Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 AM 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 most 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 fairly strong 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. Overall Northern Plains into western U.S. troughing should amplify at least a little around midweek as another upper ridge builds over the Eastern Pacific. The Northeast storm 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... Relative to typical guidance spread and error for forecasts 5-7 days out in time, most models and ensemble means were fairly well clustered for the eastern North America evolution as of the 12Z/18Z cycles. A somewhat less extreme/northward trend in ECMWF upper low track, six-hourly GFS runs shuffling around the guidance average, and a deeper trend in the ensemble means (leading to at least an implied upper low if not a closed one such as depicted in the new 00Z GEFS mean) have led to a forecast upper low track across the upper/eastern Great Lakes and into New England. This evolution reinforces a recent stronger trend for low pressure between the Great Lakes and James Bay as of early Monday and a slight northward adjustment from continuity for secondary development over/near New England by Tuesday. CMC details have differed somewhat. Taken as a whole, recent GFS/ECMWF/CMC runs illustrate the possibilities of redevelopment and/or looping of low pressure. The new 00Z ECMWF provides a significantly different adjustment that would lessen effects over the Northeast. 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 models can have difficulty in resolving important details of amplifying shortwaves from higher latitudes until much closer to an event. The GFS/ECMWF/UKMET along with the GEFS/ECMWF means are 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. Including the new 00Z run, the CMC has been on the eastern side of the guidance spread. A general model/mean average is reasonable and consistent for the shortwave reaching the Northwest and then the Plains Tuesday-Wednesday and for downplaying uncertain shortwave details upstream (such as the stronger 12Z ECMWF feature nearing the West Coast early Wednesday). Based on the 12Z/18Z guidance comparisons, the updated forecast started with the 12Z/18Z GFS and 12Z ECMWF along with some 12Z UKMET for about the first half of the period and then transitioned to a blend of those models along with the 18Z GEFS/12Z ECMWF means. ...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 southern Canada low 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 in westward-facing terrain from the central Appalachians northeastward. There is still enough uncertainty in system evolution to allow for other possibilities regarding precipitation types/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 during Sunday-Monday and continuing over the Great Lakes and Mid-Atlantic/Northeast through at least Tuesday. 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 broader upper trough over the West by Wednesday will likely bring highs down to near or slightly below normal levels at that time. 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