Extended Forecast Discussion NWS Weather Prediction Center College Park MD 248 AM EDT Wed Oct 19 2022 Valid 12Z Sat Oct 22 2022 - 12Z Wed Oct 26 2022 ...Overview... A significant pattern change remains likely for the medium range period compared to the West upper ridge/East upper trough pattern in the short range. Upper troughing will strengthen in the West this weekend and progress toward the central U.S. into 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 and front, warmer than average temperatures are forecast to shift from the central U.S. into the Northeast, with mainly dry conditions in the East 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... While model guidance is agreeable that troughing will amplify in the West this weekend, predictability and confidence quickly decrease early next week as models diverge. Much of the model spread arises from differences in the potential phasing of the trough forming in the northern stream and a southern stream upper low offshore of California and Mexico. This has been an area of considerable disagreement over the last few days in operational guidance and ensemble members. The ECMWF runs have been mostly consistent with leaving these two features in their separate streams through at least Monday, leading to a faster northern stream trough/forming upper low while the southern stream low spins in the eastern Pacific before eventually coming into the Southwest. Meanwhile GFS runs had generally been on the more phased side, creating one trough that steadily progresses east with in-between timing...until the 18Z GFS run that was pretty similar to the 12Z ECMWF. The CMC and UKMET both phase the energies pretty quickly to create a deep Great Basin/Four Corners low Sunday-Monday. Models with more stream separation like the ECMWF and 18Z GFS indicate a faster northern stream cold front of course, and keep precipitation in the north-central U.S. early next week, less so in the south-central U.S. like other guidance and the ensemble means. Ensemble members remain quite variable, which does not lead to good confidence in any particular solution. The 00Z model guidance including the ECMWF came in more on the phased side, though once again not as aggressive with the phasing as the UKMET and CMC. Meanwhile, at least model clustering has improved somewhat with mid-upper energy moving northward near the East Coast this weekend into Monday with perhaps a weak surface low, for some rain chances especially along the coast but potentially spreading more inland into the Northeast around Sunday-Monday. There are still some detail differences with this relatively small-scale feature though. And models have shown some waffling on timing of possibly a couple of rounds of additional energy coming into the West behind the initial developing trough. GFS runs/GEFS ensemble members have shown more ridging across the West in between the troughing features than the bulk of the EC guidance suite for Tuesday-Wednesday. Thus the WPC forecast could use a multi-model deterministic blend for the beginning of the forecast timeframe, but the increasing model differences led to increasing usage of the ensemble means through the period. This forecast attempted to give a nod toward the more separated stream solutions but not as aggressively as the 12Z ECMWF/18Z GFS, which was likely good given the somewhat more phased 00Z runs. Relatively large changes to many forecast variables including the QPF may be necessary over the next few days given the current low predictability extending even into days 4-5. ...Sensible Weather and Potential Hazards... Precipitation is likely across the much of the West by Saturday as the upper trough amplifies and the first surface cold front moves through, for one of the first major cool season-type precipitation events since much earlier this year. Precipitation will be enhanced across favored windward terrain of the Cascades, 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. Another couple of rounds of precipitation could continue/reform early next week in the Northwest but with even more uncertainty. Precipitation may also be possible across the Southwest for the weekend but will be dependent on the timing and track of the southern stream low and its eventual possible phasing. By early next week, precipitation 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, while there may be some snow on the backside of the surface low into the northern High Plains, but the specifics of both 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. for the weekend ahead of the developing western upper trough. Highs and lows of 10-20F above normal, with some spots higher, will be widespread across the Plains to Upper Midwest. Meanwhile, an area of below normal temperatures will expand from the West Coast states into the Great Basin and Rockies this weekend, with lows around 5-15F below average and highs 10-20F below average. Temperatures should cool to near normal and even below normal for the north-central U.S. next week as the upper trough and cold front(s) move through. This will push warmer than average temperatures into the northeastern U.S. by Monday-Wednesday. 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