Extended Forecast Discussion NWS Weather Prediction Center College Park MD 158 AM EST Thu Jan 30 2025 Valid 12Z Sun Feb 02 2025 - 12Z Thu Feb 6 2025 ***Prolonged atmospheric river event expected for the Pacific Northwest this weekend into next week*** ...General Overview... The overall upper level flow pattern across the continental U.S. is expected to be quasi-zonal for the upcoming weekend, while at the surface an arctic airmass oozes southward across the northern Plains and northern Rockies, but its southward progress should be limited by a building upper ridge over the south-central states. A slow moving upper level low situated northwest of the Washington Coast, combined with a fast mid-upper level flow and shortwave passages, will sustain a prolonged atmospheric river event across northern California and western Oregon with several inches of rainfall expected. Meanwhile, broad cyclonic flow aloft over the Great Lakes into the Northeast should be in place for the first half of next week. ...Guidance/Predictability Assessment... There is good overall synoptic scale agreement in the 00Z guidance among the deterministic solutions to begin the forecast period Sunday. There are still some timing and amplitude differences seen in the guidance with northern stream shortwaves and the arrival of the arctic front over the Northern Plains, but the overall trend has been for the arctic front to make less southward progress compared to previous days. At the time of fronts/pressures preparation, the 18Z GFS was farther south with the cold front across the Plains and Ohio Valley, whereas the CMC/ECMWF/UKMET were farther north and had more support from the AI guidance, so the forecast was hedged more in the ECMWF/CMC/UKMET direction through Tuesday. Looking ahead to the middle of next week, the 00Z CMC is quicker to drop the upper trough southward across the West Coast/Intermountain West compared to the other guidance. The use of the ensemble means increased to about half by Thursday to account for increasing model differences by this time. ...Weather/Hazards Highlights... The most impactful weather event during this forecast period will be the prolonged atmospheric river that will bring extended periods of moderate to heavy rainfall for central and especially northern California. An anomalous moisture flux oriented nearly orthogonal to the terrain ahead of a slow moving front is likely to produce widespread 2-4 inch rainfall totals during the Day 4 period Sunday, and therefore a Slight Risk of excessive rainfall is planned between Santa Rosa and Eureka. This continues going into Monday (Day 5) with heavy rainfall over the coast and adjacent terrain, and also the foothills of the central/northern Sierra where a Slight Risk is planned, with an additional 2 to 3 inches of rain during this time. Additional heavy rainfall is likely across the state going into the middle of next week as the moisture moves very slowly southward. Snow levels will initially be rather high with this event over California, but they should fall a little towards mid-week with heavy snow becoming more likely for the higher terrain of the Sierra. Elsewhere across the country, heavy snow is expected for the higher terrain of the Intermountain West and the northern Rockies as the Pacific moisture advects farther inland, with a couple feet of accumulation likely for the highest elevations over the medium range period. Light to moderate snow is likely from the northern Great Lakes to northern New England on Sunday into Sunday night as a surface low passes through the region. Temperatures are expected to be pleasantly mild for early February across much of the continental U.S., and highs reaching 15 to 25 degrees above normal are possible from the southern Plains to the Midwest states for this weekend. It should be close to seasonal averages for New England on Sunday, and the arctic front starts to bring much colder weather starting in Montana on Sunday, and then across the Dakotas and into Minnesota going into early next week, but is probably going to be halted in its southward progress by the time it reaches the central Plains due to a developing ridge across the southern Plains by midweek. Above average readings are forecast for the southern U.S. through the entire period. Hamrick 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 (QPF), excessive rainfall outlook (ERO), winter weather outlook (WWO) probabilities, heat indices, and Key Messages can be accessed from: 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