Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 PM EST Sun Jan 01 2023 Valid 12Z Wed Jan 04 2023 - 12Z Sun Jan 08 2023 ...More heavy precipitation expected to reach the West Coast with the most significant episode Wed-Thu in California... ...Guidance/Predictability Assessment... Models and ensembles are now unanimous in bringing a rather deep cyclone closer toward but off the Pacific Northwest coast on Thursday, with the CMC having a storm track closest to the coast. The bulk of the moisture ahead of the cyclone is forecast to be precipitated over California starting on Wednesday, culminating on Thursday. Guidance has been very consistent in this regard. Meanwhile in the eastern U.S., models have been slowly pushing the cold front faster toward the East Coast, leading to a lowering threat of heavy rain across the Southeast than in previous forecasts on Wednesday. Across the Northeast, models are slowly converging midway between the slower ECMWF solution and the faster GFS solution in depicting a sheared low pressure wave sliding off the New England coast with lingering surface troughing extending west into the Great Lakes as colder air arrives from eastern Canada on Thursday. A lingering upper wave/low may interact with the surface trough and the cold air to deliver a period of snow or mixed precipitation across the interior Northeast into Friday. By next weekend, a couple of Pacific frontal systems are forecast to bring the next surge of moisture into the West Coast with uncertain system timings. Meanwhile, lingering upper-level troughing moving through the Intermountain West is forecast to reach the central Plains next weekend and would help develop a relatively weak frontal wave that is forecast to track generally toward the Great Lakes. Given better model agreement in the medium range time scales, a composite blend of the 06Z GFS, 00Z ECMWF, with some contributions from the 00Z Canadian model seems to offer a reasonable forecast starting point for the medium-range forecasts today. Increased incorporation of the ensemble means appears compatible with forecast continuity into next weekend. ...Weather/Hazard Highlights... There is some chance for snow in marginally cooled air from the lower Great Lakes through the northern Northeast through midweek with system passage. Meanwhile to the south, a wavy frontal system will develop and spread some moderate rain up the Eastern Seaboard midweek, with uncertain additional/modest frontal wave formation offshore possibly lingering some light QPF into the Mid-Atlantic/coastal Northeast into later next week. Prior to this, much milder than average temperatures will linger across much of the Eastern Seaboard into midweek. Temperatures of around 15-30F above average will be common with widespread record warm morning lows forecast for Wednesday. It remains the expectation that additional rounds of precipitation are also expected to reach the West Coast this week in a very wet and active pattern in the Pacific to start 2023, but still with a focus primarily on California. This latest episode of heaviest precipitation is expected to culminate Wednesday-Thursday as a system approaches from the Pacific with another potential Atmospheric River event to shift steadily southward across the state over time. Many areas are likely to be sensitive to additional rainfall by then given the ample recent rainfall in addition to the rain expected during the short range period. This will be followed by an additional chance for an uptick in precipitation with additional system approach latter week. WPC experimental medium range Excessive Rainfall Outlook (ERO) "slight" risk areas have been issued for the Wednesday-Thursday episode. There is also a multi-day threat for heavy snow that will focus along the Sierra this week, but also spread northward through the Cascades and with some gusto inland across the Intermountain West toward the Rockies over time with system translation. This threat is addressed in the WPC Winter Weather Outlook (WWO) and other products. System/energy translation and associated/modest surface system genesis is then expected to work downstream across the central to east central U.S. heading into next weekend, but with limited QPF potential. Kong/Schichtel 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, experimental excessive rainfall outlook, winter weather outlook probabilities and heat indices 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