Extended Forecast Discussion NWS Weather Prediction Center College Park MD 156 AM EST Mon Jan 02 2023 Valid 12Z Thu Jan 05 2023 - 12Z Mon Jan 09 2023 ...Renewed heavy precipitation threats to breach the West Coast with the most significant episode Wednesday-Thursday in California... ...Guidance/Predictability Assessment and Weather/Hazard Highlights... Models and ensembles agree to bring a rather deep cyclone toward but off the Pacific Northwest coast on Thursday. The bulk of the atmospheric river moisture ahead of the cyclone is forecast to be precipitated while shifting southward over California with culmination on Thursday. Many areas, especially burn scars, 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. WPC has issued an experimental medium range Excessive Rainfall Outlook (ERO) "slight" risk area along with high probabilities for Sierra snowfalls in the WPC Winter Weather Outlook (WWO). Guidance also shows some favorable flow into the Pacific Northwest for especially southward facing terrain and periodic shortwave/terrain enhanced snow potential farther inland across the Intermountain West toward the Rockies with system translations. By the weekend, a couple of uncertain Pacific lows/frontal systems are forecast to bring the next surges of moisture into the West Coast of note, but are not expected to be with the gusto of the lead event. Guidance also remains varied with emphasis in depiction of a low pressure wave sliding off the New England coast as 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. Meanwhile, lead upper-level troughing moving through the Intermountain West is forecast to reach the central Plains by Saturday and help develop a modest frontal wave that is subsequently forecast to track generally toward the Great Lakes then off the East Coast by Monday. Compared to the models and considering system development, the 01 UTC NBM seems to be missing the modest precipitation potential that may produce some light wintry weather over the Midwest as well as a trailing area of showers down trough the south-central U.S. as well as downstream into the Mid-Atlantic. WPC QPF and associated weather grids were adjusted to accommodate. Overall, a composite blend of the GFS/ECMWF/Canadian/UKMET seems to provide a reasonable forecast basis Thursday into Friday before WPC guidance preference switches to the still compatible GEFS/ECMWF ensemble means amid modestly growing forecast spread through the rest of this medium range period. This solution maintains reasonable WPC product continuity and consistency. 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