Extended Forecast Discussion NWS Weather Prediction Center College Park MD 340 PM EDT Thu Apr 21 2022 Valid 12Z Sun Apr 24 2022 - 12Z Thu Apr 28 2022 ...Significant late season winter storm affecting the northern tier into Sunday along with potentially heavy rainfall over portions of the Southern Plains and Mid Mississippi Valley... ...Weather Pattern Overview... An amplified flow pattern will exist early in the period Sunday-Monday as the Northern Plains/Upper Midwest deep low lifts into Canada while a strong Southeast ridge extends northward through much of the eastern U.S. The storm system's associated trailing cold front could bring a heavy rainfall threat over portions of the Southern Plains to the Ozarks and Mid Mississippi Valley. Most guidance expects gradual progression of the mean pattern with time, ultimately leading to troughs near each coast and a broad central U.S. upper ridge. Both troughs could contain an embedded upper low. Much below normal temperatures may persist for multiple days over the far northern parts of the Plains while well above normal readings are possible over the East during Sunday-Monday and over the Great Basin/Southwest into parts of the Plains by Tuesday-Thursday. ...Model Guidance Evaluation... Overall a consensus/average of 00Z and 06Z models early in the period followed by a blend of models and 06Z GEFS/00Z ECMWF means provided a reasonable depiction of the most likely forecast scenario through the period. Guidance still exhibits some spread for the departing northern tier storm, again with the GFS fastest and CMC slowest, while the ECMWF continues to provide the most consistent intermediate solution. Clustering is gradually improving though. Differences for the trailing upper trough crossing the central-eastern U.S. during the first half of the week have come down to mainly low-predictability detail issues. Meanwhile there is good agreement that energy from the initial northern tier upper low will continue northeastward through Canada, while trailing energy crossing the Great Lakes and southeastern Canada develops an upper low that reaches New England by early next Thursday. Latest models and means show better than average clustering for this potential upper low relative to typical day 7 forecasts, with associated low pressure tracking close to the New England coast. On the other hand, models and individual ensemble members continue to show a lot of spread for specifics of individual shortwaves within the mean trough whose axis settles over the eastern Pacific/West Coast from about Tuesday onward. These differences affect the forecast of coverage/amounts of precipitation over the West and eventually the surface pattern as far east the Plains by day 7 Thursday. By late in the period the main theme of models/means is to keep the upper trough axis closer to the West Coast than the past three ECMWF runs through 00Z. Most new 12Z model runs are still within the majority cluster while the ECMWF has adjusted closer to consensus. ...Sensible Weather/Threats... The strong/significant winter storm affecting the Northern Plains will be ongoing at the start of the medium range period with the surface low likely tracking across northern Minnesota on Sunday. Wraparound precipitation underneath/around the closed upper low will likely continue snow showers, and a strong/tight pressure gradient should produce strong winds that would continue blowing snow and reduced visibility at times. Conditions improve by early next week as the low weakens and tracks over southern Canada. Meanwhile to the south, the trailing cold front will intersect with warm/moist air from the Gulf and a strengthening low level jet Sunday night into Monday morning should focus a few mesoscale convective systems that have the potential to train and repeat over the same areas. The latest forecast guidance suggests potential for multi-inch totals during the Day 4 period (ending 12Z Monday April 25) across an area from northeastern Texas through eastern Oklahoma, western/northern Arkansas, southern Missouri, and perhaps extending a little east from there. The Day 4 Excessive Rainfall Outlook maintained a Slight Risk area highlighting this region with only slight refinements for new guidance. Stream flows/soil moisture/precipitation anomalies are above normal for much of this area due to recent heavy rainfall. Guidance is still in the process of resolving the timing and wave details of the surface front so additional refinements in the most likely coverage and magnitude of rainfall totals are certainly possible. The front should accelerate as it continues eastward so rainfall across the eastern U.S. should be lighter during Tuesday-Wednesday. By Wednesday-Thursday the upper low forecast to approach/reach New England and associated low pressure could focus some precipitation over the region, with coverage and amounts sensitive to the exact track of the system. A portion of this precipitation could be snow in the higher elevations. Unsettled weather with generally light to moderate precipitation will spread across areas from the northern half of the West Coast into the northern Rockies as a series of shortwaves move through the mean trough south of a closed upper low positioned off British Columbia. Timing and coverage with each episode of rain/high elevation snow will take additional time to resolve given the typical lower predictability of medium to smaller scale of features contained within the mean flow. The weekend storm system and flow on either side of the associated cold front will bring contrasting temperature extremes to the central and eastern states into early next week. Northern Plains/Upper Midwest areas will see unseasonably chilly readings with highs up to 20-30F below normal while more moderate below normal anomalies will spread across the rest of the central U.S. Areas near the Canadian border may stay below normal through Thursday. Ahead of the cold front, expect many areas to see temperatures 5-20F above normal and potentially even warmer anomalies for morning lows over the Great Lakes on Sunday. The exception to the eastern warmth should be for the coastal Northeast areas and at times into the Mid-Atlantic where surface high pressure may keep temperatures on the cooler side. A front meandering to the south could lead to a sharp contrast between cool air to the north and warmth to the south and west, increasing the challenge of temperature forecasting over the region. Through the period a warming trend will spread across the southern two-thirds of the West and into the central/southern Plains with plus 10-20F anomalies for highs becoming common by Tuesday-Thursday. In contrast, the upper trough nearing the West Coast will support slightly below normal highs from the Pacific Northwest/northern California into the northern Rockies Tuesday onward. The upper trough and possible embedded upper low reaching the eastern U.S. should lead to below normal readings over the region by midweek with the Great Lakes into central Appalachians being the most likely area to see highs 10-20F below normal for one or more days. Rausch/Taylor 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 Hazards: - Heavy rain across portions of the Middle/Lower Mississippi Valley, the Southern Plains, the Ohio Valley, and the Tennessee Valley, Sun, Apr 24. - Flooding possible across portions of the Central Plains, the Upper/Middle Mississippi Valley, the Upper Mississippi Valley, and the Northern Plains. - Flooding occurring or imminent across portions of the Southeast and the Lower Mississippi Valley. - High winds across portions of the Central Plains, the Upper/Middle Mississippi Valley, and the Northern Plains, Sun, Apr 24. - Much below normal temperatures across portions of the Upper Mississippi Valley and the Northern Plains, Sun-Tue, Apr 24-Apr 26. WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, experimental excessive rainfall outlook, winter weather outlook probabilities and heat indices are at: 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