains on Sunday, with highs generally only reaching the low 60s underneath the upper trough. Meanwhile farther east, above normal temperatures by 5-15 degrees are forecast for the south-central Mississippi Valley northeast to the Ohio Valley and Great Lakes region ahead of a front. The above average temperatures will decrease in scope into Monday, focusing mainly over the Ohio Valley and then gradually moderating there too. The Eastern Seaboard could see below average highs into early next week. On the other hand, building ridging in the West will allow for some warmer than average temperatures by 5-15 degrees, bringing temperatures above 100F to the Desert Southwest once again. Kebede/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