Extended Forecast Discussion NWS Weather Prediction Center College Park MD 310 AM 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 begins 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... Model guidance begins in somewhat good agreement with the upper pattern at the start of the period Saturday, consisting of an upper low over Arizona/Utah, low amplitude northern stream troughing over the north-central U.S., ridging from the Southern Plains northeast to the Midwest, and lingering troughing in the East. But even over the weekend, some guidance quickly diverges particularly in the west-central U.S. Some models phase the northern stream and southern stream upper low quickly by Sunday. The 12Z CMC was particularly egregious with this while the 00Z CMC is not as dramatically different, but still phases the streams well before the GFS/EC consensus. This leads to the CMC and UKMET having considerable QPF differences, which was not favored. Then additional energy digging on the west side of the trough as it moves into the central and east-central U.S. is also uncertain. Some guidance pulls it far south enough to create an upper low in the southern stream somewhere across the West, but with varying placement. Then fortunately the 00Z model guidance consensus is better with the timing of the upstream pattern in showing ridging across the Northwest early week with troughing coming in by next Wednesday. Model and ensemble guidance continues to show the potential for tropical development in the Caribbean, with a tropical system being pulled north next week. GFS and CMC deterministic runs and many of their ensemble members are faster to pull the system north than the EC deterministic and ensemble members. 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 began with a blend favoring the 12/18Z GFS and 12Z ECMWF, with small proportions of the GEFS and EC ensemble means. Did not include the CMC and UKMET given their differences. As the period progressed, gradually increased the proportion of ensemble means to over half by Days 6-7 given the increasing spread. Continue to make changes to the frontal structures in the central U.S. as the pattern evolves. ...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 centered over Nebraska/Kansas and vicinity. The heavy rain is forecast to shift slightly northeastward on Sunday, prompting a Marginal Risk for parts of Nebraska/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. Antecendent conditions are generally dry, but heavy rain rates could pose flash flooding issues regardless. Moderate to heavy rain is expected 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. 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. 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. 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