Extended Forecast Discussion NWS Weather Prediction Center College Park MD 204 PM EST Thu Nov 18 2021 Valid 12Z Sun Nov 21 2021 - 12Z Thu Nov 25 2021 ...Potential continues for a strong cold front and significant storm to affect the East next week... ...Overview... For the early part of next week, a complex low pressure system is forecast to develop near the Great Lakes region and track eastward toward the Northeast U.S./Southeast Canada while the upper-level pattern amplifies aloft. The cold fronts associated with this storm system will bring colder temperatures and possibly gusty winds to the East, while heavy rain is a possibility in the warm sector in northern New England, with snow possible on the backside of the low in the interior Northeast, especially in lake effect/terrain enhanced areas. Continue to monitor forecasts as details may change. Farther west, a southern stream upper low initially southwest of California should combine with a Pacific shortwave and yield an upper trough aligned from the Northern Plains into northwestern Mexico by Thanksgiving, bringing some light to moderate precipitation to the Pacific Northwest and some higher elevation snow in the Rockies around Monday-Wednesday, and developing into a possible heavy rain threat on Thanksgiving across parts of Texas and the Lower Mississippi Valley as Gulf moisture interacts with a Plains front. ...Guidance/Predictability Assessment... The first notable feature to watch is an upper trough with its axis initially over central North America on Sunday that pushes eastward and deepens considerably as additional energy streams in and a ridge initially over the the West Coast tracks east as well. The vorticity/energy coming in that causes the trough to dig is still aloft over or near Alaska at this time, so the details of the trough/eventually developing closed low as well as the associated surface low still have lower predictability several days out in time. Models have been waffling on if/where an embedded upper low may form between Sunday and early Tuesday, but there is a decent consensus that an upper low should reach along or just offshore the Mid-Atlantic or New England coast by later Tuesday or Wednesday. At the surface, there is reasonable continuity with low pressure consolidating over the Upper Great Lakes/southeastern Canada Sunday-Monday. After that the forecast gets more complicated, as multiple surface low centers may wrap underneath the spinning upper low that may get blocked from moving away from the Northeast due to a North Atlantic ridge. For the 00/06Z model cycle, GFS runs and the ECMWF are quite similar to each other in terms of upper and surface low placement through at least Tuesday. However, cluster analysis and spaghetti plots of ensemble members from the ECMWF, GEFS, and CMC ensembles show that these deterministic models are on the slower/deeper end of the model spread despite their good agreement with each other. The GEFS and EC ensemble means are fairly well aligned through the period, and appear the best proxy for a middle ground solution in terms of timing and position, since the GFS and EC are slower and the 00Z CMC takes a bit faster track. The 12Z GFS and ECMWF appear a bit faster and thus closer to consensus. For the West, model guidance is reasonably agreeable with both the southern stream low in the Pacific west of California and it opening up and possibly combining with the northern stream around Tuesday as a shortwave comes into the Pacific Northwest, though with the 00Z UKMET remains on the slightly slower side. After that, models are still varying from run to run with timing of the trough's movement, and newer 12Z guidance coming in seems to maintain or reproduce more flow separation with a slower track of the trough/possible closed low in the Southeast, which was first seem in the 00Z CMC. For the updated medium range forecast, a deterministic model blend was used initially, favoring the GFS runs and the 00Z ECMWF. As the period progressed, phased in more ensemble mean guidance and ended up with a fairly even blend of the 00Z ECMWF and its ensemble mean and GFS runs and the 06Z ensemble mean. This maintained good continuity with the previous forecast. ...Weather/Hazards Highlights... The consolidating northern tier U.S./southern Canada low pressure will bring a leading front and stronger trailing front into and through the East early in the calendar week. Precipitation (mostly rain) will develop along and east of the Mississippi Valley on Sunday. Most rain should be light to moderate but locally more intense activity may be possible over parts of the Ohio/Tennessee Valleys. As the front and possible waves of low pressure head into New England, the trend continues to be for somewhat lower precipitation totals over western/southern areas but maintain the potential for some heavy rainfall over Maine, aided by strong Atlantic inflow. Snow is still most likely in the cold air behind the system, to the lee of the Great Lakes and along westward-facing terrain from the central Appalachians northeastward. There is still enough uncertainty in system evolution to allow for other possibilities regarding precipitation types and amounts. Meanwhile confidence is higher in a period of breezy/gusty winds from the Upper Midwest through the East Sunday into Tuesday and then possibly lingering over the Northeast thereafter. Across the northern tier the combination of low temperatures and wind would produce cold wind chills. The Pacific Northwest into northern Rockies should see mostly light to perhaps locally moderate precipitation during the first half of next week with a shortwave/cold front arriving from the Pacific. The combination of this energy aloft and the opening/progression of the upper low southwest of California may spread some moisture into other parts of the West by Tuesday/Wednesday, but still with modest confidence for precipitation coverage/amounts. Around Thanksgiving the western front is currently expected to reach far enough into the Plains to begin interacting with Gulf moisture to produce an expanding area of rain along and north of the western Gulf Coast. Some of this rain may become heavy. The strong cold front crossing the East early in the week will bring well below normal temperatures that should be most pronounced between Monday and early Wednesday when there should be decent coverage of highs/morning lows 10-15F below normal, with some pockets of highs possibly 15-20F below normal over southern areas. Temperatures may fall below freezing rather far into the South. Readings should rebound toward normal by Thanksgiving. Portions of the Rockies/Plains will see a brief warm period early in the week with some plus 10-15F or so anomalies. A cold front moving from the West into the Plains during Tuesday through Thanksgiving will bring moderately below normal highs in its wake. Tate/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