Extended Forecast Discussion NWS Weather Prediction Center College Park MD 256 PM EDT Thu Mar 18 2021 Valid 12Z Sun Mar 21 2021 - 12Z Thu Mar 25 2021 ...Overview... The weather pattern will become more active once again as a large scale mean trough becomes established over the Plains/west-central U.S. over the course 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 significant differences at least on a regional scale. ...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 one point of relative agreement is that the system should drift closer to the East Coast than depicted in recent GFS runs, favoring a compromise among the still diverse array of other solutions. The 12Z UKMET provides a new northern extreme with a track just off Cape Cod by early Wednesday while remaining consensus keeps it closer to 35N latitude or so by that time. Models/ensembles continue to show initial western U.S. trough energy separating with what remains over the CONUS consolidating into a vigorous shortwave that ejects through the Rockies and northern Plains/Upper Midwest Monday-Wednesday. However there are noticeable disagreements for timing and strength. Overall a guidance average provides good continuity for resolving these differences. GFS runs have been particularly deep with the upper level and surface reflections of this system compared to most other guidance but the general strength/shape of the upper feature also favor being stronger than the weak side of the envelope for the surface low. The combination of the GFS depth of this system and the model's more open/progressive depiction of upstream energy dropping into the West early-mid week (a change from yesterday's continuity) ultimately lead to faster progression of height falls on the leading side of the mean trough aloft. Remaining guidance, including the latest parallel GFS runs, offer continued support for an upper low near or a little south or southwest of the Four Corners as of early Wednesday, possibly opening up into a High Plains trough by early Thursday. The next Pacific feature forecast to start reinforcing the mean trough by days 6-7 Wednesday-Thursday compares quite well in the guidance relative to typical spread for that time frame. The combination of above considerations led to starting with an operational 00Z/06Z model blend with somewhat less GFS input than average for the first half of the period, followed by a transition to a model/mean blend that placed about two-thirds emphasis on the CMC/ECMWF/ECMWF mean relative to the GFS and GEFS mean. ...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. Best potential for one or more days of highs 10-15F or so below normal will be over the eastern Great Basin and central/southern Rockies. 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 a northern Plains/Upper Midwest through southern High Plains orientation Sunday, 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. The upper trough initially along the East Coast will support a cool Sunday over the extreme Southeast. 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