Extended Forecast Discussion NWS Weather Prediction Center College Park MD 155 AM EST Fri Nov 24 2023 Valid 12Z Mon Nov 27 2023 - 12Z Fri Dec 01 2023 ...Overview... Upper troughing will generally dominate the pattern over parts of the central and eastern U.S. through much of next week, with some embedded shortwaves and frontal systems. A western Atlantic surface low on Monday should quickly track through/close to Maine with some moderate to locally heavy precipitation likely across New England. Behind this, favorable westerly flow and chilly conditions sets the stage for enhanced lake effect snows through at least the first half of the week. An initial ridge over the Western U.S. to start the week should be replaced by mean troughing as a couple of shortwaves slide through, with a possible stronger one reaching the Southwest by the end of the week promoting some ridge building downstream across the Gulf Coast/Southeast. With the exception of the Great Lakes, the weather forecast across the CONUS looks fairly quiet for the entire medium range period. ...Guidance Evaluation/Predictability Assessment... On the large scale, there is modest agreement in the presence of maintained troughing across the Eastern U.S., but a lot of spread in the details of individual shortwaves rounding the base of the trough. This affects frontal and surface wave positions mid to later in the week, with more uncertainty than desired. There is also growing uncertainty out west with the smaller scale placement of energy moving into the region, through there is a little bit better consensus on some deepening of a trough over the Southwest later in the week. Given above average disagreement, felt introducing the ensemble means into the WPC forecast blend as early as day 3/Monday was prudent in an attempt to smooth these differences out. Was able to use some component of the GFS/ECMWF for some added system definition, as these were the operational models closest to that of the ensemble means. Gradually increased the percentage of ensemble means in the overall blend from 20 to 60% throughout the week. For the most part, this maintained fairly good continuity with the previous WPC forecast. In terms of QPF, or lack there of, the only major change from the NBM was to increase quite a bit QPF in the Great Lakes given the low bias it typically has with lake effect precipitation. ...Weather/Hazards Highlights... A surface low just off the Mid-Atlantic coast on Monday looks to track northward near or over New England. This could spread some moderate to locally heavy precipitation into parts of New England, but with a general lack of instability and highly anomalous moisture combined with how fast it will be moving, probabilities for any adverse flooding/flash flooding impacts are less than the threshold for even a marginal risk on the Day 4/Monday Excessive Rainfall Outlook. In the higher terrain of northern New England, this precipitation should fall as snow. Behind this system, favorable westerly cold flow over the still relatively warm Great Lakes should promote a period of accumulating lake effect snow especially downwind of Lakes Erie and Ontario on Monday-Tuesday. Models are coming into better agreement on a favorable lake effect event, but still with plenty of uncertainty in the amounts and exactly where the heaviest snowbands could set up. Probabilities in the Day 4 winter weather outlook were increased from the previous WPC shift to show this potential. Lake effect snow will be possible deeper into next week as well but with likely lighter amounts. Elsewhere across the country, conditions look mostly dry through next week though some light precipitation may accompany a shortwave into the West around mid week. Precipitation chances along the Gulf Coast region may begin to increase again by late week as a warm front lifts northward through the Gulf of Mexico. Much of the lower 48 should see near to below normal temperatures especially during the first half of next week. Decent coverage of highs and lows 10-15F below climatology can be expected from the Great Basin into the central/southern Plains early in the week, shifting into the eastern U.S. Tuesday-Wednesday while the west-central U.S. begin to slightly moderate back towards normal. An exception to the below average temperatures may be across the northern Plains, where by Tuesday and beyond lows in the 20s and highs in the 40s are expected, which are around 5-15F above normal. Lows of a few degrees above average and highs near average in Florida on Monday are likely to cool Tuesday and beyond after a frontal passage. Santorelli 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