Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 PM EST Sat Dec 24 2022 Valid 12Z Tue Dec 27 2022 - 12Z Sat Dec 31 2022 ...A strong atmospheric river could cause notable heavy rain/flooding for the West Coast along with heavy mountain snow in the West next week... ...Overview... Guidance remains agreeable and consistent in showing the establishment of a new long-term mean pattern aloft, with dynamics from a strong storm reaching Washington/Vancouver Island on Tuesday being the first of multiple pieces of energy to feed into a developing Rockies/Plains mean trough while a mean ridge builds over the East Coast/western Atlantic after midweek in the wake of a departing shortwave. Flow ahead of the Tuesday storm will bring a strong atmospheric river with a long fetch of tropical moisture into the West during the first half of next week. This event will likely produce heavy rain, potentially causing flooding especially for coastal regions of southern Oregon and California, along with heavy snow possible in higher elevations of the West. Trailing Pacific frontal systems and possible waves will likely bring additional rounds of precipitation into the West through late next week. Farther east, the new mean pattern will promote multiple days of deep-layer southerly flow from the Gulf, spreading precipitation over an increasing portion of the east-central U.S. late in the week with best potential for heavy rainfall over the Gulf Coast/Lower Mississippi Valley region. Meanwhile the pattern evolution will bring a pronounced warming trend to above normal temperatures across the central-eastern U.S., leading to above normal readings over many areas after midweek. ...Guidance/Predictability Assessment... As has been the case lately, the models and ensemble means agree well for the large scale mean pattern but continue to exhibit meaningful detail differences for individual features from the eastern Pacific, ultimately feeding into the Rockies/Plains mean trough. Guidance has also had difficulty in resolving the specifics of flow aloft and surface features across the eastern half of southern Canada with some influence on northern parts of the lower 48. A model consensus approach early in the period followed by a transition toward a model/mean mix later in the forecast maintains good continuity for the mean pattern and some individual features, though latest runs produce some unavoidable adjustments for specifics from the Plains into southern Canada (with future changes likely as well). Regarding the rapid sequence of eastern Pacific systems, the spread has narrowed a bit for the Tuesday storm reaching the Pacific Northwest/Vancouver Island with most solutions now in the low 960s-low 970s mb. The UKMET remains the weakest solution, while the past couple ECMWF runs are strongest but not as deep as the 12Z/23 run that was in the 950s. Guidance is showing signals for a possible wave along a warm front approaching the coast by early Thursday, but with a lot of spread for track/strength of the wave. The 00Z ECMWF/CMC were similar but stronger than other guidance and not surprisingly the new runs differ with the details. At least modest waviness fits within the theme of the ensemble means, so the blended approach that yielded such a wave provided a good starting point. Then by Friday another feature may approach the coast, with the new 12Z ECMWF/UKMET developing a fairly strong system but remaining guidance depicting a mere front with possible triple point. The favored blend leaned to the more conservative latter scenario for the time being. Yet another feature may approach on Saturday, with decent model/mean clustering in principle but again a lot of spread for details. Overall confidence is fairly low for exact details within the broad area of mean low pressure expected to develop over the central U.S. during the latter half of the week, with specifics becoming increasingly sensitive to shortwave details that have low predictability several days out in time. This lack of confidence extends to the pattern along and either side of the eastern half of the Canadian border. However confidence is much higher for the general pattern of increasing flow from the Gulf between Plains low pressure and East Coast/Atlantic high pressure from Wednesday onward. ...Weather/Hazard Highlights... A series of Pacific fronts/systems should bring a multi-day period of precipitation to much of the West. A strong to extreme atmospheric river with a moisture fetch all the way from the western Pacific should be in progress over the West at the start of the period early Tuesday and sink southward into midweek. This event will likely produce high rainfall rates and amounts particularly along the southern Oregon and California coasts with heavy rain/snow along the favored terrain of the Sierra Nevada as well. A Slight Risk of excessive rainfall has been maintained in WPC's experimental Day 4 Excessive Rainfall Outlook due to flooding concerns with this rain. High winds are also possible across portions of the West Coast and Great Basin--especially with the strong storm tracking into Washington or Vancouver Island on Tuesday. By Day 5/Wednesday, locally heavy rainfall could become hazardous for parts of southern California into Arizona as the moisture axis shifts south. Guidance suggests a rapid succession of additional Pacific fronts/systems may bring significant rain and mountain snow to portions of the West through the latter half of the week into the weekend, but with low confidence for the evolution or track for each system. These systems are not expected to be as intense as the initial one, but could still produce significant precipitation over some areas that are hydrologically sensitive so monitor forecasts for specifics as the details come into better focus over coming days. East of the Rockies, most areas should be generally dry Tuesday-Wednesday other than some light precipitation chances across the northern tier. By the latter half of the week, moisture inflow from the Gulf of Mexico is forecast to return to the Mississippi Valley ahead of one or more Plains systems and supporting upper trough. Within this expanding moisture shield, rainfall could become locally moderate to heavy by Friday-Saturday with highest rainfall totals currently expected to be approximately from the Gulf Coast through the Lower Mississippi Valley and into the Tennessee Valley. Temperatures across the eastern half of the lower 48 will stay 10-15F below average on Tuesday (perhaps locally colder for lows in the Midwest) but gradually moderate toward normal by around midweek. Above normal temperatures by 15-25F for lows and 10-20F in terms of highs over the West/High Plains Tuesday should migrate into the central and eventually the eastern U.S. by late week as the upper ridge reaches the East Coast. The wet/snowy pattern over the West will tend to keep daytime temperatures within a few degrees of normal from Wednesday onward while morning lows remain 5-15F above normal by late in the week. Rausch/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, experimental excessive rainfall outlook, winter weather outlook probabilities and heat indices are at: 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