Extended Forecast Discussion NWS Weather Prediction Center College Park MD 148 AM EST Wed Jan 11 2023 Valid 12Z Sat Jan 14 2023 - 12Z Wed Jan 18 2023 ...Unsettled and Active Pattern Continues for Western U.S. with focus on California Atmospheric Rivers... ...Overview and Guidance/Predictability Assessment... The latest guidance continues to show good agreement on the overall flow pattern across the U.S. staying fairly amplified during the medium range period (Saturday-Wednesday). This features an initial trough exiting the East Coast this weekend with secondary low pressure development expected to be well off the coast with little impact to parts of the Northeast. Out West, the parade of upper shortwaves into the West will continue with these systems expected to move inland across the Southwest with likely downstream cyclogenesis across the central Plains. This deepened low should track generally into the Midwest and then the Great Lakes early next week. The next system into the West, possibly more amplified than the first, is also expected to spawn another area of low pressure in the central Plains by next Wednesday. Most of the guidance differences right now reside in the timing and details of these individual shortwaves into the West Coast, and continue to show a lot of run to run variability, lending to low confidence forecasts. There is good agreement with the first shortwave into California this weekend, but questions as it moves downstream on strength. The next system moves into the West around Monday, with greater questions on placement and timing of low pressure along the coast and inland. Through the 12z/18z guidance (yesterday...Jan 10), the ECMWF was the most amplified coming inland and consequently farther south with the surface low. The GFS and CMC, along with the ensemble means, suggest this system is displaced a little farther north. These kinds of differences do have big implications for late period rainfall into the Northwest and California. Lots of timing questions as the shortwave progresses into the Southwest/southern Plains next Tuesday-Wednesday, as well as a lot of uncertainty surrounding another possible shortwave towards the Northwest next Wednesday. A quick look at the new 00z guidance (available after forecast generation time) shows generally the same pattern/issues as described above and provide little in the way of any kind of trends. The WPC forecast used a equal blend of the deterministic solutions through day 5/Monday which helped to smooth out some of the smaller scale details. For days 6 and 7, trended more towards the mostly agreeable ensemble means to help mitigate some of the larger scale differences. This approach maintained good continuity with the previous WPC forecast as well. ...Weather/Hazard Highlights... Unsettled and active weather is expected to continue across portions of the West as a series of atmospheric rivers move onshore. The first in this series of extended range storms will arrive this weekend, and is expected to bring another round of widespread heavy rain across northern and coastal California with heavy snowfall into the Sierra Mountains as well. Worsening flooding or delayed improvement of ongoing issues is possible across this very over-saturated coastal central to southern California. As the energy moves inland on Sunday, rain and higher elevation snows are possible in the Southwest, and eventually return flow out of the Gulf of Mexico may increase precipitation chances into the Plains/Midwest as well. An almost constant fetch of onshore moisture into the Northwest and parts of the California through the period will keep rainfall, though less intense, in the forecast early next week before another possible heavier storm arrives around next Tuesday. In the East, precipitation ahead of the trough should be well off the coast by this weekend, except for some possibly light wrap back snows across northern New England which is dependent on proximity of secondary low development to the coast. Above normal temperatures will move east with time from the Central U.S. this weekend into the Midwest and eventually the East early next week. The greatest anomalies should focus across the Deep South into the Midwest with daytime highs 10 to 20 degrees F above normal. Even more impressive will be the very warm nighttime lows, with readings 20 to 25 degrees F above normal generally across the same region from Sunday onward. Daytime highs across the West and Southwest should be near or below normal amidst plenty of clouds and rainy conditions. Santorelli 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