Extended Forecast Discussion NWS Weather Prediction Center College Park MD 158 AM EST Wed Jan 04 2023 Valid 12Z Sat Jan 07 2023 - 12Z Wed Jan 11 2023 ...Significant heavy precipitation threats for California to renew this weekend and early next week... ...Guidance/Predictability Assessment... The overall upper level pattern over much of the continental U.S. will be a bit more complicated than usual, with multiple uncertain embedded shortwaves within relatively well defined northern and southern stream flows. The eastern Pacific will continue to be extremely active with with a series of deeepened low pressure systems approaching the West Coast with more atmospheric river events expected. The upper level ridge over the Intermountain West for most of the forecast period will tend to limit how far inland/diminshed these storm systems get into at least early next week. Accordingly, the WPC medium range product suite was primarily derived from a composite blend of mid-larger scale compatible guidance from the 18 UTC GFS/GEFS mean, 12 UTC ECMWF/Canadian/ECMWF ensemble mean, WPC continuity and the 01 UTC National Blend of Models (NBM). WPC preferred blend weighting shifts in favor of the more compatible ensemble means at longer time frames by Tuesday in an effort to best mitigate rampant smaller scale variances consistent with predictability. The latest 00 UTC guidance suite generally remains in line with this forecast plan. ...Weather/Hazard Highlights... The WPC experimental Day 4/5 Excessive Rainfall Outlooks (EROs) renews Saturday and Sunday "slight" risk areas centered over northern California. The threat for additional rounds of heavy rainfall with ample runoff issues that may expand southward are likely to persist through early next week in a dangerously wet pattern with multiple rivers of moisture working inland ahead of a series of uncertain/deepened eastern Pacific storms. This continues a pattern favorable to produce a major widespread episode. Many areas, especially burn scars, are likely to be sensitive to additional rainfall given the ample recent and forecast rainfall. High probabilities for Sierra snowfalls are also depicted in the WPC Winter Weather Outlook (WWO) Saturday-next Wednesday. Guidance also shows some favorable flow into the Northwest for especially southward facing terrain. There is also more modest shortwave/terrain enhanced snow potential to work into the Great Basin, perhaps especially early-mid next week with a better maintained system push inland across the region. Meanwhile, upper-level troughing moving through the West is forecast to reach the Plains and help develop a modest surface frontal wave to subsequently track with some uncertainty eastward across the Mid-Mississippi Valley to the Mid-Atlantic Saturday into Monday. Compared to the models, the NBM still seems to be missing some modest precipitation potential that may produce some light wintry weather for parts of the Ohio Valley through the Appalachians as well as a trailing area of rainfall down trough the south-central U.S. this weekend and modestly to the Southeast into early next week. WPC QPF and weather grids were accordingly adjusted. Guidance has trended heavier with activity across eastern TX and the lower MS Valley where some downpours may lead to local runoff issues. 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