Extended Forecast Discussion NWS Weather Prediction Center College Park MD 158 AM EST Thu Feb 13 2020 Valid 12Z Sun Feb 16 2020 - 12Z Thu Feb 20 2020 ...Overview and Guidance/Predictability Assessment... The consensus of latest guidance shows western U.S. troughing aloft eventually becoming sharper, possibly closing off an embedded upper low, in response to an amplifying northeastern Pacific ridge that builds into western Canada and the northwestern U.S. by Tue-Wed. Initial shortwave energy emerging from the West early next week should produce a Plains through Northeast system during the first half of the week, with a trailing upper trough reaching the eastern states by next Thu. Upper ridging should persist between Cuba and the Yucatan Peninsula, likely strengthening/expanding from Mon into Wed. Once again the models and ensembles show various detail possibilities within the above large scale pattern evolution during the Sun-Thu period, leading to lower than desired confidence for some aspects of the sensible weather forecast over the lower 48. The most immediate question mark is the handling of shortwave energy moving into the West on Sun. There is a fair amount of spread regarding distribution/timing of what may be multiple impulses within the overall trough, leading to differences in timing/strength/track of the Mon-Wed Plains through Northeast system. Based on 12Z-18Z guidance available for this forecast update there was enough agreement to start with a blend of 18Z GFS and 12Z ECMWF/UKMET/CMC to yield a solution fairly close to continuity. However new 00Z runs are more diverse, especially counting the UKMET that holds enough energy westward to result in a weak/suppressed solution at the surface. Also 00Z guidance is suggesting that leading energy will be less amplified than in the 12Z ECMWF/ECMWF mean. Potential influence from southern Canada upper troughing may provide another complexity in the forecast by Tue-Wed. Regarding eastern Pacific/western U.S. evolution Tue-Thu, the majority cluster had trended even stronger over the past day with the upper ridge building toward/into the Northwest. The previously weak/progressive GEFS mean has adjusted most of the way toward consensus but still appeared to be a little too weak through day 6 Wed. GEFS trends left the 12Z CMC ensembles as the only progressive solution through the 12Z-18Z cycles. However the 00Z ECMWF has reversed the trend. Meanwhile most operational models and now the 00Z GEFS mean are signaling that an upper low should close off for a time between the California coast and the Great Basin. The 18Z/00Z GFS runs represent the western part of the envelope while the 12Z ECMWF had one of the most sheared solutions. New 00Z ECMWF trend upstream furthers the idea of a mere open trough over southern parts of the West. Incorporating 20-50 percent ensemble means by days 6-7 Wed-Thu (more ECMWF mean) while maintaining operational input for the rest provided a good representation of the expected pattern evolution based on 12Z-18Z guidance without committing too strongly to a specific solution. Also this blend helped to resolve some timing/amplitude differences with the upper trough reaching the East. ...Weather/Hazard Highlights... Over the West, expect a leading shortwave aloft/surface system to produce rain and mountain snow from the northern half of the West Coast into the northern-central Rockies Sun-Mon with activity tapering off from west to east. This event may produce a brief period of enhanced activity over favored terrain. Upper ridging moving toward/into the Northwest should promote fairly dry weather over the region through Thu. The forecast over southern areas is more uncertain Tue-Thu, with coverage and intensity of precipitation depending on specifics of the upper low that may form and how far northward flow aloft can bring eastern Pacific moisture. An upper low track on the western side of the spread as per the GFS, currently a lower probability scenario, would produce some precipitation over parts of California. On the other side of the envelope a sheared scenario aloft could lead to very little precipitation over southern areas. At this time the greater probability is for at least some precipitation (rain or snow depending on location/elevation) over the southern Rockies/High Plains. High pressure building into the Plains may produce some low level upslope flow over this region as well. There is still decent agreement that a system emerging over the Plains by Mon and then tracking northeastward should produce a broad precipitation shield with various weather types. However there is still meaningful uncertainty over precise system strength and track, tempering confidence in where the rain-snow line will be and the intensity of precipitation. At least some potential for wintry precip types will extend from the northern half of the Plains through the Northeast. The Northeast appears to have the greatest margin for error, with some snow possible whether from warm advection ahead of the system or from a farther south storm track. Expect rain to fall elsewhere. There is still some signal for relatively higher rainfall totals between the Southern Plains and Tennessee Valley but important details aloft affecting frontal timing are still unresolved. The warmest temperature anomalies during the period will be over the central/eastern states Mon into early Wed within the warm sector of the Plains through Northeast system. Morning lows may be as high as 15-25F above normal with anomalies for daytime highs generally at least a few degrees less extreme versus normal. Cold air dropping southward behind this system will bring highs of 10-20F or so below normal into the central-southern Rockies and High Plains starting on Mon but more so Tue-Thu. A modified form of this chilly air should spread across the eastern half of the country Wed-Thu. Rausch Additional 3-7 Day Hazards information can be found on the WPC medium range hazards 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