Extended Forecast Discussion NWS Weather Prediction Center College Park MD 155 AM EST Fri Jan 06 2023 Valid 12Z Mon Jan 09 2023 - 12Z Fri Jan 13 2023 ...Heavy rain and mountain snow threats for California to renew again next week... ...Guidance/Predictability Assessment... The weather pattern over much of the continental U.S. remains quite active through next week, with multiple embedded shortwaves aloft within relatively well defined northern and southern stream flows. The highlight continues to be a very active eastern Pacific with a couple of deepened low pressure systems expected to impact the West Coast with lead atmospheric river moisture allowing for both significant and impactful QPF amounts. There is good agreement for the first system (Monday-Tuesday) but more uncertainty in especially the timing of another system later in the week. These western U.S. shortwaves should progress inland through the week, amplifying east of the Rockies and eventually off the Southeast coast. Good enough agreement across the board through day 5 to utilize a blend of the latest 12z/18z deterministic guidance for the latest WPC forecast. Guidance continues to trend stronger/more amplified for a shortwave across the south-central U.S. late week, with increasing QPF trends across the Deep South. There are some notable differences in track though as the 12z ECMWF is farther south with the upper low than the GFS. The 12z CMC from Jan 5 was slower than these pieces of guidance and the ensemble means and was not favored for this system, but did note the 00z CMC (available after forecast generation time) did come in quicker and more in line with the other guidance. The WPC forecast trended towards a majority ECENS/GEFS ensemble mean blend for days 6 and 7 amidst growing forecast spread. Overall, the forecast remained fairly consistent with the previous shift. ...Weather/Hazard Highlights... The big weather story through next week will remain out West as yet more atmospheric rivers take aim at an already over saturated California. The WPC experimental Day 4 Excessive Rainfall Outlook (ERO) continues a broad slight with embedded moderate risks across much of northern and central California on Monday, where rainfall amounts of several inches are expected. Many areas are already or will be very sensitive to additional rainfall given the ample recent and shorter term forecast rainfall. Snow levels look fairly high with this event, but still the higher terrain (generally above 8000') of the Sierras likely will see some very heavy snowfall measured in feet Monday into Tuesday. After this, the next system looks to impact northern California again by Wednesday-Thursday with again impactful and possibly heavy rainfall. The initial system in California should track east during the middle of next week with guidance continuing to favor modest snow accumulations over the parts of the Great Basin and the Rockies. Then, it has potential to spawn moderate central Plains cyclogenesis later next week where potential lead return Gulf inflow into the MS Valley and vicinity may offer an opportunity for an expanding area of late period rainfall across portions of the Deep South/Tennessee Valley. There remains some signal, though low confidence still, for snow and ice potential on the northern periphery of the precipitation shield as the moisture feeds into a cooled antecedent high pressure airmass that may dig in advance of this system front Canada into the north-central through Eastern U.S. Elsewhere, a leading short wave and frontal boundary through the Southeast should allow for some light to moderate rainfall as it passes through. The Northwest should see some precipitation as well associated with the various east Pacific systems, though not nearly as impactful as farther south into California. Temperatures across the south-central Plains to Midwest/Southeast should stay warm through next week, with anomalies 10-15 degrees above normal possible. 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, 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