Extended Forecast Discussion NWS Weather Prediction Center College Park MD 150 AM EST Tue Nov 20 2018 Valid 12Z Fri Nov 23 2018 - 12Z Tue Nov 27 2018 ...Overview... A very strong closed high north of England is forecast to retrograde westward across southern Greenland later this week and settle over northern Quebec, favoring blocked troughing over the eastern third of the lower 48. In the west, troughing will pass through late this week and be replaced by ridging Sun-Mon ahead of another system next Tuesday. ...Guidance/Uncertainty Assessment... Models and ensembles continue to perform adequately enough that a deterministic blend sufficed for the first couple days of the period though the 12Z UKMET was most different (quite strong) from the rest after Friday and was dropped from the blend. Timing the systems Fri-Sat has been most problematic over the past few days thus a blended/consensus starting point remained the best option. By Saturday, low pressure will develop and lift along the Southeast coast but the models continue to diverge in how to track the system. Preferred to follow the ECMWF ensemble mean early next week as it has been more stable than the GEFS mean, taking the low off the DelMarVa early Sunday and then eastward as the incoming ridging from the north blocks its continued northeastward movement (discounted the much father north 12Z ECMWF per this reason). To the west, second trough and then upper low will dip into the mid-Mississippi Valley Sunday and lift into the Great Lakes, taking another system northeastward on the west side of the Appalachians. By next Mon/Tue, with the upper ridging firmly in place over Canada, this system should have no choice but to scoot eastward and stretch out E-W over the Northeast with the triple point low likely taking over from about the DelMarVa northward. This could be a significant 1-2 punch for much of the East this weekend and early next week. ...Weather Highlights/Threats... Continued wet for the west as the second of two systems (the first in the short range) moves through on Friday/Saturday with plenty of snow at higher elevations. Additional precipitation will be likely next week ahead of the next Pacific system, but favored over coastal areas of WA/OR by then. In the east, lead system on Friday will tap the Gulf for moisture and even as the northern front weakens into the Great Lakes, precipitation will expand eastward as the Gulf surface low organizes. Rain will spread northeastward with some wintry weather on the north and northeast side as the cold airmass retreats (models really struggle with this evolution). Heaviest rain may be along the I-81/I-95 corridor from SC up to the Mid-Atlantic to the northwest of the surface low track. Will get only a brief respite ahead of the next system lifting into the Lakes which has the potential to be slower and stronger than the previous system as it gets blocked from a typical exit through southeastern Canada. Temperatures will moderate in the east after a cold Black Friday but a cold front out of Canada will bring in colder air again next week with a large area of max temp negative anomalies (5-15F below average) from the Rockies eastward. Fracasso WPC medium range 500 mb heights, surface systems, weather grids, quantitative precipitation, winter weather outlook probabilities and heat indexes are found 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