Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 PM EST Sat Nov 27 2021 Valid 12Z Tue Nov 30 2021 - 12Z Sat Dec 04 2021 ...Overview... Periods of relatively low amplitude upper troughing are expected for parts of the central and eastern U.S. as shortwaves move quickly through the mean flow, and these impulses could lead to light precipitation from the Midwest to Northeast, especially for favored lake effect and terrain areas. Meanwhile in the western/central U.S., upper ridging will be in place for much of the period, causing milder than average temperatures with potentially record-breaking warmth. ...Guidance/Predictability Assessment... The 00Z/06Z model guidance begins the medium range period Tuesday with reasonably good agreement with the overall pattern, though slight differences remain with shortwaves tracking through southern Canada and the Great Lakes even early in the period. But the general consensus is for shortwave energy to quickly dig through the central and eastern U.S. Wednesday into Thursday, while additional energy and a surface low track Canada and into the Great Lakes region by Thursday. Meanwhile there are minimal differences regarding ridging in the West, and southern stream compact lows in the eastern Pacific that show some placement and strength differences may not have much of an effect on the CONUS. A multi-model deterministic blend of the 00Z ECMWF, CMC, and UKMET and 06Z GFS was utilized early in the period as guidance was fairly well clustered. Greater model differences arise later Thursday and through the end of the week as shortwave energy stemming from the lower predictability higher latitudes and/or over the Pacific track into and across the U.S. and Canada. At this forecast lead time it is difficult to tell how these relatively small and low predictability features will evolve, and deterministic guidance and ensemble members vary greatly with which shortwaves become the ones to dig considerably and create meaningful changes to the large scale flow. Run to run variability remains high particularly with the deterministic ECMWF runs, but the GFS runs do not necessarily agree with the ensemble means either. For the latter part of the medium range period, trended quickly toward a GEFS and EC ensemble mean blend, since these were better aligned with each other and lessened individual model differences. The main change to the previous forecast was to track fronts somewhat faster across the central and eastern U.S. per the uncertain but overall model consensus. This also lowered expected QPF across the Mid-South with shorter resonance time of the front. ...Weather/Hazards Highlights... As rounds of shortwave energy pass over the north-central to northeastern U.S., expect generally light precipitation across those areas. With the west and northwesterly flow, favored lake enhancement areas as well as upslope terrain in the interior Northeast could see enhanced totals, and at this time it appears downwind of Lake Ontario into the Adirondacks should see the highest precipitation amounts. Meanwhile, periods of moist onshore flow into the Olympic Peninsula and the northern Cascades should lead to generally moderate rain and snow amounts, especially Tuesday into Wednesday. Snow levels are uncertain at this point with the ridgier GFS suite showing rain and less precipitation reaching into Washington, while the EC and CMC are wetter and have lower snow levels, but at least the highest terrain should have snow. Northern parts of the Rockies will also see light to moderate precipitation in this pattern. Other than these two highlighted regions, a majority of the country will be generally dry other than increasing moisture for the latter part of the week from southern Texas northeastward into the Mid-South. Temperatures are expected to be considerably above normal for much of the country next week thanks to ridging overhead. Both highs and lows are forecast to be 20-30 degrees above average for northern and central portions of the High Plains and expanding into the Mississippi Valley Wednesday and Thursday. A few locations may reach or exceed daily maximum and minimum temperature records with these mild temperatures. As the flow flattens during the late period, above normal temperatures should spread toward the East Coast for the latter part of the workweek after a cooler than average start. Tate 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