Extended Forecast Discussion NWS Weather Prediction Center College Park MD 238 AM EDT Fri Mar 19 2021 Valid 12Z Mon Mar 22 2021 - 12Z Fri Mar 26 2021 ...Overview... The weather pattern will become more active during the medium range period as a large scale mean trough becomes established over the Plains/west-central U.S. through much of next week. Ahead of this trough, portions of the Plains and Mississippi Valley (and possibly a little eastward) should see increasing potential for significant rainfall while Pacific features dropping into the mean trough will bring episodes of rain and mountain snow from the Northwest into the Rockies. Meanwhile, an upper trough and possible embedded low will support a western Atlantic system, with continued uncertainty for its exact track and how much precipitation it may bring to areas along the East Coast. Guidance is still fairly agreeable with the evolving pattern but there are remaining differences in the details. ...Model Guidance/Predictability Assessment... The system off the East Coast continues to be troublesome in terms of model spread, with predictability on the mid-lower side of the spectrum as the upper support essentially becomes embedded within a developing large-scale ridge. The GFS continues to be the fastest/farthest east model. The latest run of the UKMET still shows a northern outlier solution, just off Cape Cod by early Wednesday. The ECMWF/CMC are in a more middle ground position. A vigorous short wave ejects out of the Rockies on Monday, lifting into the Upper Midwest/Great Lakes by Wednesday while another follows on its heels (maybe a bit farther south) Wednesday-Friday. Both systems exhibit notable timing/intensity differences, with the GFS consistently faster than the other models (GFSp is fast too, but still slower than the operational GFS). A third system enters the Pacific Northwest next Thursday, with a possible closed low over the Southwest by Friday. However, there is significant model spread with how deep this system gets by then, so confidence is low. For early period, WPC used a blend of the deterministic models with more weighting towards the ECMWF. Due to increasing uncertainty late in the week, the blend incorporated more of the ensemble means (ECENS and NAEFS) Thursday-Friday. This overall solution worked well across the board, and also fit with previous shift continuity as well. ...Weather/Threats Highlights... The developing mean trough aloft with periodic reinforcement will tend to favor below normal temperatures over much of the West into the Rockies, moderating somewhat by the end of the week. The reinforcing shortwaves will bring multiple episodes of rain and mountain snow to areas from the Pacific Northwest eastward/southeastward through the West into the Rockies. Most of this activity should be of light to moderate intensity but some localized heavier areas will be possible. Consolidating upper dynamics ejecting from the West should promote a central Plains through Upper Midwest system early-mid week. This system may produce a focused area of enhanced snowfall over or near the Colorado Rockies with some snow possibly extending into the central High Plains and then northeastward on the cold side of the surface low track. Farther south, improving Gulf inflow ahead of the trailing cold front should promote areas of heavy rainfall from the central Plains/Midwest into the Lower Mississippi Valley with lighter rainfall extending northeastward. The system could produce brisk winds depending on its exact strength. The next shortwave digging into the West by midweek should hold the frontal boundary over the southern Plains/Lower Mississippi Valley for a time, with continued flow of moisture from the Gulf promoting a threat of heavy rainfall over the Lower Mississippi Valley and vicinity into Thursday. Areas along the East Coast may see a period of rainfall and strong winds with the western Atlantic system. However confidence is low for magnitude and duration given current guidance spread, as possibilities for some locations range between dry conditions and fairly heavy rainfall. Expect the warmest temperature anomalies to progress eastward from the Northern Plains/Upper Midwest on Monday, reaching the eastern states by mid-late week. The northern third of the central/eastern U.S. will be most likely to see highs of 10-20F above normal on one or more days. Anomalies for morning lows should be somewhat higher with broader southward coverage of readings 10F or more above normal, along with some northern locations seeing some plus 20-25F anomalies. Sunday lows over parts of the Dakotas and Minnesota could be near daily record warm values if they hold for the calendar day. Santorelli/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, 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/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml