Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM EDT Thu Apr 11 2024 Valid 12Z Sun Apr 14 2024 - 12Z Thu Apr 18 2024 ...Deepening Plains storm early next week likely to bring a severe weather threat along with strong winds and areas of heavy rainfall... ...Overview... With some continued spread in the details, guidance agrees in principle for a deep upper low to be crossing California around the start of the period early Sunday. Continued progression of this feature should support a deepening storm system over the Plains by early next week, with various hazards including severe weather and bands of heavy rainfall along with a broad area of gusty winds. Behind this storm, an 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 in emphasis for above normal temperatures. ...Guidance/Predictability Assessment... After some waffling in the guidance over the past day, new runs seem to be improving their clustering with some aspects of the forecast while maintaining spread with others. For the California into Plains upper low/storm development, some models had nudged a little faster recently while the GFS stayed on the slower side, but adding in the new 00Z runs there seems to be improved clustering between the two sides. Among the 12Z/18Z runs, a composite of operational runs (more GFS/ECMWF relative to the UKMET/CMC) provided a good intermediate solution with reasonable continuity. ECMWF-initialized machine learning models (MLs) on average continue to support low pressure having a central depth in the mid-upper 980s mb early Tuesday, which is a little weaker than what some dynamical model runs are showing. GFS runs have been on the deep side of the spread for the surface low as it reaches near the Canadian border by Wednesday. Detail differences continue for the digging western trough, including its amplitude at a specific point in time as well as the location and duration of any embedded upper low. A general model blend, with modest ensemble mean inclusion late in the period, offers a good intermediate starting point given the spread. Note that the new 00Z GFS seems to extend its northern tier trough axis east of consensus late in the period. After a rebound in spread in yesterday's 00Z runs, guidance has returned to prior consensus for the general evolution of the weak wave emerging from the Great Lakes on Sunday, along with some lingering Northeast upper troughing in its wake which should support the trailing front settling into the Mid-Atlantic. ...Weather/Hazards Highlights... The upper low crossing California around Sunday should produce some areas of light to locally moderate rain and high elevation snow from California into the northern Rockies, while strong winds are possible over some areas as the feature passes through. Then expect multiple hazards over the central U.S. early next week as the upper low helps to deepen a Plains system Monday-Tuesday. The Storm Prediction Center is already highlighting an area of severe potential over the Plains while the storm should produce a broad area of gusty winds, especially over the central-southern High Plains. Meanwhile abundant moisture and instability associated with this dynamic storm should produce some areas of intense rainfall over parts of the Midwest and Plains. Guidance shows a fair amount of scatter with the details and some areas have dry antecedent conditions, but the overall threat appears sufficient to introduce a Marginal Risk area in the Day 5 (Monday-Monday night) Excessive Rainfall Outlook, extending from parts of the Upper Midwest/eastern Dakotas into the south-central Plains. Continued northeastward progression of the storm should concentrate heaviest rainfall over the Upper Midwest by Tuesday. The upper trough digging into the western U.S. and eventually extending into the northern Plains early-mid week should tend to focus precipitation over the northern half or so of the Rockies and High Plains for two or three days. 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. The surface front ahead of this trough may become parallel to the upper low over the central U.S. around the end of the period next Thursday. The combination of decelerating progression and some Gulf inflow may lead to increasing rainfall near the front around that time. Over the East, a Great Lakes wave and frontal system on Sunday may produce an area of rainfall over parts of the eastern Great Lakes, Upper Ohio Valley, and northern Mid-Atlantic/far southern New England. This activity may produce some pockets of moderate to heavy rain but so far it appears sufficiently progressive and localized not to warrant an area in the ERO. Then mostly light rain may reach the Northeast by Tuesday-Wednesday ahead of a warm front anchored by the Plains storm. Plains/Mississippi Valley upper ridging will support well above normal temperatures across the central U.S. Sunday-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 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 Sunday into Monday with the upper low crossing the region. 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-Thursday. Less extreme cool anomalies would extend farther southwestward. Rausch 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