Extended Forecast Discussion NWS Weather Prediction Center College Park MD 202 AM EST Wed Nov 29 2023 Valid 12Z Sat Dec 02 2023 - 12Z Wed Dec 06 2023 ...Wet pattern for the Northwest to include heavy snow threats inland from the Cascades to the north-central Intermountain West/Rockies... ...Overview and Guidance Evaluation/Predictability Assessment... An active weather pattern is upcoming through the medium range time scales. An amplified trough will be reinforced by a couple of shortwaves as it moves from the interior West to the East Coast Saturday-Monday. Amplified ridging builds behind this across the West and eventually the Plains early next week as the next system moves in to the West Coast from the East Pacific next Wednesday. The guidance continues to be quite agreeable on the large scale/synoptic pattern but still with a lot of uncertainty in the details. Run to run variability and timing of individual shortwaves through the Midwest and East this weekend/early next week, which translates to even some regional sensible weather/precipitation uncertainties especially across the East. Out West, models agree that an Atmospheric River may impact the West Coast this weekend, but still show considerable differences in placement of the heaviest QPF. The latest NBM still seems too wet in spots, and the WPC forecast tempered that down somewhat. The WPC forecast tonight used a purely deterministic model blend for days 3 and 4. After this, began increasing use of the ensemble means amidst differences in timing of shortwaves into the East and trough amplification. There is better agreement actually as the next trough moves into the East early-mid next week, but larger scale timing differences with the next trough into the West at the same time necessitated more influence from the ensemble means days 6-7, although with still 50 percent of the deterministic GFS and ECMWF to help maintain system definition. This maintained pretty good continuity with the previous WPC forecast as well. ...Weather/Hazards Highlights... Upper-level shortwave energy will move over the Mid-South while a warm front lifts north out of the Gulf of Mexico late this week, and an amplified trough shifts out of the West this weekend providing a renewed surge of moisture. This will continue to support a moderate to heavy rainfall threat for the Gulf Coast and Southeast into Saturday, where there is a marginal risk for excessive rainfall within an area of anomalous moisture (PWs 2-3 standard deviations above normal) and instability. It is possible an embedded slight risk will be needed at some point, but there is still some uncertainty on exact amounts. Rainfall may continue into Sunday and Monday as well across the Southeast, but should be lighter in intensity. Much of the Eastern U.S. looks unsettled through next week, albeit with higher uncertainty on rainfall amounts given shortwave differences. Some of the precipitation could be in the form of snow across parts of the Great Lakes and Northeast. Out West, precipitation will be ongoing as the period begins Saturday. Some modest rainfall totals are likely along the coast, with snow farther inland across the higher elevations of the Cascades, northern Sierras, and northern Rockies. A small marginal risk was added to tonights Day 4/Saturday ERO across far northwest CA/southwest OR. The stronger system/potential AR moves in Sunday and with snow levels rising, could bring a more widespread heavy rainfall threat along the Oregon coast and western slopes of the Cascades. A marginal risk was included for this area on the Day 5/Sunday ERO. The next system in the East Pacific early next week brings another rain/mountain snow threat, though at this point looks to focus farther north across mainly Washington state. Temperatures across the East this weekend may be moderately above normal, with slightly below normal temperatures across the interior Southwest. The Western U.S. should trend warmer with time and especially by next Tuesday-Wednesday in the Northern High Plains/Plains where daytime highs could be 10 to 20 degrees above normal. Meanwhile, the Eastern U.S. should trend back towards normal/slightly cooler than normal next week as reinforced troughing affects the area. 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