Extended Forecast Discussion NWS Weather Prediction Center College Park MD 209 PM EST Sat Nov 18 2023 Valid 12Z Tue Nov 21 2023 - 12Z Sat Nov 25 2023 ...Pattern Overview... Confidence remains fair at best given that the latest runs of guidance continue to diverge on the track and evolution of the system expected to track northeastward from the Mississippi Valley from Tuesday onward, spreading a broad area of precipitation over the East just ahead of the Thanksgiving holiday. Additionally, there is potential for trailing waviness due to any central U.S. into northwestern Mexico trough energy that may happen to consolidate and then eject eastward/northeastward. A shortwave trough is depicted to enter the Pacific Northwest by the later periods and in combination with southern Canada/northern tier U.S. flow should yield a mean trough with an axis extending southwestward from the Northern Plains/Upper Midwest. Temperatures will be variable leading up to Thanksgiving but there will be a general colder trend spreading south through the center of the country. ...Guidance Evaluation/Predictability Assessment... The latest guidance has continued to favor a northward shift with an upper trough/low passing north of the Great Lakes with a weaker low pressure over the eastern U.S. The placement of scattered to widespread precipitation across the eastern U.S. continues to vary from the Mid-Atlantic to northern New England. The resultant precipitation will likely be beneficial to portions of the Southeast and Mid-Atlantic where conditions have been drier of late. The GFS persists in developing a secondary surface system across the central U.S. as digging northern energy extends the trough further southwest into the Southern Rockies/Southwest... the UKMET is trending toward that solution as well. Both were deemed extremes and were not utilized for this forecast. Solutions continue to vary for the upper trough moving into the West, with minimal confidence in any specific solution by late in the period. This lack of confidence applies to the surface pattern farther east, with consecutive operational runs varying considerably by next Saturday. Given no appreciable improvement in the guidance, the WPC blend maintained continuity by using equal weights of the past two cycles of the ECWMF, GEFS means and the EC/CMC Ensemble means. ...Weather/Hazards Highlights... A broad area of precipitation is expected to affect the eastern U.S. Tuesday into Wednesday with some possible adverse effects on holiday travel. During the period covered by the Days 4-5 Excessive Rainfall Outlooks (Tuesday through Wednesday night), the best potential for heaviest rainfall should extend from near the central Gulf Coast northeastward through the Mid-Atlantic and parts of southern/eastern New England. The new Day 4 outlook maintains the prior Marginal Risk, with a modest adjustments near the Southeast and Central Appalachians. Initially dry soil conditions should temper runoff issues somewhat within most of the Marginal Risk area, but still expect some locations where drainage is impaired (due to urbanization/leaves/etc) to have some sensitivity to periods of heavier rain. The Day 5 Marginal Risk area over eastern New England, with the most likely forecast scenario having a frontal wave promoting a period of strong onshore flow ahead of it to enhance rainfall totals. As with areas to the south the prior day, relatively dry ground conditions may help to mitigate runoff issues to some degree but localized sensitivity may still exist. Elsewhere, snow will be possible over parts of interior New England at the start of the Tuesday-Wednesday event. A period of lake effect snow will be possible after system passage, with duration and totals dependent on how quickly the systems lifts away. Winds should be brisk behind the system but not as strong as they would have been based on some earlier forecasts of a stronger Great Lakes into Canada storm. With very uncertain details, one or more waves could produce additional areas of rainfall across the southern/eastern U.S. after midweek. Over the West, a system moving into the region late in the week may bring mostly light totals across northern areas, possibly extending into the Plains by Friday. There is some potential for locally heavier snowfall in parts of the northern Rockies around Thanksgiving, but still with a lot of uncertainty in the details. The most prominent temperature anomalies during the period will be with colder air dropping south through the Plains/eastern slopes of the Rockies around next Friday-Saturday, with an expanding area of highs 10-15F below normal. Otherwise, moderately below normal readings will progress eastward through the southern tier and eastern U.S. from Tuesday into Thanksgiving, while an initial upper ridge over the West will support above normal temperatures into midweek before a gradual cooling trend. Campbell/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 forecast, excessive rainfall outlook, winter weather outlook probabilities, heat indices and Key Messages 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/#page=ero https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml https://www.wpc.ncep.noaa.gov/#page=ovw