Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 PM 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 into next week*** ...General Overview... The overall upper level flow pattern through the medium range period is expected to be dominated by a developing Rex block over western North America, fast quasi-zonal flow across much of the Continental U.S., and deep troughing over central and eastern Canada. A slow moving, upper level, cut-off low situated northwest of the Washington Coast, combined with the fast mid-upper level flow downstream, will sustain a prolonged atmospheric river event across northern California and southwestern Oregon. Meanwhile, a cold front in between broad cyclonic flow aloft over the Great Lakes and Northeast and a developing ridge over the Bahamas and the far southeast U.S. will separate cold Arctic air to the north and above normal temperatures to the south through the middle of next week. ...Guidance/Predictability Assessment... Overall, there remains good synoptic scale agreement within the 00Z/06Z/12Z deterministic guidance to begin the forecast period Saturday. This is particularly the case for the strength and position of the cut-off upper low northwest of Washington this weekend into early next week. Elsewhere, as the forecast progresses into next week, differences in timing and amplitude of several shortwaves within the fast mid and upper level flow along the U.S./Canada border continue to result in uncertainty, particularly with regards to the development of a northern stream mid-latitude cyclone and its attendant Arctic cold front. Combined with differences in strength and amplitude of a ridge over the Bahamas and the far Southeast U.S., questions remain with how fast and how far south the Arctic front will progress from the northern Plains into the central and eastern U.S. Similar to the last forecast package, blended model preferences skewed more to the ECMWF, the GFS, and their respective ensemble means for the latter half of the period. ...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 3-6 inch rainfall totals between Santa Rosa and the Oregon state line during the Day 4 period Saturday. Given good model agreement outside of the GFS, the inherited Slight Risk of excessive rainfall was not only maintained, but expanded farther south along the northern California coast and farther east into the higher terrain. The risk of heavy rain continues through Day 5 on Sunday with reasonable model agreement so decided to leave the inherited Marginal Risk for northern California and the foothills of the central and northern Sierra as is. Additional heavy rainfall is likely across the state through the middle of 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. Miller/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