####018006775#### FXUS02 KWBC 020700 PMDEPD Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM 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... ...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, especially the newer 00Z cycle, is in reasonably good agreement with an upper low coming through the Great Basin as the period begins Sunday that shifts east Monday and then pivots northward and slows 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 Tuesday, though the details of exact strength and track and QPF waffle. A model blend favoring the deterministic GFS, ECMWF, and CMC with small proportions of the GEFS and EC ensemble means worked well for the first few days of the forecast. By Wednesday and Thursday, model spread increases as the upper low gets trapped between ridging to its east and an upper high to its northwest in a blockier pattern. In general the 12/18Z model cycle took the surface low a bit farther north while the older and newer 00Z runs had it more suppressed. Reinforcing energy may dive in from western Canada into the Northwest and complicate matters. CMC runs generally have this energy weaker but entering the U.S. sooner, allowing the troughing to progress eastward, while GFS runs are stronger but delayed with the energy and leave it in the West. The CMC initially appeared to be an outlier but many of the EC- based AI/machine learning models were similar to it. The 12Z ECMWF was more like the GFS but the newer 00Z run is more similar to the CMC and AI models. So there is certainly some remaining spread with this. The WPC forecast shifted toward favoring more GEFS and EC ensemble means late in the period given the increasing model spread. ...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 will embed a 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. 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 possibility for the first excessive heat days of the year. 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 $$