108 FXUS02 KWBC 250657 PMDEPD Extended Forecast Discussion NWS Weather Prediction Center College Park MD 257 AM EDT Thu Apr 25 2024 Valid 12Z Sun Apr 28 2024 - 12Z Thu May 02 2024 ***Severe thunderstorms and heavy rain expected for the Plains and Midwest Sunday into Monday*** ...General Overview... An amplified upper level flow pattern is forecast to be in place across the Continental U.S. this weekend, with a building ridge across the East and a trough across the Rockies. A strong shortwave ejecting across the Central Plains will spur surface cyclogenesis and multiple rounds of heavy rain and strong/severe thunderstorms, mainly from central Texas to Iowa/Missouri. Very warm conditions for this time of year are expected for the eastern half of the U.S. going into the beginning of next week, and colder temperatures out West will support some lingering snow for the higher terrain of the Colorado Rockies. A second trough is then likely to develop along the West Coast by the middle to end of the next week with the ridge axis building across the Plains. ...Guidance/Predictability Assessment... The 00Z model guidance suite features good overall agreement across the U.S. on Sunday with similar timing on the main shortwave across the central High Plains, so a multi-deterministic model blend works well as a starting point in the forecast process. Greater differences are apparent across the Northern Rockies and Dakotas by Tuesday, with the 00Z ECMWF trending stronger with an upper low tracking near the Canadian border. By Wednesday, the 00Z ECMWF becomes more out of line with the consensus and the machine learning models with the upper low/trough near the Pacific Northwest, so the GFS and GEFS mean serve as a better depiction here. There is also disagreement on the timing of a trough and cold front across the Great Lakes and Northeast by the end of the forecast period Thursday, with the CMC/GFS faster and the ECMWF slower. The use of the ensemble means accounted for about half of the forecast blend by next Thursday. The QPF forecast started with about 50% NBM and 1/6 each of the GFS/ECMWF/CMC through Monday, and then trending to about 2/3 NBM and removing the CMC for Tuesday and Wednesday. ...Weather/Hazards Highlights... The overall synoptic environment will be quite conducive for multiple episodes of excessive rainfall across portions of the central and lower Mississippi River Valley. There will likely be multiple mesoscale convective systems that develop from the ArkLaTex region to Missouri, and there is a good chance of training convection given the slow overall progression of the frontal boundary and parallel flow to the trailing frontal boundary. For the Day 4 period Sunday, the existing Slight Risk area from the previous Day 5 will be expanded to include more of eastern Texas where the QPF signal in the GFS/ECMWF has increased. There is now a multi- model signal for some 3-5 inch totals where storms train over the same areas, with locally higher totals possible. There is still a good chance for a future upgrade to Moderate Risk for portions of this region if these model trends continue. The WPC forecast for QPF remains on the lower end of the guidance spectrum given the influence from the NBM. A Slight Risk area is also planned on Day 5 centered over northern Louisiana and into portions of adjacent states, generally southeast of the Day 4 outlook area. The convergence boundary is likely to linger across the region on Monday and fire new rounds of convection, followed by an abatement in the coverage and intensity of the convection going in Tuesday over this region. Elsewhere across the U.S., widespread moderate to locally heavy rain is expected from Minnesota to northern Michigan on Sunday as the surface low lifts northeastward, and a Marginal Risk area is planned to account for that on the Day 4 outlook. Colder temperatures moving in across the Intermountain West and the Rockies behind this storm system will likely result in continuing snow showers for the higher mountain ranges of Wyoming and Colorado, although becoming lighter. Rain and mountain snow is also likely to make a return to the Pacific Northwest for the beginning to middle of next week as the next shortwave trough arrives. By Wednesday, showers and storms could once again increase in coverage from Texas to the Central Plains. Temperatures will also be making weather headlines as a major warm-up is expected for a wide expanse of the country east of the Rockies. Highs are expected to rise well into the 80s from the Central/Southern Plains to the Ohio Valley and then reaching the East Coast by Monday, perhaps reaching 90 degrees in some cases. These readings are on the order of 10-20 degrees above normal for late April, and there may be some daily record highs set. Overnight lows will also be summer-like with lows only falling into the 60s for many of these same areas. A modest cool-down is likely by mid- week from the Great Lake to the Ohio Valley and Northeast, and still remaining warm and humid across the south-central U.S. to the Carolinas. The opposite holds true for the Intermountain West on Sunday with readings 5-15 degrees below average, before things moderate going into early next week. Hamrick 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 $$