Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 PM 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 New England with some moderate to locally heavy precipitation likely there. 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 West 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, and then perhaps some wet weather entering the south-central U.S. by Thursday, the weather forecast across the CONUS looks fairly quiet for the entire medium range period. ...Guidance Evaluation/Predictability Assessment... While the pattern on the synoptic scale showing troughing in the eastern half of the U.S. through at least the middle of next week is in good agreement, the details of individual shortwaves affecting frontal and surface wave positions still have lower confidence than desired. The first forecast challenge with this is the timing and placement of a nor'easter affecting New England at the start of the period Monday. Most of the models in the 00/06Z cycle sped up the surface low a tad, putting it over eastern Massachusetts 12Z Monday. The GFS runs, UKMET, and many GEFS members all showed this, while the CMC and CMC ensemble were even a bit faster, but the ECMWF suite was a bit slower, especially the deterministic 00Z EC. Since even the ECMWF-based machine learning models were faster than the EC, the WPC forecast favored the faster trend, most like the middle ground GFS/GEFS solutions. The newer 12Z ECMWF is faster than its 00Z run but still not quite as fast as other guidance. The low should track quickly into Canada Monday-Tuesday. After that there is increasing uncertainty in the details within the trough pattern. Meanwhile across the West, fortunately models have converged somewhat on the pattern, with Southwest U.S. energy that was complicating the pattern appearing weaker now. So the timing of the upper low tracking east from the Pacific has improved though the GFS runs are farther south than other guidance. By the latter part of the week though, additional energy coming into the West shows increasing spread. Similar to the previous shift, used a blend of mainly deterministic models early in the forecast period but with some ensemble mean component to smooth out individual model differences. Gradually increased the percentage of ensemble means in the overall blend to half throughout the week. For the most part, this maintained fairly good continuity with the previous WPC forecast, though some changes in fronts were made. 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 looks to track northward near or over New England on Monday. This could spread some moderate to locally heavy precipitation into parts of New England with a quick push of heavy rainfall in coastal areas, 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 given the small scale of the heavy snowbands, minor shifts in location of the bands would provide considerably different conditions and accumulations over slightly different areas. 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 midweek. Precipitation chances along the Gulf Coast region to perhaps as far north as the Middle Mississippi Valley 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 through the first half of next week. Decent coverage of highs and lows 10-15F below climatology can be expected from the Four Corners states into the central/southern Plains and Mississippi/Ohio Valleys on Monday, shifting into the eastern U.S. Tuesday-Wednesday while the west-central U.S. moderates back toward normal. However, the interior West looks to cool back down during the latter half of the week due to troughing aloft. 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. By Thursday and especially Friday, southerly flow off the warm Gulf of Mexico looks to ramp up and increase temperatures, especially lows, in the south-central U.S. Lows of a few degrees above average and highs near average in Florida on Monday are likely to cool Tuesday-Wednesday after a frontal passage, before slowly rebounding. Tate/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