Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM 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 low entering the Central U.S. to start the period Tuesday will support a deepening surface low pressure system which will track 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 down from Western Canada should ultimately evolve into a well organized upper low near/over the Upper Great Lakes by later in the week with deeper troughing moving across the Eastern third of the Nation next weekend. 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 are some model run-to-run timing variability still, but enough agreement to support a purely deterministic model blend. The next system dropping out of Canada into the Upper Great Lakes later next week shows much more uncertainty on its evolution and placement, which translates to differences in surface front progression across the Eastern U.S. into next weekend. The models/ensembles generally agree that ultimately a closed low forms over/near the Great Lakes, but the 12z/Apr 12 ECMWF was more the outlier solution showing an elongated trough/upper low stretching from central Canada towards the Great Lakes, which caused a much faster progression of the Great Lakes upper low due to additional energy from Canada pushing it eastward (and weaker ridging over the West). The GFS and CMC were more compact, and this sort of solution had better agreement from the ensembles and the ECMWF-initialized AI/ML models as well. And interestingly enough, the new 00z ECMWF run tonight (available after forecast generation time) came into better agreement with the GFS/CMC and ensembles. Due to the increasing uncertainty in the late periods, the WPC blend trended towards the ensemble means late period, with some influence still from the GFS for better system definition. ...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 from the Plains on Monday into 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. Guidance continues to show a fair amount of scatter with the details, and some areas have dry antecedent conditions that could limit the potential, but the overall threat remains sufficient to show very broad Marginal Risk areas on both the Days 4 and 5 (Tues-Wed night) Excessive Rainfall Outlooks across portions of the Northern Plains/Upper Midwest into the MS-Valley/Ohio Valley/Appalachians. An embedded Slight Risk could be added if and when models narrow in on a focus for heavy rainfall, especially across parts of the Upper Midwest on Tuesday, but for now there is still too much spread to be confident in the placement. 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 eventually 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 meaningful snow will be possible over at least the higher elevations, and some snow may extend into the High Plains by midweek as colder air reaches the region. Blowing snow is also a concern that may cause impacts. Farther south, by around Thursday or so, the surface front ahead of this trough may become parallel to the upper low over the south-central to east-central U.S. The combination of decelerating progression and some Gulf inflow may lead to increasing rainfall near the front around that time. 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 the upper trough digging into the West and then including the northern Plains will likely start to bring below normal highs into the Northwest early next week, followed by readings 10-15F below average over the northern Rockies/High Plains by next Wednesday-Friday. Less extreme cool anomalies may extend farther southwestward underneath amplified troughing late week-next weekend across much of the eastern half of the nation. 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