Extended Forecast Discussion NWS Weather Prediction Center College Park MD 156 AM EST Tue Jan 10 2023 Valid 12Z Fri Jan 13 2023 - 12Z Tue Jan 17 2023 ...Unsettled and Active Pattern Continues for Western U.S. with focus on California Atmospheric Rivers... ...Overview and Guidance/Predictability Assessment... The overall flow pattern over the CONUS later this week through early next week will remain mostly amplified, but progressive. At the start of the period (Friday), a strong ridge of high pressure will build into the Plains while downstream over the Midwest, an elongated amplified trough or closed low will be pushing a strong surface low through the Lower Great Lakes as a cold front pushes off the East Coast. Very uncertain deepening low pressure off the Mid-Atlantic could bring some potential for wraparound precipitation/snows to parts of mainly the interior Northeast. Back to the West, one atmospheric river will be ongoing across the Northwest, but should largely spare over saturated California from more rain, at least until the weekend when the next system pushes ashore. A brief ridge may build on Sunday, but the next in this relentless parade of storms early next week will bring another round of rain and heavy mountain snows to California. For the Eastern U.S. system, there remains a lot of variability in the details, especially this weekend as southern stream energy drives additional surface low cyclogenesis off the Mid-Atlantic coast. The deterministic models exhibit quite a bit of spread both with respect to timing and proximity of the low to the coast which has big implications for possible wrap back snows across the Northeast and possibly into parts of the Mid-Atlantic. The 18z (Jan 9) GFS was not used after day 4 because it was much too fast and well east with the system off the coast, while the better bulk of guidance suggested the low pulls more northward. A quick look at the new 00z guidance coming in (available after the forecast production time) shows again a lot of variability in this system once it gets off the coast suggesting a very low confidence forecast after day 4. Out West, an initial shortwave into the West Coast which drifts east into the Plains this weekend and eventually the Midwest by early next week shows fairly good agreement in the models, though with some minor timing/strength differences. A general model blend took care of these issues. By Monday, a compact closed low looks to approach the Northwest coast and move inland over the Intermountain West on Tuesday and there are some greater uncertainties noted here with timing, strength, and interactions between northern and southern stream energies. The WPC forecast for tonight used a general model blend days 3-4, but quickly trended towards the ensemble means the second half of the period in an attempt to mitigate the detail differences in the deterministic models. This generally lined up well with the previous shift continuity 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 highly amplified pattern mentioned above will help focus the best moisture transport and rainfall along the immediate northwest California coast into Friday, and further up the Pacific Northwest coast affecting coastal regions of Oregon and Washington. However, by this weekend, another strong weather system approaches central California and will bring another round of widespread precipitation. This system will impact a larger area of California with even more heavy rainfall threats. Worsening flooding or delayed improvement of ongoing issues is possible again across a very over-saturated coastal central to southern California while the higher elevations of the Sierra see heavy snowfall. 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/lower Mississippi Valley as well. Meanwhile, back to the East, most of the precipitation will likely have exited the East Coast by early Friday, except for some lingering, and locally heavy, rainfall along the New England Coast. Any snowfall with this system likely remains confined to higher elevations and locations well inland. As a secondary low pressure area develops off the Northeast coast, there is some potential for wrap back snows across parts of the Northeast into Saturday, but this evolution and threat is highly uncertain and dependent on exact proximity of the low to the coast. Parts of the Northeast could be well above normal to start on Friday, but should trend back to normal this weekend. Generally warmer than normal temperatures are expected across the Central U.S. through the period, particularly by later this weekend into early next week where parts of the Plains and into the Midwest could see high temperatures 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 across the Mississippi Valley and into the Midwest Monday-Tuesday. 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