Extended Forecast Discussion NWS Weather Prediction Center College Park MD 257 AM EDT Fri Apr 12 2024 Valid 12Z Mon Apr 15 2024 - 12Z Fri Apr 19 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 over Nevada/California to start the period Monday will progress eastward eventually supporting a deepening low pressure system over the Plains by Tuesday. 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 digging down from British Columbia into the Northwest U.S. early next week should ultimately evolve into a broader positively tilted mean trough from south- central Canada through the West. Expect this trough to bring colder temperatures and multiple days of snow potential especially to the northern half of the Rockies/High Plains. Meanwhile upper ridging over the Plains on Sunday will shift eastward and settle near the East Coast Tuesday-Thursday, leading to a corresponding shift for above normal temperatures. ...Guidance/Predictability Assessment... Models continue to show good agreement on the upper low over CA/NV as it slides eastward inducing strong cyclogenesis in the Central U.S. early next week. Including the new 00z runs tonight, there remains some uncertainty in the exact placement and timing of the surface low and much more uncertain low development off the Northeast Coast next Thursday as the upper low weakens and a shortwave moves through New England later in the week. Behind this, another sharp trough/possible closed low will skirt the Northwest/Northern Plains mid-week, eventually shearing with energy continuing eastward through the Upper Great Lakes, and energy hanging back over the Northwest next Thursday-Friday. Lots of uncertainty in the evolution of this though. The blend for tonights WPC forecast used an equal blend of the deterministic models days 3-5 (minus the CMC which was slower than the other guidance with the Central U.S. surface low). After this, increased the ensemble means to 60 percent of the blend by Day 7, along with the ECMWF and GFS to maintain some system definition. Overall, this maintained good continuity with the previous WPC shift as well, through Thursday. ...Weather/Hazards Highlights... Precipitation will be increasing in coverage across the Rockies/Central U.S. with multiple weather hazards as the upper low helps to deepen a Plains surface low Mon-Tues. The Storm Prediction Center is already highlighting areas of severe potential for the Plains on Monday and for the Mid-Mississippi 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 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 (Monday-Tuesday night) Excessive Rainfall Outlooks progressing from the northern/central Plains into the Upper Midwest/Mississippi Valley. 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 MS Valley on Tuesday where models are showing some agreement for higher QPF (which should be fairly progressive). Behind the low, the storm should produce a broad area of gusty winds, especially over the southern/central High Plains Monday and into the north-central High Plains 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 on the backside of the central U.S. surface low, 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. By next Thursday or so, the surface front ahead of this trough may become parallel to the upper low over the central U.S. The combination of decelerating progression and some Gulf inflow may lead to increasing rainfall near the front around that time. Plains/Mississippi Valley upper ridging will support well above normal temperatures across the central U.S. into Monday with some northern/central areas seeing highs 20-25F above normal. Thereafter, the pattern evolution will lead to a broad area of above normal temperatures over the eastern half of the country into the southern Plains, 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 southwestern U.S. will see below normal highs through Monday with the upper low crossing the region. Then 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 and followed by readings 10-20F below average over the northern Rockies/High Plains by next Wednesday-Friday. Less extreme cool anomalies may extend farther southwestward. Santorelli/Tate 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