Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 AM EST Sat Nov 25 2023 Valid 12Z Tue Nov 28 2023 - 12Z Sat Dec 02 2023 ...Overview... An upper low over/near Hudson Bay will allow for general troughing with some embedded shortwaves Tues-Thurs across the Great Lakes and Northeast. This pattern would support a multi-day period of enhanced lake effect snows, ongoing from the end of the short range period, locally heavy at times. An initial ridge over the West on Tuesday should be replaced by mean troughing as a closed low drops into the Southwest with a reinforcing amplified shortwave/additional embedded low Friday-Saturday. This should promote ridge building downstream across the Southeast and eventually the East Coast by next weekend. Much of next week should be relatively quiet and dry, with the exception of the Great Lakes and increased moisture and precipitation moving into the south-central U.S. by late in the week. ...Guidance Evaluation/Predictability Assessment... While the models on the synoptic scale show relatively good agreement, at least the first half of the period, there is still lower confidence than desired in the details of individual shortwaves through the Great Lakes/Northeast affecting frontal and surface wave positions. Out West, there are some placement differences with the intitial upper low moving into the West Wednesday-Thursday and then timing/strength of the energy through the mid-south later in the week (with implications for possible late period heavy rainfall across the lower Mississippi Valley and Gulf Coast). Greater uncertainty arises with additional sharper shortwave energy towards the Pacific Northwest Thursday-Friday, which some guidance shows another upper low closing off near California before it slides inland. This should generally act to amplify troughing further across the West late period, but there is still considerable uncertainty in the timing and details. The WPC blend for tonight was able to utilize a purely determinisitic model blend (between the ECMWF, GFS, and UKMET...the CMC was too sharp with a shortwave through the Great Lakes early on) days 3-5. After that, blended in increasing amounts of the ensemble means to smooth out differences out West late period. Generally this maintained good continuity with the previous WPC forecast. ...Weather/Hazards Highlights... Favorable westerly cold flow over the still relatively warm Great Lakes will continue to promote a multi-day period of accumulating lake effect snow especially downwind of Lakes Erie and Ontario, with the best chance for possibly heavy totals on Tuesday. Models show agreement on a favorable lake effect event Monday into Tuesday (and possible additional beyond), 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. 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 look to increase again by late week as a warm front lifts northward through the Gulf of Mexico bringing possibly at least locally heavy rainfall. Much of the south-central to Eastern U.S. should see near to below normal temperatures through the first half of next week with decent coverage of highs and lows 10-15F below climatology. The west-central U.S. should be moderating back toward normal mid week with the interior West cooling 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