990 FXUS02 KWBC 150756 PMDEPD Extended Forecast Discussion NWS Weather Prediction Center College Park MD 256 AM EST Mon Dec 15 2025 Valid 12Z Thu Dec 18 2025 - 12Z Mon Dec 22 2025 ...Another significant atmospheric river will impact the Pacific Northwest, particularly on Thursday... ...Overview... As the medium range period begins Thursday, an upper trough will be present atop the north-central U.S. accompanied by a surface low pressure system, spreading some precipitation to the eastern third of the lower 48. After that trough pivots east into Friday, quasi- zonal flow will again set up across the country with generally fast-moving and shallow shortwaves. Upstream, strong westerly flow will force another atmospheric river (AR) into the Northwest/northern California on Thursday. Additional flooding is possible especially in areas that have seen excessive rainfall recently, and heavy snow is likely in the higher terrain. This AR should drift southward on Friday but additional rounds of precipitation are possible in the Northwest into the weekend and early next week. Temperature-wise, the flat flow will allow for above average temperatures for most of the CONUS as astronomical winter begins, though the northern tier can expect colder periods with shortwaves moving through. ...Guidance/Predictability Assessment... Model guidance shows minor spread with the initial trough in the north-central U.S. on Thursday moving eastward into late week and its accompanying surface low. The 12Z and new 00Z ECMWF are a tad slow compared to consensus with the low initially, and UKMET runs become slow to pivot the trough axis eastward. Still, a multi-model blend including the ECMWF/GFS/CMC runs worked well for this feature. Deterministic and AI models also show reasonable agreement regarding Thursday's AR moving into the Pacific Northwest, focusing in Oregon in particular. Since even relatively small model differences in the orientation and placement could be impactful, will continue to monitor. Behind this AR, the flat flow with shallow troughs/ridges makes it hard to determine model outliers. For the 12/18Z cycle, some differences did arise regarding potential weak troughing across the northern tier over the weekend. The ECMWF and GFS (especially the 12Z) were supporting slightly deeper troughing across the northern Rockies/Plains Saturday moving into the Great Lakes by Sunday, but their AI counterparts and the CMC were more zonal. The CMC even ended up out of phase with minor ridging over the Great Lakes on Sunday. Fortunately it appears the 00Z guidance is converging better, showing a shallower solution for this trough with more agreeable timing across the models. Then, northeast Pacific energy may approach the Northwest once again early next week. The 00Z GFS seems to be faster/deeper with this energy entering the Northwest compared to other guidance including the AI-GFS. The WPC forecast increased the proportion of ensemble means as the period progressed to account for these model differences, with means comprising half the blend Day 6 and more Day 7 as model spread increased for the details of the low amplitude pattern. ...Weather/Hazards Highlights... The Pacific Northwest can expect heavy precipitation on Thursday as a moderate to strong AR impacts the region. Forecast guidance is in reasonably good agreement that the AR will be directed primarily at Oregon, with global models indicating 5-7+" of QPF given the strong IVT. Thus a Slight Risk remains in place for the Day 4/Thursday ERO across western Oregon into far southern Washington. The exact positioning of the AR will make a difference in terms of how much overlap there is with the previous couple of extreme AR/rainfall events and thus the sensitivity of the antecedent conditions; the farther north the AR ends up, the more overlap. On Friday, the AR is forecast to gradually move southward into northern California with the IVT starting to weaken. A Marginal Risk is in place across portions of western Oregon and northern California for Day 5/Friday. Meanwhile, heavy snow is likely into the Cascades and into the northern Rockies with the ample moisture combining with sufficiently cold temperatures. Lingering precipitation is possible into Saturday across the West Coast states and interior West, though with lesser amounts. But moisture surges are once again possible on Sunday for additional heavy precipitation, though it will take additional time to refine the details. The upper trough moving from the north-central U.S. into the Great Lakes and through the Northeast Thursday and Friday will provide forcing for a Midwest to southeast Canada surface low pressure/frontal system. Some moisture will be pulled north from the Gulf ahead of the cold front, allowing for broadening precipitation chances across the eastern third of the U.S. Thursday. Most of this precipitation is likely to be rain except in the Great Lakes to interior Northeast. Most precipitation should be pulling away on Friday, but with some lake effect precipitation possible behind the main system. Light to moderate precipitation (mostly rain) may get renewed across the east-central U.S. over the weekend, though with the details uncertain. Warmer than average temperatures are forecast across the western and central U.S. through much of the period. The highest anomalies will likely be across the Plains most days, with temperatures generally 15-25 degrees above normal, locally even higher. On Friday highs in the 60s should expand into western Nebraska and South Dakota, and by Saturday temperatures in much of Texas should reach the upper 70s to low 80s. The eastern U.S. can expect more transient above average temperatures around Thursday and perhaps again into next weekend, but with periods of near to below normal temperatures in between. Meanwhile the northern tier is forecast to see shots of cold air with shortwaves and cold surface highs behind cold fronts. 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 $$