Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 PM EST Wed Nov 29 2023 Valid 12Z Sat Dec 02 2023 - 12Z Wed Dec 06 2023 ...Multi-day wet pattern for the Northwest to also include heavy snow threats inland from the Cascades to the north-central Intermountain West/Rockies... ...Overview and Guidance Evaluation/Predictability Assessment... A complex and active weather pattern is projected to continue through the weekend and into next week. An amplified trough will be reinforced by a couple of shortwaves as it moves and periodically reinforced 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 uncertainty in the details. Run to run variability and timing of individual shortwaves through the Midwest and East this weekend/early next week still translates to even some regional sensible weather/precipitation uncertainties especially across the East. However, forecast spread has improved some since yesterday, bolstering confidence to a degree. 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 given uncertainties. That said, the depicted pattern will be quite wet and snowy and offers periods of enhanced winds with progressive passage of dynamic systems. Accordingly, the WPC medium range product suite was primarily derived from a composite blend of best clustered guidance from the 00 UTC GFS/ECMWF/UKMET/Canadian and ECMWF ensemble mean for the coming weekend into Monday. Replaced the UKMET in this blend with the 00 UTC GEFS mean through early-mid next week to address growing forecast spread. Overall, the 06 UTC GFS/GEFS were a bit of an outlier with shortwaves, but the 12 UTC guidance cycle is now back more in line with earlier 00 UTC guidance. WPC product continuity was reasonably maintained. ...Weather/Hazards Highlights... Out West, precipitation will be ongoing as this forecast period begins Saturday, with a protracted wet event into the Northwest continuing well into next week, with growing totals and impacts possible. Some modest rainfall totals are likely along the coast, with potentially heavy snows with gusty winds also farther inland across the higher elevations of the Cascades and northern Sierras through the north-central Intermountain West/Rockies but perhaps especially this weekend from around the Grand Tetons to the northern Wasatch. A small marginal risk for excessive rainfall was maintained for the Day 4/Saturday ERO across far northwest CA/southwest OR given guidance QPF trends in a wet pattern. 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 is 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. Downstream, ejecting shortwave energy and a leading/wavy frontal system with an enhanced Gulf moisture surge are expected 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 and instability. Rainfall may continue into Sunday and Monday as well across the Southeast, but guidance is quite mixed and overall trending less significant. Much of the Eastern U.S. looks unsettled through next week, albeit with higher uncertainty on rainfall amounts given shortwave differences. However, the approach and passage of several shortwaves and advance of an amplified main upper trough should spawn several wavy fronts. Some of the precipitation should be in the form of snow across parts of the Great Lakes and Northeast. Northern New England terrain will focus snowfall threat given longevity of pattern and cold air availability later weekend/early next week. 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. Schichtel/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