Extended Forecast Discussion NWS Weather Prediction Center College Park MD 311 PM EDT Sat Apr 13 2024 Valid 12Z Tue Apr 16 2024 - 12Z Sat Apr 20 2024 ...Deepening Plains surface low likely to bring severe and heavy rainfall threats ahead of it and strong winds and notable snow behind... ...Overview... A closed upper low entering the central High Plains to start the period Tuesday will support a deepening surface low pressure system that tracks into the Upper Midwest/Great Lakes Wednesday-Thursday. Various hazards are likely with this storm system, including severe weather and bands of heavy rainfall on the warm side along with a broad area of gusty winds. Behind this storm, another upper trough/low digging into the Northwest Tuesday should ultimately evolve into a well-organized upper low near/over the Upper Great Lakes and amplify troughing in the eastern third of the nation by late week. Behind this, upper ridging looks to build over the Northwest/western Canada. ...Guidance/Predictability Assessment... Model guidance remains reasonably agreeable on the large scale/presence of these features, but still with plenty of disagreement in the timing and details. With the initial low out of the Plains early-mid week, there is some model run-to-run timing variability still, but enough agreement to support a purely deterministic model blend. The next system tracking from the Northwest into the Upper Great Lakes shows more uncertainty on its evolution and placement, though with some model convergence in recent cycles. Around midweek, most models are more agreeable than a day ago regarding how much troughing will get into the northern Rockies/High Plains, affecting snow totals there. The 00Z CMC was the main outlier in showing less troughing, but the 12Z CMC was in better agreement. There were also some differences Wednesday in when this energy consolidates into a closed upper low, with GFS runs being slower to close it off, but the 12Z GFS has come into better alignment with that timing. Models then show this upper low tracking eastward through south- central to southeastern Canada Thursday-Saturday, though with variations on the exact timing as well as the southern extent of the trough amplitude into the U.S. At least recent model cycles do not show too many axis differences with the trough/low compared to the 12Z 4/12 run of the ECMWF for example. The details of Northwest ridging and Southwest southern stream troughing could also use more time to determine. Thus the WPC forecast gradually transitioned from a multi-model deterministic blend favoring the GFS and ECMWF early in the period to a half model/half ensemble mean blend by Day 7. ...Weather/Hazards Highlights... Precipitation will be ongoing across the Rockies/Central U.S. with multiple weather hazards as the upper low helps to deepen a Plains surface low into Tuesday. The Storm Prediction Center has been highlighting eastward progressing severe thunderstorm potential reaching the Lower to Mid-Mississippi Valley to western Ohio Valley on Tuesday given the abundant instability and moisture present with this system. These ingredients could also produce heavy rainfall with intense rates capable of flash flooding over parts of the Plains and the Upper Midwest. Excessive Rainfall Outlooks continue to show Marginal Risks stretching from the Mid-Lower Mississippi Valley Tuesday into the Tennessee and Lower Ohio Valleys Wednesday along with the potentially severe storms, while farther north along the low track the ample lift should provide support for possibly heavy rainfall. Watching a corridor from eastern Nebraska and South Dakota into parts of Iowa and southern Minnesota for an embedded Slight Risk on Tuesday, where heavier totals are likely to maximize. This area will likely see exceedance of Flash Flood Guidance values there, but in this region of the country this does not often result in notable flooding impacts, so held off. Additionally, this area has seen normal to below normal soil moisture/streamflow conditions, another mark against sensitivity to flooding. Behind the low, the storm should produce a broad area of gusty winds focusing mainly across the north- central High Plains on Tuesday. The next upper trough digging into the western U.S. and extending into the northern Plains early-mid week, along with another frontal boundary, should tend to focus precipitation over the northern half or so of the Rockies and High Plains for a few days. Thus notable snow will be likely over the higher elevations. Chances for meaningful snow in lower elevations are decreasing, but blowing snow is a concern. Then as systems progress, rain and convection are forecast to move into the eastern U.S. into late week, while lingering across the south-central U.S. as the frontal system stalls. Expect a broad area of above normal temperatures over the eastern half of the country into the southern Plains Tuesday-Thursday, with advancing frontal systems slowly trimming the western side of the warmth. Expect highs in the eastern U.S. to be up to 10-15F or so above normal with anomalies for morning lows tending to be several degrees higher. The upper trough digging into the West and then including the northern Plains will likely start to bring below normal highs into the Northwest by Tuesday, followed by readings 10-15F below average over the northern Rockies/High Plains by next Wednesday-Thursday while gradually expanding farther south and east behind the secondary cold front late week. Tate/Santorelli 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