Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 PM EDT Thu Apr 11 2024 Valid 12Z Sun Apr 14 2024 - 12Z Thu Apr 18 2024 ...Deepening Plains surface low likely to bring severe and heavy rainfall threats ahead of it and strong winds and notable snow behind... ...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 low pressure system over the Plains by early next week. 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, 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 for above normal temperatures. ...Guidance/Predictability Assessment... Model agreement is rather good early next week, indicating an upper low atop California/Nevada as the period begins Sunday and its eastward progression toward the central U.S. by early Tuesday. There are some slight differences in the surface low position and depth as it consolidates late Monday into Tuesday, but nothing too egregious. By Wednesday GFS runs have been on the deep side of the spread for the surface low as it reaches near the Canadian border, but the newer 12Z run seems in better agreement. The second upper trough/low of note will be the trough digging in the Northwest by Tuesday and deepening/expanding southeast Wednesday-Thursday. This feature shows relatively more spread in amplitude as well as the location and duration of any embedded upper low. The 00Z GFS seemed a little east/fast with the feature Tuesday-Thursday, but the 06Z was more in line with other guidance. By Wednesday and especially Thursday, increasing spread is noted with with how much energy makes its way east toward the first upper low (over the Great Lakes at that time) and how much stays put or digs in the West, while energy potentially diving southward from north-central Canada complicates matters. The 00Z ECMWF and CMC were farther south with the depth of the troughing in the West compared to their newer 12Z runs. For the early part of the period, WPC maintained a multi-model blend of the deterministic guidance favoring the 00Z ECMWF and 06Z GFS. Gradually transitioned to a blend of half models/half ensemble means (ECMWF and GEFS) by Day 7 amid increasing spread and uncertainty. ...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 high winds are possible over parts of southern California and Nevada 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 surface low Monday-Tuesday. 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. These ingredients could also produce heavy rainfall with intense rates that could cause flash flooding concerns over parts of the Midwest and Plains. Guidance shows a fair amount of scatter with the details, and some areas have dry antecedent conditions that could limit flood potential, but the overall threat remains sufficient to show 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. An embedded Slight Risk could be added if and when models narrow in on a focus for heavy rainfall. Continued northeastward progression of the storm should concentrate heaviest rainfall over the Upper Midwest by Tuesday. 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. 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 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 Sunday into 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-Thursday. Less extreme cool anomalies may extend farther southwestward. Tate/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