Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 PM EST Fri Dec 23 2022 Valid 12Z Mon Dec 26 2022 - 12Z Fri Dec 30 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... The models and ensemble means have been consistently showing a significant upper level pattern change over the course of next week. The eastern Pacific trough/western U.S. ridge/eastern trough configuration at the start of the week should move eastward with time, with a mean trough ultimately settling over the Rockies/High Plains while a ridge builds over the East Coast. Vigorous systems ahead of the Pacific trough, and in particular a storm reaching the Washington coast or Vancouver Island by Tuesday and its trailing front, 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. Additional rounds of precipitation are likely across much of the West through late next week. Meanwhile farther east, the pattern change will bring an end to lake effect snow that lingers into Monday, while flipping temperatures over the eastern half of the country from well below normal on Monday to above normal after midweek. Moisture should also begin to flow northward through the Mississippi Valley by late in the week. ...Guidance/Predictability Assessment... The updated forecast maintained the approach of using an operational model composite for about the first half of the period followed by a transition to a combination of operational models and ensemble means. This provided good continuity for most aspects of the forecast. There are some notable shortwave differences that develop within the Rockies/Plains mean trough by Thursday-Friday (latest GFS runs leaning slow/amplified, CMC on the faster side), leading to some bouncing around for most likely positions for waves along a mean frontal boundary, while a fair amount of spread also arises for specifics of southeastern Canada/New England flow along with associated surface details. It will likely take some time to resolve the details over those areas during the latter half of the week. In general the 00Z GFS compared a little better to other guidance than the 06Z run during most of the period, so the former run provided the GFS component in the blend. On Tuesday a guidance average looks reasonable given the ongoing detail/track differences (most of which are within typical spread/errors for four days out in time) for the strong storm reaching affecting the Pacific Northwest. For this feature, the new 12Z runs have widened the depth spread somewhat valid 12Z Tuesday, with the ECMWF now in the 950's mb versus the UKMET being in the 970's while other models are generally in the 960's. ...Weather/Hazard Highlights... A series of northeastern Pacific 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 is likely to affect the West and peak Monday-Tuesday to produce considerably high rainfall rates and amounts particularly along the Oregon and California coasts. Slight Risks of excessive rainfall have been maintained in WPC's experimental Days 4-5 Excessive Rainfall Outlooks due to flooding concerns with this rain. Farther inland in higher elevations, this could also lead to heavy snow in the Cascades/Sierra Nevada into the Rockies. High winds are also possible across portions of the West Coast and Great Basin--especially with the strong storm tracking into or near the Pacific Northwest Tuesday. Rounds of precipitation are likely to continue in the West through the rest of next week with one or more additional frontal systems, but this activity should not be as intense as early in the week. Elsewhere, lingering lake effect snow is a possibility through Monday and possibly Tuesday in overall westerly flow, while a quick clipper could cause light snow across parts of the Middle Mississippi Valley/Ohio Valley on Monday. Conditions should remain generally dry across the central and eastern U.S. other than that, until the latter half of the week when moisture inflow from the Gulf of Mexico returns to the Mississippi Valley ahead of a cold front and upper trough. Within this expanding moisture shield, rainfall could become locally moderate to heavy by Friday. Temperatures across the eastern half of the lower 48 will stay 10-20F below average on Monday 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 Monday-Tuesday should migrate into the central and eventually most of the eastern U.S. as the upper ridge reaches the East Coast. 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