Extended Forecast Discussion NWS Weather Prediction Center College Park MD 255 PM EDT Thu May 2 2024 Valid 12Z Sun May 05 2024 - 12Z Thu May 09 2024 ...Heavy rain will continue in the south-central U.S. into Sunday with increasing rainfall across the northern Plains/Mississippi Valley on Monday... ...Overview... An upper low that is strong for this time of year is forecast to move through the West early next week and track east into the Plains and Midwest as the week progresses. This will support cooler than normal temperatures, gusty winds, and precipitation including higher elevation snow in the West early in the week. Rain that could be locally heavy then shifts into the northern Plains to Midwest. Farther south, a subtropical jet coming into the south- central U.S. and the western side of a front stalling will lead to rain and thunderstorms in the southern half of the Plains and Mississippi Valley into Sunday, potentially in some areas that have received ample rainfall recently and/or will during the short range period. Then mean upper ridging will lead to warming temperatures in much of the central and eastern U.S. next week. ...Guidance/Predictability Assessment... Recent model guidance, including the newer 12Z cycle, continues to show reasonably good agreement with an upper low coming through the Great Basin as the period begins Sunday shifting east Monday and pivoting northward before slowing over the northern Plains. A strong surface low (that models show could break low pressure records for May) will accompany it. Model spread was well within typical ranges through about Monday-Tuesday, though the details of exact strength and track and QPF remain uncertain. A model blend favoring the deterministic guidance continued to work well for the first few days of the forecast. By midweek, model spread increases as the upper low gets trapped between ridging to its east and an upper high to its north in a blockier pattern, with some guidance wanting to hold the upper low back longer than others. There is also additional energy which drops into the West this period too, which may complicate the pattern. The ECMWF (both 00z and 12z runs) are strongest with holding the northern Plains low back. Given the model variability, transitioned fairly quickly toward a 60/40 ensemble mean to deterministic guidance blend late period. This maintained good continuity with the previous WPC forecast as well. ...Weather/Hazards Highlights... As the upper low tracks into and through the West, widespread precipitation is likely. The higher elevations of the Sierra Nevada, north into the Cascades, and east into the Intermountain West/Rockies will see some May snow. The trough/low aloft and frontal boundaries could also produce some high winds across the Great Basin to Rockies to High Plains. Precipitation is forecast to move steadily eastward through early next week, reaching the northern Plains to Midwest Monday and Tuesday. Rainfall on the west side of the strong surface low could be heavy across eastern Montana into the Dakotas given strong upper-level support for lift. However, this region is not particularly sensitive to flooding concerns, and instability should be limited wrapping around the low. Thus a Marginal Risk is in place in the Day 5/Monday ERO for this area given the potential for unusually heavy rain but without widespread significant flooding impacts. The Marginal Risk extends southeast into the Upper/Middle Mississippi Valley, where more instability will be in place for higher rain rates, but storms will likely be faster-moving. The Storm Prediction Center also has portions of the Plains delineated with potential for severe weather on Monday. Rainfall is forecast to shift east into the Great Lakes and Ohio Valley Tuesday-Wednesday. Farther south, the subtropical jet looks to be active coming into the south-central U.S. Sunday, focusing convection in a moist and unstable environment. Many areas in the south-central U.S. will have quite saturated antecedent conditions because of recent and upcoming short range events, so this will be a factor for flooding as well. Though there is still some variability, models seem to be converging on a position for heavier rainfall in eastern Oklahoma and vicinity, so the ERO for Day 4/Sunday includes an embedded Slight Risk within the broader south-central U.S. Marginal for that area. Fortunately into the workweek, rain chances should lessen for those waterlogged areas. The Eastern Seaboard can also expect some rain on Sunday, with perhaps a nonzero chance of locally heavy rain, though antecedent conditions are much drier across the Mid- Atlantic into the Northeast. Much cooler than average temperatures by around 15-25 degrees for highs will be present in the West on Sunday underneath the upper low. The anomalously cool temperatures should moderate somewhat as the week progresses, but spread into the northern High Plains after a warm day on Sunday. But farther southeast, mean upper ridging will cause warm temperatures to spread across the south-central and east-central U.S. into the Eastern Seaboard. 90s are forecast to become more widespread across the southern tier, with temperatures over 100F (and even higher heat indices) into southwest Texas and the possibility for the first excessive heat days of the year. Santorelli/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