683 FXUS02 KWBC 170736 PMDEPD Extended Forecast Discussion NWS Weather Prediction Center College Park MD 236 AM EST Wed Dec 17 2025 Valid 12Z Sat Dec 20 2025 - 12Z Wed Dec 24 2025 ...Additional rounds of heavy precipitation likely for the West Coast... ...Overview... A shallow upper trough is likely to move across the central to eastern U.S. over the weekend within relatively flat flow. Upstream, low amplitude ridging is forecast to move through the interior West and the central U.S. early next week, strengthening into the middle of next week with a south-central U.S. upper high setting up in response to deepening eastern Pacific troughing. The mean westerly and later southwesterly flow into the West Coast will continue to bring rounds of atmospheric rivers (ARs) causing heavy precipitation to northern California in particular, with some precipitation pushing into the Intermountain West/Rockies as well. Temperature-wise, unseasonably to record-breaking warm temperatures are forecast across the Southwest to Plains, though the northern tier can expect colder periods with shortwaves moving through. ...Guidance/Predictability Assessment... Model guidance remains in good agreement on the large scale with the pattern described above. CMC runs have shown a shallower solution compared to consensus with the initial trough moving through the northern tier, but the newest 00Z run aligns better with other guidance. Then there is good agreement for ridging to build across the Rockies to Plains while northeast Pacific troughing deepens. This will provide mean southwesterly flow into California and the Northwest for additional AR potential. The details of the exact axis/orientation/placement for these ARs will be impactful, but are still uncertain. Overall, a multi-model blend worked well for the early part of the period, and added some ensemble mean guidance to the blend for the latter part of the period to temper individual model differences. ...Weather/Hazards Highlights... Moist inflow is forecast to affect northern California as the medium range period begins Saturday, especially by late in the period as another AR approaches. Continue to show a Marginal Risk for isolated flooding in the Day 4 ERO. By Sunday the next AR is forecast to arrive in earnest across northern California with IVT increasing. Rain rates are forecast to increase with the moisture surge, with global models showing pockets of 3 hour rain amounts of over an inch. This potential combined with this being the third day of moderate to heavy precipitation in a row allows for a Slight Risk issuance for northern California in the new Day 5/Sunday ERO. Some precipitation is also possible farther north into the Pacific Northwest, but with lesser amounts. Additional rounds of moderate to heavy precipitation are likely into Monday-Tuesday as well. For these events, moisture is forecast to spill over into the Intermountain West and northern/central Rockies, which could produce heavy snow in higher elevation areas with sufficiently cold temperatures. Shallow shortwaves moving through the northern tier could bring light precipitation across the north-central and northeastern U.S. through the weekend and into early next week, including some light snow. Farther south, some weak to moderate Gulf moisture may stream into frontal systems across the south and east-central U.S. and lead to precipitation (mostly rain) chances across the Lower Mississippi, Tennessee, and Ohio Valleys over the weekend into early next week. Warmer than average temperatures are forecast across the interior western and central U.S. through much of the period. The highest anomalies will likely be across the interior West into the Plains most days, with temperatures as much as 20-30 degrees above normal. Highs in the 70s and 80s across the Southwest to Texas could set daily records. Meanwhile, the northern tier is forecast to see shots of cold air with shortwaves and cold surface highs behind cold fronts. Below zero lows are possible in portions of North Dakota and Minnesota, while highs could be periodically below average as well. The eastern U.S. can expect more transient above and below average temperatures as fronts pass through. Tate 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 $$