Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 AM 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... Latest model and ensemble guidance remains similar for the evolving large scale pattern through Thanksgiving while displaying typical spread and run-to-run variability for the details. A Pacific upper ridge moving into the West/Plains during the first part of the calendar week will support an amplifying eastern U.S. trough whose axis should reach near the East Coast by early Tuesday. It is still up for debate where one or more embedded lows may close off over the Great Lakes or southern Canada early in the week but there is a more common theme of an upper low over or just offshore New England by Wednesday. Thereafter, how quickly the upper low departs will depend on the strength and westward extent of North Atlantic upper ridging. The evolution aloft and potentially complex surface low pressure/frontal evolution may bring a heavy rain threat to northern New England in the warm sector and a period of well below normal temperatures, brisk to strong winds, and lake effect/terrain enhanced snow behind the strong cold front. Continue to monitor forecasts as details may change. Farther west, the combination of a Pacific shortwave reaching the West by Tuesday and an opening upper low initially southwest of California should yield a positively tilted upper trough aligned from the Northern Plains into northwestern Mexico by Thanksgiving. These features and energy rounding the upstream ridge may produce periods of precipitation over the Pacific Northwest while rain may expand/intensify along and north of the western Gulf Coast on Thanksgiving as Gulf moisture interacts with a Plains front. ...Guidance/Predictability Assessment... For the evolution across eastern North America, over recent days the ensemble means (and model runs closest to them in principle) have provided a fairly steady anchor for the general forecast with fairly modest adjustments over consecutive runs. Latest GEFS/ECMWF means have achieved similar solutions for the East Coast upper trough by early Tuesday by way of the ECMWF mean nudging faster and GEFS mean somewhat slower. Both the operational models and means have recently shown a trend for a sharper/deeper trough in response to a slower/stronger trough reaching the West and stronger intervening ridge. Models have been waffling on if/where an embedded upper low may form between Sunday and early Tuesday. However 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. Thus far any stray southern solutions for this upper low (some ECMWF runs, 18Z GFS) have reverted back to the north toward the larger clustering in subsequent runs. A building upper ridge over the North Atlantic and Greenland poses the risk of slowing down the system for a period of time. Latest GFS runs have started to show this and the new 00Z ECMWF has trended much slower late in the period. 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 some models show a frontal wave tracking northward over the western Atlantic while one or more other waves evolve over or reach areas from over/east of New England into the Canadian Maritimes, in response to the upper dynamics. Enough upper ridging to the north could keep this system near New England through or beyond midweek. Many of these details have lower predictability several days out in time, especially since the supporting energy is still over or near Alaska at this time. The 12Z/18Z GFS, 12Z ECMWF/CMC, and the ensemble means provided a reasonable cluster of guidance for the upper trough reaching the West by Tuesday (as mentioned, a little deeper/slower than seen yesterday; UKMET runs have been slow) and the eastern Pacific upper low opening up to the south, as well as for the overall trough that continues eastward through the rest of the period. However by Thanksgiving there is a fair degree of uncertainty over how energy will be distributed within the trough. Also new 00Z runs add other options, such as a stronger upstream shortwave reaching the West in the GFS and much more flow separation in the CMC. Based on guidance comparisons through 12Z/18Z, the updated forecast incorporated the 12Z/18Z GFS, 12Z ECMWF, and some 12Z CMC for the first half of the period and then transition to a model/ensemble mean blend. GFS input tilted more toward the 12Z run once the 18Z version strayed south of consensus with the upper low near the East Coast, while the 12Z GEFS mean provided a slightly stronger solution than the 18Z run for the system near New England. ...Weather/Hazards Highlights... 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 head into New England, recent trends have lowered precipitation totals somewhat over western/southern areas but maintain the potential for some heavy rainfall over Maine, aided by 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 brisk to strong 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 may 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. 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