Extended Forecast Discussion NWS Weather Prediction Center College Park MD 221 AM 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... Troughing moving into the West as well as a deep surface low and frontal system will bring a strong atmospheric river with a long fetch of tropical moisture into the region around Monday-Tuesday, with 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, there could be lingering lake effect snows downwind of the Great Lakes particularly Monday, while below normal temperatures gradually moderate closer to and then above normal for the central and eastern U.S. as next week progresses. Generally dry conditions are likely until moisture increases across the Mississippi Valley for the latter part of next week. ...Guidance/Predictability Assessment... Model guidance shows reasonably good agreement for a fairly progressive pattern during the medium range period as shortwaves move through an eastern trough that looks to eject east into the Atlantic on Tuesday while in the West, troughing will deepen and push toward the central U.S. by the end of the period Friday with ridging building ahead of it. In both the East and West, minor shortwave differences were generally handled by a multi-model blend of deterministic guidance through the first half of the period. Some slight increases in model spread occur toward later next week--for example, GFS runs tend to be slower with the western trough moving east due to some stream separation compared to more phased other guidance--and increasing proportions of the GEFS and EC ensemble means were used for days 6-7, but still with a majority of deterministic models given the fairly good model clustering. This led to good model continuity with the previous forecast as well. ...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 added to WPC's experimental Days 4-5 EROs 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. Rounds of precipitation are likely to continue in the West through much of next week though at this point look to be not 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 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. 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 the eastern U.S. underneath the strengthening upper ridge. 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