Extended Forecast Discussion NWS Weather Prediction Center College Park MD 249 PM 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 amplifies 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 next week. 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... The latest model guidance remains 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. By early next week however, 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 before slowly drifting into the Southwest. The CMC, GFS, and UKMET on the other hand, indicate some degree of phasing between the two streams eventually showing one trough that steadily progresses east. The CMC and UKMET, at this point, may be too quick to phase these systems and developes a closed and very deep upper low over the Southwest. The Ensemble members remain quite variable and lead to low confidence in any particular solution, but the GFS does seem to be the better comparison for the ensemble means. Models have also come into better agreement with the next shortwave into the West around Monday-Tuesday, though with some differences in the details which should shake out as the feature mentioned above comes into better focus. The WPC forecast today leaned more on the GFS over the ECMWF, along with the ensemble means, especially later in the period. The CMC/UKMET were not included after day 4 due to the more extreme/deeper solution (not that its not a viable solution, just not preferred to go that deep yet). This maintained good consistency with the previous WPC forecast. ...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 to possibly heavy rainfall are likely 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. Santorelli/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