Extended Forecast Discussion NWS Weather Prediction Center College Park MD 155 AM EST Thu Jan 12 2023 Valid 12Z Sun Jan 15 2023 - 12Z Thu Jan 19 2023 ...Unsettled and Active Pattern Continues for Western U.S. with focus on California Atmospheric Rivers... ...Overview and Guidance/Predictability Assessment... The first in a series of shortwaves into the West Coast this period (Sunday-Thursday) will move onshore on Sunday, and quickly move into the Central U.S. by Monday. Modest surface low development is expected with this system as it moves into the Midwest and eventually Great Lakes. Some minor timing differences, but models generally show good agreement with this system allowing for a general model blend for tonights manual forecast progs. Quickly on its heels, another shortwave moves into Southern California around Monday-Tuesday next week this time with plenty more uncertainty. The UKMET and ECMWF were farthest south with the system into California with the GFS and CMC noticeably north. This northern idea seemed better represented by the ensemble means as well. The UKMET, being the farthest south, was not used in tonights blend. As the system proceeds into the Southwest and the Central U.S. early next week, it's likely to spawn an additional modestly deepening low pressure system which tracks into the Midwest next Thursday. There is more uncertainty with respect to amplitude of the shortwave into the central U.S., which obviously has implications for system strength mid to late next week. The WPC progs favored a blend closer to the (expectedly weaker) ensemble means to mitigate the differences, but did incorporate 50 percent total of the stronger ECMWF and less amplified GFS for a little bit of added system definition. A look at the incoming 00z guidance (available after forecast generation time) shows the GFS has trended towards the stronger 18z (yesterday) and todays 00z ECMWF with a closed low over the Mid-Mississippi Valley next Thursday, although the 00z CMC still shows a weaker shortwave. This possible trend towards a stronger solution will need to be monitored over the coming days. Yet another shortwave looks to target the Pacific Northwest around Wednesday next week, but there is again a lot of timing and strength differences with this, so a blend towards the better agreeable ensemble means was reasonable for now for this late period system. ...Weather/Hazard Highlights... The West will remain active as a series of shortwaves continues to direct unneeded additional heavy rainfall and mountain snows towards particularly California, but also the Pacific Northwest. The first in this series of extended range storms will arrive on Saturday just before the start of this forecast period, and the heaviest of the precipitation should be over by Sunday as the overall upper system moves inland. Still, any lingering rainfall over an already very over-saturated California may continue to worsen flooding or delay improvement of ongoing issues. Rain and higher elevation snows are possible in the Southwest on Sunday, and eventually return flow out of the Gulf of Mexico may increase precipitation chances into the Plains/Midwest as well early next week. The next in the series should arrive early next week again possibly exacerbating problems across California as it delivers more moisture into the region. Heavy rainfall could develop out into the Plains-Mississippi Valley-Midwest again along the attendant cold front next Wednesday, possibly heavy if a stronger storm manifests. There could be a chance for some snow/ice too in the northern periphery of the storm, amounts again highly dependent on storm strength. 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 15 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