Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 PM EST Mon Nov 23 2020 Valid 12Z Thu Nov 26 2020 - 12Z Mon Nov 30 2020 ...Multiple wet days could lead to a few inches of rain accumulating from eastern Texas toward the Southeast through early next week... ...Overview... A mid- to upper-level shortwave trough embedded in the large-scale northeasterly upper flow, along with a surface low pressure system, are forecast to track across the Northeast Thanksgiving Day, helping cause rain showers along the Eastern Seaboard. Meanwhile, a larger-scale trough should move through the western and central CONUS. Its northern and southern streams are likely to separate by the weekend; northern stream troughing moves quickly toward the Northeast while a potentially closed upper low in the southern stream lingers over the south-central CONUS, causing possibly heavy rain centered over eastern Texas to the Lower Mississippi Valley and the Southeast. The separate streams may combine again around Mon as additional northern stream energy enters the north-central CONUS, and the trough axis should shift east toward the Midwest. ...Guidance Evaluation/Preferences... Overall, model guidance was in generally good agreement with the pattern described above. The main differences have to do with the degree of separation that develops in the northern and southern streams over the weekend and the timing for joining the streams back up, as well as regarding the potency of the southern stream upper low. The 00Z and 06Z GFS runs remain on the weaker side of the solutions with this energy, but do seem to be slowing down a bit with its movement compared to earlier runs, which better matches other guidance. The WPC forecast for fronts/pressures consisted of a multi-model deterministic blend early in the forecast period, but heavily favoring the 00Z ECMWF solution and lessening the influence of the weaker GFS. The 00Z ECMWF clustered well with the 00Z UKMET and CMC. As the forecast progressed, generally the same strategy was followed with gradually more weight on the 00Z EC ensemble mean. This maintained good continuity with the previous WPC forecasts. ...Sensible Weather and Hazards... For Thanksgiving Day, showers (mainly in the form of rain) are possible across the Eastern Seaboard ahead of a cold front as a surface low tracks across the Great Lakes and New England Thu before exiting Fri. The energetic trough/closing low tracking across the West should lead to snow chances in the Central/Southern Rockies late in the week, with some potential for light snow to spread into southern/central parts of the High Plains toward the weekend. However, the likely bigger threat with this potent upper low will be multiple days of rainfall as moisture from the Gulf of Mexico streams in ahead of the low. Areas from the eastern half of Texas to the Lower Mississippi Valley can expect persistent enhanced rainfall through the weekend, leading to rain totals of a few inches. A surface low/frontal system is forecast to develop and strengthen in the Mississippi Valley over the weekend as the upper low pivots eastward, moving into the Great Lakes region by Mon. Precipitation should continue along and ahead of this system, with rain spreading to the Southeast/Southern Appalachians. Precipitation may also spread northward across the Ohio Valley/Mid-Atlantic/Northeast early next week, but with more uncertainty in amounts at this point due to model spread. Elsewhere, generally light precipitation is forecast for the Pacific Northwest for this time of year, but precipitation chances could increase by Mon as renewed troughing approaches behind an amplifying ridge into the Rockies. Cooler than normal temperatures by 5 to 10 degrees can be expected underneath the troughing across the Intermountain West/Rockies through Fri. As the trough and cold front shift eastward, the Southern Plains will transition to cooler than normal by the weekend after a warm Thu. Cooler weather spreads to parts of the Mississippi Valley to the Ohio/Tennessee Valleys early next week. Temperatures of 5 to 15 degrees above average are possible for the East through Sat, moderating closer to normal early next week. The north-central U.S. should see periods of above normal temperatures, with Sat being the most anomalously warm day with highs 15 to 20 degrees above normal. Tate Additional 3-7 Day Hazards information can be found on the WPC medium range hazards 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