Extended Forecast Discussion NWS Weather Prediction Center College Park MD 214 AM EDT Wed Apr 20 2022 Valid 12Z Sat Apr 23 2022 - 12Z Wed Apr 27 2022 ...Significant late season winter storm increasingly likely for Northern Plains this weekend and potentially heavy rainfall threat to some areas farther south... ...Weather Pattern Overview... A highly amplified pattern is expected over the forecast period highlighted by an anomalous closed upper low coming out of the West into the Plains this weekend while strong ridging anchors over the Southeast. A deepening surface low will track from the Rockies toward the Upper Midwest bringing 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. Meanwhile, much above normal temperatures will be found downstream underneath the Southeast ridge and finally, another Pacific upper trough should approach the northwestern U.S. by the beginning of next week. ...Model Guidance Evaluation... A blend of the latest available deterministic guidance was used for this forecast cycle - mainly the 12Z ECMWF, 18Z/12Z GFS, 12Z CMC, 12Z UKMET. For Days 3-4, a blend mostly consisting of the ECMWF/GFS was used for the strong low pressure system in the Plains where the deterministic runs and ensemble clustering continues to improve such that confidence is above normal. Further south along the trailing frontal boundary Sunday into Monday - there remains some timing and speed issues with how fast the front kicks southeast. Some suggestion that the front will hang out/slow across portions of the southern Plains to Ozarks but some forecast runs show a much more progressive scenario though the setup favors a slow solution so the forecast blended some of the slower guidance. By days 6-7, forecast uncertainty grows substantially with how the phasing or lack of shortwave energy over the Northeast occurs and its progression potentially blocked by ridging to the northeast. Another storm system is likely to approach the Pacific Northwest toward the end of the period. ...Sensible Weather/Threats... A strong to potentially significant storm system is increasingly likely to emerge out of the Rockies and track toward the Northern Plains and Upper Midwest later this week into the weekend. The best potential for significant totals of snow and blowing snow are favored from northeast Wyoming and eastern Montana to 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 if model trends continue. While there is some uncertainty in the speed and track - the guidance is increasingly becoming clustered on the idea of a high impact late season winter storm for portions of the region where the latest Winter Weather Outlook shows high probabilities for several inches of snow. As the low lifts north of the Great Lakes, its associated frontal boundary extending southward through the central U.S. is likely to slow, stall or become nearly stationary late in the weekend into early next week across portions of the southern/central Plains toward the Ozarks. Warm, moist southerly air intersecting the boundary will likely lead to strong thunderstorms that may train or repeat over the same areas Sunday into Monday. Several mesoscale convective systems repeating over the area could lead to large footprint of multi-inch totals and the latest guidance suggests the best potential for flash flooding and excessive rainfall over portions of southern Missouri, northwest Arkansas, northeast Oklahoma and adjacent areas. A Day 5 Experimental Slight Risk ERO was included for this region given the potential - where 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. 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. The West will see more variable temperatures, with minus 5-15F anomalies late week into the weekend but followed by a west-to-east moderating trend with increasing coverage of above normal readings early next week. Clouds/precipitation may bring below normal highs to the Pacific Northwest next Tuesday though. A major warming trend is likely for the central/southern Plains on Friday and then much of the eastern half of the nation going into the weekend. Central Plains locations could see highs up to 20-30F above normal on Friday. Plus 10-20F anomalies should be more common farther east. Also humidity levels should increase in response to deep southerly flow ahead of the central U.S. storm system. One exception to the eastern warmth should be over the Northeast/northern Mid-Atlantic where cool surface high pressure/ridging may prevail for multiple days. A back door front expected to settle over the Mid-Atlantic could provide a sharp contrast between cool air to the north and warmth to the south and west. 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 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