Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 AM EST Sat Nov 18 2023 Valid 12Z Tue Nov 21 2023 - 12Z Sat Nov 25 2023 ...Pattern Overview... Confidence in various aspects of the forecast has decreased dramatically over the past day as guidance has diverged for important details from early in the forecast period that starts early Tuesday. This affects specifics of a 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, as well as potential trailing waviness due to any central U.S. into northwestern Mexico trough energy that may happen to consolidate and then eject eastward/northeastward. Farther west, there is loose agreement and continuity with a shortwave trough reaching the western U.S. by late week but with detail differences that affect precipitation distribution and amounts from the West into the Plains. By the end of the period next Saturday, the average of guidance suggests that the combination of the western shortwave and southern Canada/northern tier U.S. flow should yield a mean trough with an axis extending southwestward from the northern Plains/Upper Midwest. Aside from the southern tier staying cool, system progression will promote variable temperatures from Tuesday into Thanksgiving. Then the pattern should favor colder temperatures spreading down through the central U.S. ...Guidance Evaluation/Predictability Assessment... In the past 24 hours the most common change in the guidance has been toward less phasing between northern stream flow and the trough extending from the Mississippi Valley southwestward as of early Tuesday. This has shifted the potential resulting upper low from the Upper Great Lakes (or even Upper Midwest per the 00Z/17 GFS) well northward, at least into northern Ontario, while resulting in weaker low pressure over the eastern U.S. in a majority of solutions. Latest runs have offered a host of possibilities for where this wave could go, from eastward off the Mid-Atlantic coast (12Z/18Z GFS) to a northeastward track through New England. To add some complications behind this system, the new 00Z GFS is on the extreme side of guidance with how much northern stream energy it digs into the central U.S., creating a second strengthening surface system, while other runs have tried consolidating energy in the northwestern Mexico part of the initial upper trough and then ejecting it, leading to yet another possible source of surface wave/QPF forcing. Meanwhile the details 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. The pronounced model spread and variability seen through the period favored a conservative forecast approach closest to continuity while assessing how guidance may try to gravitate toward any of the alternate solutions currently offered. This led to starting the updated forecast with a blend of the past two ECMWF runs through the 12Z/17 run and 18Z GEFS mean plus 12Z ECens/CMCens means. ...Weather/Hazards Highlights... In spite of uncertainty with specifics, continue to expect a broad area of precipitation 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 southwestward extension into southern Mississippi based on latest guidance suggesting increased potential for an axis of heavy rain to extend into that area. 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 outlook introduces a 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. 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