Extended Forecast Discussion NWS Weather Prediction Center College Park MD 701 AM EST Fri Nov 26 2021 Valid 12Z Mon Nov 29 2021 - 12Z Fri Dec 03 2021 ...Overview and Guidance/Predictability Assessment... Multiple shortwaves will track across the CONUS during the extended period; which will reinforce the mean trough over the eastern U.S. Over the West, an amplified ridge will begin to weaken as a couple of shortwaves move through and into the central U.S., which may also help to flatten the overall upper pattern some by mid to later next week. Generally speaking, the global guidance and the ensemble means have a fair handle on the evolution of this pattern albeit with uncertainty in the specific details. The impulses moving across the northern U.S. and Great Lakes regions pose timing and magnitude differences that essentially would affect a surface low deepening off the Northeast coast. The guidance is favoring an eastern shift as the surface low skirts near the New England coast. Once again the UKMET is positioned notably south of the rest of the clustered guidance and was also as much as 10 mb deeper with the low tracking across the Canadian Prairies/Northern tier U.S. Therefore was considered an outlier for this issuance. As noted in the previous forecast, the CMC which showed a strong shortwave moving into the West diving south to meet up with energy over northern Mexico to create a closed low differently than consensus. The CMC was usable in the initial periods. The WPC blend utilized a combination of the 12/18Z GFS, ECWMF and CMC as a starting point then gradually ceasing use of the CMC while increasing the inclusion of the GEFS and ECWMF means. This approach helped maintain a degree of continuity as well as minimizing individual model differences. ...Weather/Hazards Highlights... Remnant moisture from the weakening atmospheric river will keep lingering moderate to locally heavy rainfall possible for the Olympic Peninsula, far northern Cascades and the surrounding areas. Relatively light precipitation is likely to continue for this region through Monday before another increase in moisture brings another round of moderate to locally heavy precipitation to western Washington around Tuesday-Wednesday, which may be mostly rain but with some uncertainty in snow levels at this point. Portions of the Northern Rockies is also expected to have light precipitation with this flow. Periodic lake effect snow/enhanced snowfall is expected for the Great Lakes regions through the extended period as northwesterly flow passes across the open lakes. A favored locations downwind of Lake Erie will likely observe enhancements through Monday while areas downwind of Lake Ontario are anticipated later in the week. In addition to the lake effect snows, parts of the Midwest and the Northeast will have light precipitation as energy passes aloft. Models have been trending down for the QPF amounts over New England that past few runs. Although there are differences in the location of the Atlantic Low, the possibility of flow wrapping into the low onshore could yield light rain or snowfall. Other than the two highlighted regions, a majority of the country will be generally dry other than some early week showers for Florida and increasing moisture for Texas midweek onward. Temperatures will generally be slightly below normal across the Eastern U.S. as the mean trough becomes reinforced, with some moderation back towards normal Wednesday and Thursday. Much of the West and central states will maintain warmth through the extended forecast thanks to the ridging over the region. A few locations may reach or exceed daily maximum and minimum temperature records, where some places could register 20 to 30 degrees above seasonal average. As the flow flattens during the late period, above normal temperatures should also begin expanding into the Midwest and South. Campbell 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