Extended Forecast Discussion NWS Weather Prediction Center College Park MD 208 PM EST Fri Nov 26 2021 Valid 12Z Mon Nov 29 2021 - 12Z Fri Dec 03 2021 ...Overview... The initial upper-level pattern as the medium range period begins Monday will consist of troughing across the East and ridging over the West, and shortwaves moving through the broader flow may serve to deamplify the pattern somewhat as next week progresses. A surface low off the Northeast coast could spread some precipitation to the region Monday, while rounds of light precipitation are possible for the Great Lakes region and interior Northeast through the week. Meanwhile, persistent moist inflow into the Pacific Northwest should cause additional precipitation there, with the heaviest likely Tuesday into Wednesday. ...Guidance/Predictability Assessment... Model agreement for the 00Z/06Z cycle was fairly good regarding the large scale pattern early in the period on Monday, though uncertainties in energy coming into the trough in the East will affect positioning and depth of a surface low off the Northeast coast. The general trend for this low has been somewhat weaker compared to yesterday, but ensemble guidance continues to show spread with the central pressure as well as its position. The 00Z UKMET continued to be a slow outlier compared to consensus and was not favored. Other models are more difficult to rule out, but the 00Z and especially the new 12Z CMC show a track somewhat slower and closer to land than what we wanted to favor at this time. The WPC forecast attempted to follow a middle ground solution of the GFS and ECMWF deterministic and ensemble mean positions for the low. But uncertainty is still fairly high with this feature for a day 3 forecast especially as relatively small wobbles in the low track closer or farther from land could cause impactful precipitation differences, and new 12Z guidance is shifting a bit westward. After that, there is fairly good agreement for potent shortwave energy to move across the Great Lakes Tuesday-Wednesday, but with more variability in its exit northeastward as well as potentially additional energy approaching that region by Thursday. Farther upstream, initially model guidance is clustered fairly well in the West, but differences increase by around Wednesday with more ridging shown in the Northwest in GFS and GEFS members compared to the deterministic ECMWF and EC ensemble members and the mean. This affects amounts of precipitation reaching western Washington from British Columbia as well as precipitation type (the ridgier GFS suite has less precipitation farther south and staying all rain, with the EC/CMC wetter and lower snow levels). Even greater differences arise in the large scale pattern by Thursday-Friday in the western and central U.S. as shortwaves either dig or pass through the broader flow to varying degrees. The EC mean and to a less extent the GEFS means end up showing a mean trough across the central U.S. on Friday, which the 00Z deterministic ECMWF and CMC were closer to compared to the 00Z and 06Z GFS runs that kept low amplitude troughing over the Northwest and Northeast instead. Uncertainty is rather high by that point given the small magnitude of the impactful shortwaves and the energy arising from high latitudes and over the Pacific. The WPC forecast was based on based on a blend of the deterministic 00Z ECMWF and CMC and 00Z and 06Z GFS early in the period. As the period progressed, weighted the GEFS and EC means more to minimize individual model differences, as well as lessening GFS influence as GFS runs were not aligned with the means by later in the week. ...Weather/Hazards Highlights... The Pacific Northwest, namely the Olympic peninsula and the far northern Cascades, should see relatively light precipitation continuing through Monday before another increase in moisture brings another round of moderate to locally heavy precipitation to western Washington around Tuesday-Wednesday, with some uncertainty in snow levels at this point. Portions of the Northern Rockies are also expected to have light precipitation with this flow. Periodic shortwave energy across the north-central to northeastern U.S. will cause rounds of light precipitation chances through those areas. Favored lake enhancement areas will see the best chance for snowfall becoming moderate, especially downwind of Lake Ontario, as northwesterly flow passes across the open lakes. Additionally, the low offshore of the Northeast could cause light wraparound rain or snowfall through Monday, with amounts depending on the low track. Other than these two highlighted regions, a majority of the country will be generally dry other than increasing moisture for Texas midweek onward and spreading into the Mid-South by Thursday/Friday. Temperatures will generally be slightly below normal across the Eastern U.S. Monday as the mean trough becomes reinforced, with moderation back towards normal Wednesday and Thursday. Much of the western and central states will maintain mild temperatures 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. Tate/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