Extended Forecast Discussion NWS Weather Prediction Center College Park MD 429 PM EDT Wed Apr 20 2022 Valid 12Z Sat Apr 23 2022 - 12Z Wed Apr 27 2022 ...Significant late season winter storm increasingly likely for the Northern Plains this weekend and potentially heavy rainfall threat to some areas farther south... ...Weather Pattern Overview... A highly amplified pattern will prevail over the weekend into the start of next week as a deep upper low tracks northeastward through the Northern Plains and a strong Southeast ridge extends an axis northward. A strong surface low forecast to be over the Dakotas on Saturday and continue northeast thereafter, along with the trailing front, will bring a wide variety of weather hazards - significant snow and wind to the Northern Plains and eventually a heavy rain threat evolving late in the weekend into early next week over portions of the Southern Plains to the Ozarks and Mid Mississippi Valley. By Tuesday-Wednesday, gradual progression of the mean pattern will bring moderate upper troughing to the East, a ridge to the Rockies/Plains, and a trough into the eastern Pacific/West Coast. Much below normal temperatures may persist for a while over the far northern parts of the Plains while well above normal readings are possible over a majority of the eastern half of the country through the weekend. ...Model Guidance Evaluation... The updated forecast emphasized a blend of the 00Z ECMWF/00Z UKMET/06Z GFS during the first half of the period, followed by gradually increasing input of the 06Z GEFS/00Z ECMWF means so that the means had half total weight by day 7 Wednesday. Recent ECMWF runs have generally provided the best intermediate timing for the northern tier storm, with GFS runs consistently faster and the CMC slow. UKMET runs have tended to be between the ECMWF and CMC but the new 12Z UKMET has adjusted to the ECMWF, leaving the CMC more in the slow minority. Meanwhile guidance has varied considerably over recent days for the amplitude and timing of the trailing upper trough that progresses through the central/eastern U.S., affecting the timing and waviness of the cold front trailing from the surface low. Over the past day most guidance has been consolidating toward greater progression than recent CMC runs. It is possible that low pressure could evolve near New England by day 7 Wednesday depending on specifics of the eastern North America upper trough. For the eastern Pacific/West Coast upper trough, from late Sunday onward there is still a lot of model/ensemble spread with respect to individual shortwaves flowing around the trough and the eventual location of the embedded upper low. By next Wednesday the new 12Z ECMWF/CMC brings the upper low closest to the Pacific Northwest while the 00Z ECMWF mean kept it well to the northwest. Prefer a compromise approach within this broad spread until better agreement emerges. ...Sensible Weather/Threats... A strong to potentially significant storm system will likely track over the Northern Plains and Upper Midwest during the weekend. The best potential for significant totals of snow and blowing snow extends from northeast Wyoming and eastern Montana through much of western/central North Dakota and far northwest South Dakota - immediately west to northwest of the surface low track. The strong winds and heavy snow combined could lead to near blizzard conditions for some areas. There is some lingering uncertainty in the speed and track but the gradually improving guidance cluster suggests good confidence in a high impact late season winter storm for portions of the region where the latest Winter Weather Outlook shows high probabilities for at least several inches of snow. The frontal boundary anchored by this storm will help to focus an area of heavy rainfall from parts of the southern/central Plains toward the Ozarks and Mid Mississippi Valley. Southerly flow of warm, moist air intersecting the boundary will likely lead to strong thunderstorms that may train or repeat over the same areas Sunday into Monday. Mesoscale convective systems repeating over the area could lead to an axis of multi-inch totals and the latest guidance suggests the best potential for flash flooding and excessive rainfall from far northeastern Texas through eastern Oklahoma, northwestern Arkansas, and southern Missouri along with adjacent areas. The Day 5 Experimental Excessive Rainfall Outlook shows a Slight Risk over this region given the potential, as stream flows and precipitation anomalies are already above normal due to recent heavy rainfall and with the potential for additional heavy rainfall leading up to Day 5. Guidance is still in the process of resolving the timing and wave details of the surface front so changes in the most likely coverage and magnitude of rainfall totals are certainly possible. Lighter rainfall will spread through the eastern U.S. during Tuesday-Wednesday as the front accelerates. Mostly light precipitation over the northern part of the Pacific Northwest to start the week should expand southward through Oregon and eastward into the northern Rockies after Monday as an upper trough approaches the West Coast. Some pockets of moderate activity are possible. Day-to-day details are still uncertain due to lower confidence in specifics of shortwaves within the overall upper trough. Expect unseasonably cold temperatures to persist over far northern parts of the Plains through the period with North Dakota in particular seeing highs 15-30F below normal for multiple days. While still below normal, anomalies may finally moderate some around midweek. The West will see more variable temperatures, with minus 5-15F anomalies over the Intermountain West/Rockies during the weekend followed by a west-to-east warming trend with increasing coverage of above normal readings. Warmest highs of 10-15F above normal are likely to extend from the Great Basin/Southwest into the central High Plains during Tuesday-Wednesday. At the same time clouds/precipitation ahead of the upper trough nearing the West Coast may bring slightly below normal highs to the Pacific Northwest. Much of the eastern half of the country will see a significant warming trend during the weekend and continuing into early next week over the East. Plus 10-20F anomalies should be fairly common for one or more days and parts of the Midwest/Great Lakes could see morning lows 20-25F above normal. One exception to the eastern warmth may be over the Northeast/northern Mid-Atlantic where cool surface high pressure/ridging may prevail for a while. A front meandering over the Mid-Atlantic could provide 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. Rausch/Taylor Hazards: - Heavy precipitation across portions of the far northern Plains, Sat, Apr 23. - Heavy rain from across portions of the central and southern Plains into the Midwest, Sat-Sun, Apr 23-Apr 24. - Heavy snow across portions of eastern Montana and western North Dakota, Sat, Apr 23. - Severe weather across portions of the central Plains west of the middle to upper Mississippi Valley, Sat, Apr 23. - Flooding possible across portions of Georgia and the northern Plains. - Flooding occurring or imminent across portions of the Southeast and central Mississippi. - Flooding likely across portions of the Southeast. - High winds across portions of the northern Plains, Sat-Sun, Apr 23-Apr 24. - Much below normal temperatures across portions of the northern Plains, Sat-Tue, Apr 23-Apr 26. - Enhanced wildfire risk from the foothills of the southern Rockies to portions of the central Plains, Sat, Apr 23. 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, 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