Extended Forecast Discussion NWS Weather Prediction Center College Park MD 158 AM EST Wed Jan 29 2025 Valid 12Z Sat Feb 01 2025 - 12Z Wed Feb 05 2025 ***Prolonged atmospheric river event expected for the Pacific Northwest this weekend*** ...General Overview... The overall upper level flow pattern across the continental U.S. is expected to become more quasi-zonal going into the weekend, while at the surface an arctic airmass edges southward across the northern Plains and northern Rockies. A slow moving upper level low situated northwest of the Washington Coast, combined with a fast mid-upper level flow and shortwave passages, will likely sustain a prolonged atmospheric river event across northern California and western Oregon. Meanwhile, broad cyclonic flow aloft over the Great Lakes into the Northeast should be in place for the first half of next week, and a broad upper ridge trying to develop over the Gulf Coast region. ...Guidance/Predictability Assessment... There is good overall synoptic scale agreement in the 00Z guidance among the deterministic solutions to begin the forecast period Saturday. There are still some timing and amplitude differences seen in the guidance with northern stream shortwave and the arrival of the arctic front over the Northern Plains, and more so in the CMC and UKMET guidance. Therefore, the model preference through the upcoming weekend was hedged more in the direction of the ECMWF/GFS and some of their respective ensemble means. Looking ahead to early next week, the upper level flow pattern becomes more zonal across the continental U.S. overall. The guidance has come into better agreement with the closed low near the Pacific Northwest. The use of the ensemble means increased to about half by Wednesday, while still keeping about 25% each of the GFS and ECMWF. ...Weather/Hazards Highlights... The most impactful weather event during this forecast period will be the prolonged atmospheric river that is likely to bring extended periods of moderate to heavy rainfall from southwestern Oregon into the northern third of California. An anomalous moisture flux oriented nearly orthogonal to the terrain ahead of a stationary front is likely to produce widespread 2-4 inch rainfall totals between Santa Rosa and the Oregon state line during the Day 4 period Saturday, and therefore a Slight Risk of excessive rainfall is planned. There should be some reduction in rainfall intensity going into Sunday across northern California, but there should still be enough moisture and forcing to produce additional 1 to locally 3 inch totals for the Day 5 Sunday, and therefore a Marginal Risk is planned for northern California and including the foothills of the central and northern Sierra. Additional heavy rainfall is likely across the state going into early next week as the moisture moves very slowly southward. 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 several feet of accumulation likely for the highest elevations over the 5 day period. Lighter snow is likely from the northern Great Lakes to northern New England as a surface low passes through the region Monday with some lake effect snow behind it. Temperatures are expected to be 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 the Mid- Atlantic and slightly below average for New England, and the arctic front starts to bring much colder weather starting in Montana on Sunday. The arctic airmass oozes southeastward 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