Extended Forecast Discussion NWS Weather Prediction Center College Park MD 258 AM EDT Mon Mar 27 2023 Valid 12Z Thu Mar 30 2023 - 12Z Mon Apr 3 2023 ***Organized low pressure system late in the week to produce some severe thunderstorms, episodes of heavy rainfall, and windy conditions across portions of the Plains and Midwest*** ...Synoptic Overview... The low pressure system that will initially be over California during the short range forecast period is forecast to cross the Intermountain West on Thursday as the upper trough axis moves inland. As the trough ejects over the western High Plains, lee cyclogenesis is expected to commence over eastern Colorado Thursday night and a well organized low pressure system will evolve over the Central Plains along a rather strong frontal boundary. This system then moves in a general northeastward direction towards the Great Lakes by Saturday and then to southeastern Canada by Sunday. Meanwhile, the next upper level trough/closed low from the Gulf of Alaska reaches the Pacific Northwest by Sunday and brings another cold front across much of the western U.S. to reinforce the cool weather pattern. This front/upper trough may also result in a surface low developing over the central/southern Plains by next Monday. ...Model Guidance/Predictability Assessment... The 00Z model guidance suite is in decent synoptic scale agreement through Friday night, and a general model blend suffices for this time period. The GFS becomes stronger by early Saturday with a closed low aloft, whereas the other guidance supports more of a positively tilted open wave aloft. The GFS solution would result in a heavier band of QPF on the northern side of the low, so this will be something to monitor in the days ahead if other guidance trends in that direction. The differences with the GFS are even more apparent by Sunday with a much stronger and slower system near the East Coast, whilst the CMC/ECMWF and most of the ensemble members support a quicker passage. For the next system reaching the Pacific Northwest, the CMC is a little faster to bring in the trough, whereas the GFS and ECMWF have a closed low that is slightly slower to arrive, but overall still decent agreement for a day 6/7 forecast. Taking these factors into account, the forecast for the weekend was primarily derived from a CMC/ECMWF/ECENS blend and a little less of the GFS while gradually increasing use of the ECENS and some of the GEFS mean going into Monday. ...Weather/Hazards Highlights... As the first trough and low pressure system from the West ejects across the Plains for the end of the week, precipitation will increase in coverage across the central and then eastern U.S. before clearing out by next Sunday. Strong to severe thunderstorms are possible in the southern half of the Plains Thursday and into large portions of the Mississippi Valley in the warm sector of the low on Friday based on recent outlooks from the Storm Prediction Center. The Day 4/Thursday excessive rainfall outlook has a Marginal Risk area in place across portions of the Upper Midwest, primarily centered over Iowa. The instability axis in the warm sector is expected to push farther north into the vicinity of a warm front from eastern Nebraska across much of Iowa, so some instances of flash flooding are possible in this region where multiple rounds of convection develop in the vicinity of the warm front. Swaths of moderate to heavy rain are forecast to reach the eastern U.S. by late week, though there continues to be model dispersion on amounts. However, there is enough confidence to place a Marginal Risk area from northern Illinois to southern Michigan on Day 5/Friday just south of the warm front, and also from central Arkansas to southern Kentucky where the axis of best instability and forcing is likely to reside. On the northwest side of the surface low, a band of snow in the deformation zone is possible from South Dakota to Minnesota and eventually the northern Great Lakes, though the positioning and snow totals will have to be refined with time as models get a better handle on the expected evolution. Breezy to windy conditions are also likely to develop across the southwestern to south-central U.S. and progressing eastward toward the Ohio Valley with time behind the cold front with a strong pressure gradient in place. Persistent troughing across much of the western U.S. will lead to cooler than normal temperatures for most of this forecast period, with highs running around 10-20 degrees below normal in many cases. The coldest weather is expected across the Dakotas, where temperatures will range from 15 to 25 degrees below average depending on the placement and day, equating to highs in the 20s to lower 30s, and lows in the 0s to low 10s across much of North Dakota for the end of the week. Upper level ridging extending from a Gulf of Mexico upper high should result in warm and humid conditions to rebuild across the southern/central Plains Thursday and spread into the Mississippi Valley Friday and the East by Saturday, with highs generally 5-15F above average and lows around 15-25F warmer than normal. A few record warm low temperatures could be tied or set. 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, experimental excessive rainfall outlook, winter weather outlook probabilities and heat indices are at: 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