571 FXUS02 KWBC 201902 PMDEPD Extended Forecast Discussion NWS Weather Prediction Center College Park MD 202 PM EST Sat Dec 20 2025 Valid 12Z Tue Dec 23 2025 - 12Z Sat Dec 27 2025 ...Major series of atmospheric rivers to fuel rounds of heavy rainfall to impact the West Coast, but especially California through the holidays... ...Overview... An amplified upper ridge will shift from the interior West/Rockies through the east-central U.S. next week and develop a south- central U.S. upper high. This will occur in response to a slow but steady eastward translation of a deepened upper trough from the eastern Pacific into the West Coast. In this impactful pattern, onshore flow will continue to direct leading atmospheric rivers (ARs) to fuel rounds of heavy precipitation affecting California in particular, but with widespread activity from the West Coast to the Intermountain West/Rockies. Temperature-wise, unseasonably to potentially record-breaking warm temperatures are forecast to spread next week from the West/Southwest through the south-central Rockies/Plains to the east-central U.S., though the northern tier states out through northern New England can expect cooler periods with some snow chances as shortwaves/frontal systems work through. ...Guidance/Predictability Assessment... Latest guidance continues to show good agreement on the large scale pattern through much of the period, but some uncertainty on the details. The most uncertain part of the forecast, and also the most impactful, is with upper troughing that deepens off the Pacific coast as it moves towards California and possibly onshore by late week. The models do agree that this will occur, but the details of energies within the trough and vort maxes towards the coast will create considerable impacts to sensible weather aspects. There continues to be a signal for a couple of ARs during the period and heavy rainfall shifting southward along the California coast, but there is some distribution/exact amounts uncertainty. Impactful timing differences emerge by Friday, with some recent model runs showing closed low development near California and issues with how quickly the energy moves inland. The ECMWF was a bit faster than consensus, but the GFS remains slow. The AI guidance is varied too, so prefer the ensemble means at this point which are closer to consensus and in the middle of the spread (though likely too weak). The WPC forecast for today used a general blend of the deterministic models Days 3 and 4, gradually increasing weighting of the ensemble means from day 5 and onward. This helps to mitigate some of the detail differences in the models and also to maintain continuity with the previous shift. ...Weather/Hazards Highlights... The biggest story during the period will focus out West with amplifying upper troughing and system genesis working toward the West Coast and persistent lead onshore flow directing Pacific moisture into the West Coast through next week. High moisture anomalies are expected with atmospheric rivers impacting northern/central California early in the week and eventually central/southern California through Christmas Eve/Christmas. Atmospheric rivers will fuel a continued wet period for especially California during the upcoming busy holiday week. The guidance signal is growing in support of a renewed deep moisture feed early- mid next week for northern-central California, with high rain rates (1+ inch per 3 hours) and areal averaged rainfall of 3"+ toward coastal areas and 5"+ towards the Sierra Nevada. A marginal risk with embedded slight risk is still highlighted across this region for the Day 4/Tuesday Excessive Rainfall Outlook, and upgrades are not out of the question as precipitation proves slow to shift from areas with already wet conditions due to short range rainfall. By about midweek and into mid-later next week, some connection to tropical Pacific moisture is likely to stream in and increasingly impact southern California as a surface low approaches the coast. Marginal and Slight Risk EROs are in place for Wednesday as well, stretching into southern California and parts of the southern Great Basin. Significant rainfall is expected, particularly for the Transverse Ranges of southern California. Higher elevations of the Sierra should see an extended heavy snow event. To the north, the Pacific Northwest can also expect organized precipitation into early next week, but with lesser amounts and intensities. Additionally, moisture is forecast to spill over the Cascades and into the Intermountain West and northern/central Rockies next week to produce heavy snow at higher elevations. Shallow shortwaves moving through the northern tier and associated surface system reflection focus will bring light precipitation across the north-central and northeastern U.S. through the early part of next week, including some light-moderate snows. Farther south, some weak to moderate Gulf moisture may stream into trailing frontal systems across the south and east-central U.S. and lead to light precipitation (mostly rain) chances across the Lower Mississippi, Tennessee, and Ohio Valleys for early to mid next week. Warmer than average temperatures are forecast across the interior West and central U.S. through much of the period. The highest anomalies will likely be across the interior West into the Plains, with widespread temperature anomalies upwards to 20-30 degrees above normal. Highs into the 80s across parts of the Southwest to Texas could set daily records. Unseasonable warmth is likely to spread east across the mid-latitudes to the Mississippi, Ohio, and Tennessee Valleys and the East over the Christmas holiday period. Santorelli/Schichtel 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 $$