Extended Forecast Discussion NWS Weather Prediction Center College Park MD 216 AM 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... 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 lasting into the beginning of the period Tuesday 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 flip temperatures from the Plains eastward from below normal to above as the week progresses. Additionally, moisture should begin to flow northward through the Mississippi Valley and increase precipitation chances by late in the week. ...Guidance/Predictability Assessment... Model guidance begins the medium range period Tuesday with reasonably good agreement with the overall pattern of upper troughing just offshore the West Coast, downstream ridging, and eastern U.S. troughing. There were some slight differences with the exact trough axis/shortwave differences in the East, with the 12Z GFS on the faster side and the 12Z CMC on the slower side, but with better agreement in the 18Z GFS/12Z ECMWF and nothing a multi-model deterministic blend could not handle. The incoming 00Z guidance appears to be better aligned with these details, and the trough quickly exits the region by day 4/Wednesday anyway. In the West, model differences remained most notable in the 12Z runs with the depth of the surface low pressure system moving toward the Pacific Northwest, with the ECMWF in the 950s mb versus the UKMET being in the 970s while other models were generally in the 960s. 00Z runs are mainly in the 960s for a bit better agreement, though the UKMET is in the low 970s. Again a multi-model deterministic blend seemed to work well early in the period to produce a reasonable consensus. As the period progresses, the western upper trough is the main concern as it moves east and slows. CMC and ECMWF runs have been showing multiple shortwaves moving through the overall trough while UKMET and GFS runs show a particular shortwave or two digging around day 5/Thursday to produce a slightly deeper and perhaps slower trough that is also indicated by many GEFS members. All are within normal spread for the forecast timeframe but the differences do lead to uncertainties with frontal positions especially in the central U.S. The WPC forecast gradually incorporated and increased the proportions of the GEFS and EC ensemble means through the period to about half given the increasing model differences but that are still within normal medium range spread. ...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 southern Oregon and California coasts. 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 or near the Pacific Northwest Tuesday. By Day 5/Wednesday, locally heavy rainfall could become hazardous for parts of southern California into Arizona as the moisture axis shifts south. Some model guidance shows another round of rain could make it into southwestern Oregon/northwestern California as well by Wednesday over areas that would be hydrologically sensitive. While there is good agreement for rounds of precipitation to continue in the West through the rest of next week with one or more additional frontal systems, timing of each precipitation event does vary after the first strong atmospheric river. The subsequent precipitation events are not expected to be as intense as the initial one. Regardless, farther inland in higher elevations, heavy snow is a threat in the Cascades/Sierra Nevada into the Rockies in these multiple rounds of precipitation next week. 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 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 10-15F 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. Tate/Rausch 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