Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Tue Oct 18 2022 Valid 12Z Fri Oct 21 2022 - 12Z Tue Oct 25 2022 ...Overview... A significant pattern change is in store for the medium range period as upper troughing lifts out of the East late week while an upper trough develops and strengthens over the West, progressing toward the central U.S. early next week. This deep trough and a couple of cold fronts will lead to areas of lower elevation rain/higher elevation snow as well as cooler temperatures over the West, and will eventually spread increasing rain chances to the Plains/Mississippi Valley. Ahead of the trough, warmer than average temperatures are forecast to shift from the central U.S. into the Northeast, with mainly dry conditions aside from some possible rain along the East Coast dependent on the positioning of a wavy offshore front and upper-level energy. ...Model and Predictability Assessment... Model guidance is quite agreeable with the overall pattern change transitioning to a western trough/eastern ridge for the medium range period. The bulk of guidance all brings notable mid-upper energy into the Northwest that deepens a trough in the West late week into the weekend, while a southern stream upper low meanders offshore southern California/Mexico. But for early next week, models are considerably more variable regarding the potential phasing of these northern/southern stream features. The ECMWF has shown not much phasing at all, leading to a faster northern stream trough and a southern stream low lingering west, while the more phased 18Z GFS and 12Z CMC solutions indicate one slower trough. Ensemble members remain quite variable and lead to low confidence in choosing either solution. Then, models end up out of phase with the upstream flow coming into the Northwest by around Tuesday, impacting potential for additional precipitation there. GFS runs have been particularly ridgy while the ECMWF has shown troughing as additional energy enters. The 00Z ECMWF and CMC have trended a bit more ridgy though, and this is an aspect of the forecast that remains quite uncertain. In the East, there remain lingering issues with models handling energy spilling into the southern stream while the bulk of the trough in the northern stream retreats considerably from its position in the short range. Recent ECMWF runs have not been as aggressive with developing a closed low in the Southeast compared to previous runs from stronger energy remaining south, but most guidance does show some energy in the Southeast or nearby, for some model convergence. But this relatively small scale feature remains on the lower predictability side and could affect surface frontal position and potential for a surface low to spin up (as in ECMWF runs and the 18Z GFS), as well as rain chances for coastal portions of the East, so additional refinements to the forecast are likely. Thus the WPC forecast was based on a blend of the deterministic guidance, particularly the 12/18Z GFS and 12Z ECMWF, for the early part of the period, but incorporated components of the GEFS and EC ensemble means by day 5 and quickly increased them to about half by days 6/7 given the differences in operational models described above. This approach maintained good continuity with the previous forecast aside from typical nudges reflecting latest trends. ...Sensible Weather and Potential Hazards... Precipitation is forecast to spread into the Northwest late week as the upper trough arrives and amplifies and the first surface cold front moves through, spreading a weak-moderate atmospheric river quickly across the West in one of the first major cool season-type precipitation events. The best focus will be over favored windward terrain over the Pacific Northwest on Friday and then extending southeastward into the northern/central Rockies and Wasatch over the weekend, with higher elevation snow and lower elevation rain. Confidence in a meaningful precipitation event remains fairly good, but exact amounts of rain/snow and timing will still need to be refined with time. Precipitation could continue/reform early next week in the Northwest but with even more uncertainty. Precipitation may also be possible across the Southwest Friday and especially Saturday depending on the timing and track of the southern stream low and its eventual possible phasing. By early next week, rain chances should generally increase across the Plains and into the Mississippi Valley ahead of the upper trough and a developing low pressure system. Areas of enhanced rainfall are possible but the specifics are not yet clear at this time. Meanwhile, some rain may be possible along or near the East Coast this weekend into Monday associated with a frontal boundary and possible low pressure system, but with low confidence on how much rain could fall onshore versus remaining offshore. Temperatures are likely to be warmer than average across the central U.S. late week into the weekend ahead of the developing western upper trough. Highs and lows of 10-20F above normal will be widespread across the Plains to Upper Midwest. As the upper trough and cold front approach, temperatures are forecast to moderate in the central U.S. as they push the relatively warm temperatures into the Great Lakes region and Northeast by Monday-Tuesday. Meanwhile, temperatures in the West will cool considerably from slightly above normal on Friday to below normal for the weekend and beyond. Lows around 5-15F below average and highs 10-20F below average at times are expected to be common initially for the West Coast and spreading across the Intermountain West and Rockies as the upper trough slowly shifts. 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