Extended Forecast Discussion NWS Weather Prediction Center College Park MD 514 PM EST Wed Nov 17 2021 Valid 12Z Sat Nov 20 2021 - 12Z Wed Nov 24 2021 ...Potential continues for a strong cold front and significant storm to affect the East early next week... ...Overview... The expected large scale pattern evolution from the weekend into the middle of next week remains on track. Guidance consensus shows flat mean flow as of early Saturday steadily becoming more amplified as a deepening upper trough heads into the eastern U.S., downstream from a Pacific upper ridge that reaches western Canada and the northwestern U.S. by Monday. Within the eastern trough, some models continue to show an upper low closing off and taking a path somewhere within an area from the Great Lakes/southern Canada into New England, supporting at least moderately strong southern Canada into Northeast U.S. low pressure, which could undergo some redevelopment during the overall system's life span. This storm remains a prominent weather focus due to its timing right before Thanksgiving but it will likely still take a while to resolve the details. Meanwhile the models and ensembles still have an upper low developing southwest/south of California during the weekend and early next week. This feature should open up by Wednesday as a Pacific shortwave reaches the West by Tuesday and additional energy could arrive thereafter. Northern Plains into western U.S. troughing should amplify at least a little around midweek as another upper ridge builds over the Eastern Pacific. Eastern North America low pressure and its trailing front will usher in a brief cold spell over the East during the first half of next week while the West and parts of the High Plains will see variable but generally above normal temperatures through Tuesday. ...Guidance/Predictability Assessment... The 00/06Z model cycle continued to show fairly good clustering in the eastern North America evolution considering typical guidance spread and error for forecasts at this lead time. One remaining issue is the possibility and positioning of a 500 mb low that could close off within the trough. After deep closed lows in previous runs, the 00Z ECMWF showed narrow and open but still deep troughing, while the new 12Z run has a closed low again. Meanwhile, the new 12Z GFS shows an open trough in the East Tuesday after 00Z and 06Z runs were closed. Ensemble means generally show narrow and deep troughing that may imply a closed low. The detail issues in the troughing could affect surface low positions and strength as well. Regardless, low pressure is forecast to track between the Great Lakes and James Bay as of early Monday and could see redevelopment and/or looping through Tuesday-Wednesday. A minor change from the previous forecast was to move the trailing cold front faster based on consensus. It is important to note that the upper dynamics supporting this system are still within flow to the north/west of an upper low over Alaska and historically the models can have difficulty in resolving important details of amplifying shortwaves from higher latitudes until much closer to an event. A blend of the deterministic guidance used at the beginning of the medium range period transitioned to an approximately even blend of the GFS/ECMWF/GEFS mean/EC mean by the latter part of the period. Farther west, GFS/ECMWF/UKMET along with the GEFS/ECMWF means are still providing the most common solution for the upper low that forms west/south of California, with some opening/progression likely to occur after early Tuesday. CMC runs have been on the eastern side of the guidance spread, though the 12Z CMC has shifted west closer to consensus. A model/mean blend excluding the 00Z CMC worked well for this feature as well as the shortwave reaching the Northwest and then the Plains Tuesday-Wednesday and for downplaying uncertain shortwave details upstream. ...Weather/Hazards Highlights... The start of the weekend should be dry over much of the country aside from rainfall persisting along the eastern Florida Peninsula and some light snow over parts of the northern/central Rockies. Expect precipitation to develop along and east of the Mississippi Valley on Sunday ahead of a leading Plains front and trailing strong cold front (anchored by northern tier U.S./southern Canada low pressure) that catches up. Most rain should be light to moderate given the frontal progression but locally more intense activity may be possible. Highest totals for Sunday-Sunday night should be in the Ohio/Tennessee Valleys. From Monday onward the most likely evolution of this system would bring more of an enhanced rainfall threat to New England, ahead of the cold front/secondary low development. The best snow potential would be 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. Confidence is somewhat higher in the potential for a period of brisk to strong winds behind the cold front sweeping through the Plains into the East, with such winds continuing over the Great Lakes and Mid-Atlantic/Northeast into at least Tuesday after strong initial southerly winds on Monday. The Pacific Northwest into northern Rockies may see mostly light precipitation during the first half of next week with a shortwave 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 parts of the West by Tuesday/Wednesday, but with low confidence for precipitation coverage/amounts at this time. The East will see chilly temperatures (mostly 5-10F below normal) lingering into the early weekend. The next surge of colder air will spread across the eastern half of the lower 48 Monday-Tuesday with the broadest coverage of highs 10-15F below normal expected on Tuesday. Some locations may also see morning lows 10F or so below normal. The West into the central/southern High Plains will see mostly above normal temperatures through Tuesday with the warmest anomalies of plus 10-15F for highs likely over the High Plains on Saturday (lingering into Sunday near the Rio Grande) and again on Tuesday. The upper trough over the West by Wednesday will likely bring highs down to near or slightly below normal levels at that time. Rausch/Tate Hazards: - Heavy precipitation across portions of Northeast, Mon-Tue, Nov 22-Nov 23. - Heavy rain across portions of the Northeast, Mon-Tue, Nov 22-Nov 23. - Flooding occurring or imminent across portions of the Pacific Northwest and the Northern Great Basin. - High winds across portions of the Northeast, Mon-Tue, Nov 22-Nov 23. - Much below normal temperatures across portions of the Northern Plains, Mon, Nov 22. - Much below normal temperatures across portions of the Southeast, Tue, Nov 23. - Much below normal temperatures across portions of the Southeast, Wed, Nov 24. 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