Extended Forecast Discussion NWS Weather Prediction Center College Park MD 256 PM EDT Wed Mar 17 2021 Valid 12Z Sat Mar 20 2021 - 12Z Wed Mar 24 2021 ...Overview... Guidance shows the establishment of a large scale mean trough aloft over the west-central U.S. next week, increasing the precipitation potential over the Plains and Mississippi Valley while also producing one or more episodes of meaningful rain/mountain snow over parts of the West/Rockies. Ahead of this developing trough expect some degree of mean ridging to settle into southeastern Canada while a fairly narrow ridge drifts from the Plains into the East. Leading shortwave energy consolidating into a trough (with possible embedded low) along the central/southern East Coast should generate a frontal wave that ultimately wraps up off the Florida/Southeast coast before tracking farther into the Atlantic. This system could bring some rain and brisk winds to coastal areas of the southern Mid-Atlantic and Southeast. ...Model Guidance/Predictability Assessment... Broadly speaking there continues to be reasonable clustering for individual features but with some detail/timing differences and the occasional stray solution. Most continuity changes are generally modest as well. Upper trough energy coming into the West this weekend should separate with the central/southern part of the trough likely ejecting northeastward through the Plains/Upper Midwest by Tuesday-Wednesday. The associated surface system could be fairly vigorous by midweek if this energy forms a closed low or negatively tilted shortwave as some guidance is advertising. A blend approach is reasonable to navigate between the GFS/GEFS (more southeastward amplitude of the shortwave latter half of the period) and 00Z ECMWF (slower than previous runs) extremes. Behind this feature there is a decent consensus that Pacific energy diving into the evolving mean trough will form an upper low near the Four Corners by day 7 Wednesday. The main considerations here were to phase out the 00Z CMC for having more southwestward amplitude mid-late period and to average the past two runs for ECMWF input late in the forecast. There is good agreement for the next shortwave approaching the Northwest next Wednesday. By early next week the 00Z UKMET became deeper than most solutions with the East Coast upper feature though the new 12Z run is similar and now joined by the farther south 12Z CMC. At that time the 12Z GFS strays farther north/northeast than the previous run or consensus. The 00Z/06Z guidance average did bring the western Atlantic surface low somewhat closer to the southeastern coast compared to continuity. A 00Z/06Z operational model blend early trending toward an even model/ensemble mean mix late, with aforementioned fine-tuning, provided a good starting point for the manual forecast. ...Weather/Threats Highlights... The West will see a series of rain/mountain snow events with the expected succession of shortwaves. This weekend the leading system should bring moderate amounts especially to favored terrain from the Northwest into the central Rockies. Lingering energy crossing southern parts of the West could eventually extend the precipitation into the southern Rockies. The next system will spread its moisture from the Northwest through much of the Rockies from late weekend into next week, with moderate to locally enhanced activity. Then around midweek the Pacific Northwest should begin to see the precipitation from another shortwave/cold front. Expect precipitation to expand in coverage and intensity over the Plains and Mississippi Valley next Monday-Wednesday. Ejection of leading western U.S. shortwave energy will coincide with increasing Gulf inflow around a surface ridge reaching the eastern states. Heaviest rainfall should progress from the central Plains eastward with time ahead of the best upper dynamics and developing surface low pressure/trailing cold front. Some snow may be possible from the north-central High Plains northeastward, within the cold sector of the developing surface system. The upstream energy digging into the West early-mid week may hold the front trailing from the Plains-Upper Midwest system over the southern Plains, which could maintain a threat for enhanced rainfall over southern parts of the Plains/Mississippi Valley. The system forecast to develop offshore Florida and then lift north/northeast may bring some rainfall and brisk winds to areas near the Southeast/southern Mid-Atlantic coast but the heaviest rain should remain offshore. Otherwise most of the eastern U.S. should be dry for most of the period. The warmest temperature anomalies during the period will gradually migrate from the northern Plains into the Great Lakes region, with max/min readings generally 10-20F above normal and on some days up to a few degrees warmer. The best potential for daily records should be for warm lows over the northern Plains/Upper Midwest on Sunday. Somewhat less extreme warm anomalies will extend farther south/southwest ahead of the mean frontal boundary with morning lows tending to have higher anomalies than daytime highs. Trailing cooler air will spread across the West and into the Rockies, bringing highs down to 5-15F below normal. The upper trough over the Southeast coast this weekend will produce highs more than 10F below normal over some areas, followed by a rebound to at least normal readings. 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