Extended Forecast Discussion...UPDATED NWS Weather Prediction Center College Park MD 259 PM EDT Wed Mar 20 2024 Valid 12Z Sat Mar 23 2024 - 12Z Wed Mar 27 2024 ***Late season winter storm becoming more likely for the Dakotas and into Minnesota and Wisconsin*** ...General Overview... The shortwave trough crossing the Southeast U.S. on Friday and into Saturday will produce a surface low over the northeastern Gulf, and then develop further as it tracks off the Southeast Coast going into the weekend, producing heavy rain and gusty winds for many coastal areas. Looking ahead to the Sunday to Tuesday time period, a pattern change evolves across the western half of the U.S. as a larger synoptic scale trough develops across the Intermountain West and Rockies. This will lead to surface cyclogenesis across the Central Plains and Midwest, bringing a late season winter storm to parts of the Northern Plains, and strong thunderstorms across portions of the South. ...Guidance/Predictability Assessment... Model guidance has been relatively consistent in both the overall pattern and trends within individual runs regarding a southern stream shortwave/subsequent Gulf Coast to southwest Atlantic low development this weekend. The latest 00Z ECMWF and CMC continue to show a low developing along the northern Gulf Coast around Saturday morning, tracking to the northeast off the coast of the Carolinas by Sunday morning. On the other hand, the GFS has maintained a solution showing a more active northern stream wave, potentially phasing with some of the energy from a faster moving southern stream system, subsequently accelerating to the north into Canada. Some energy remains to the south but maintained/additional surface low development takes place far offshore with no remaining sensible weather impacts for the East Coast. The UKMET maintains only a southern stream system similar to the ECMWF/CMC, however the low progression is slower. Then, mid-period early next week, attention turns westward as an amplifying western trough leads to lee cyclogenesis along the Rockies, with the deepening low eventually lifting northeast through the Plains/Upper Midwest. Once again, the 00Z ECMWF and CMC have a similar solution in terms of timing and track of the surface system through the Upper Midwest, traversing the region Sunday through Tuesday, while the GFS is faster and brings the low into Canada by as early as Tuesday morning. Available data from the suite of machine learning models from the EC supports a solution with similar track/timing to the ECMWF/CMC, both for the East Coast system as well as the central U.S. low, favoring a slower movement while traversing the Upper Midwest. The ICON also favors the ECMWF/CMC solution off the East Coast, though does indicate more uncertainty with the system over the central U.S., taking a more southeasterly track compared to the ECMWF and CMC. The updated WPC forecast uses an initial composite blend of the deterministic guidance but with preference given to the ECMWF and CMC given better consensus support for their handling of the East Coast and central U.S. systems. There is an increasing contribution from the ECens and GEFS means by the middle of the period as uncertainty regarding the timing and intensity of the central U.S. system grows, favoring a greater percentage of the ECens given the GEFS shows a faster trend similar to the GFS, which has less consensus support. A 50/50 blend of the deterministic and mean guidance is used by the end of the period given increasing spread handling upstream energies over the West. ...Weather/Hazards Highlights... On Saturday, the core of heaviest rainfall lifts northeast and the best prospects for heavy rainfall would be from eastern North Carolina to the Delmarva and into coastal New Jersey and southern New England, with a Marginal Risk encompassing the region. A Slight Risk is included from the northern Delmarva to central New Jersey, where a stronger model signal exists for a few inches of rainfall during the Day 4 period. On Sunday, a Marginal Risk area is planned for portions of the Central Plains with a corridor of heavier rain likely developing from northern Kansas to eastern Nebraska as the surface low tracks by. For the West, there is a Marginal Risk planned on Saturday for coastal portions of northwestern California and southwestern Oregon with moisture flux from the Pacific, producing 1-3 inches of QPF here. Attention then turns to the central U.S. as a strong surface low is forecast to develop in the lee of the Rockies, and then tracks towards the Upper Midwest by Monday morning. There continues to be an upward trend for significant snowfall accumulations (8"+) from eastern Montana to the Dakotas, Minnesota, and northern Wisconsin, as this region will be on the colder side of the low with a deformation zone developing, and producing bands of heavier wet snow with gusty winds. There has been a northwestward trend for the higher snowfall totals across the Upper Midwest compared to yesterday. A swath of potentially impactful freezing rain may also occur, most likely to the southeast of the heavy snow swath from eastern Nebraska northeastwards through Iowa and into southwestern Wisconsin. Timing/track uncertainty may change the eventual location of the impactful winter weather, but the risk for disruptions to daily life and hazardous travel is apparent. Multiple rounds of showers and thunderstorms, some of which could be strong to severe, can be expected in the warm sector of the system, beginning from the Southern Plains Sunday, spreading eastward to the Mississippi Valley Monday and the central Gulf Coast to the Ohio Valley Tuesday. There are still question marks about how much instability will exist ahead of the front as the system progresses eastward, even though the kinematic environment will be more favorable for potential severe weather. Snow showers are also expected to continue across many of the Western U.S. mountain ranges with the upper trough building in. In terms of temperatures, winter-like readings will continue across the north-central U.S. with a strong arctic high situated over central Canada providing cold northerly flow. Meanwhile, a pleasant warm-up is likely ahead of the developing storm system from the Deep South to the Ohio Valley. Putnam/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