Extended Forecast Discussion NWS Weather Prediction Center College Park MD 254 AM EDT Thu Sep 15 2022 Valid 12Z Sun Sep 18 2022 - 12Z Thu Sep 22 2022 ...Overview... The upper level flow pattern during the medium range period will be characterized by an anomalously deep trough/low for this time of year over the eastern Pacific/western U.S. and eventually shifting slightly eastward next week, while ridging amplifies across much of the central and eastern U.S. ahead of it. This will promote cool temperatures and some precipitation coming into parts of California and the Northwest beginning this weekend, and then moisture increasing ahead of a possibly forming tropical system in the eastern Pacific could lead to showers and storms in the Four Corners states possibly beginning Monday and especially Tuesday-Wednesday of next week. Meanwhile warmer than average temperatures will be the norm across the central and eastern U.S. into the early part of next week, before a northern stream trough edges a cold front east and considerably cools the north-central CONUS. Rounds of rain are possible along the northern tier with progressive shortwaves and frontal boundaries passing through, while a meandering front should continue to allow for scattered thunderstorms across Florida. ...Guidance Evaluation/Predictability Assessment... Model guidance has been persistent for the last few model cycles showing a closed upper low centered just off California as the medium range period begins Sunday. The trend overall has been for this low to move more slowly inland into the West as the week progresses, as most models show some separation between it and the northern stream tracking more quickly eastward, but not uniformly. The 12Z CMC continued to hold out for a more phased solution and thus more progressive in the south than the overall consensus, but the newer 00Z CMC has jumped on board for stream separation. The 12Z GFS was more phased as well compared to the 18Z run. The ECMWF continues to show stream separation but just seems to keep getting slower in tracking the low inland, with the 12Z run somewhat slower than other guidance and the 00Z run even more so. The 18Z GFS seemed like the best middle ground solution for the 12/18Z model cycle. The WPC forecast blend used a multi-model deterministic blend favoring the 18Z GFS and 12Z ECMWF early on, then eliminated the 12Z CMC and lessened the weighting of the EC in favor of the GEFS and EC ensemble means as the forecast period progressed. This blend also worked for overall ridging farther east and was reasonable for an initial shortwave trough moving from the north-central to northeastern U.S. Sunday-Tuesday, and with northern stream energy progressing eastward midweek atop the ridge. ...Weather/Hazards Highlights... Upper troughing across the eastern Pacific and western U.S. will promote a cool season type precipitation pattern early next week, with rain and even some snow in the highest elevations across California into the Northwest. Some heavy rain may occur in northern California on Sunday in particular, and burn scars would be particularly vulnerable to flooding concerns and debris flows. The Sierra Nevada could see snow early next week, though the best chances for snow with this activity are in the Northern Rockies by Tuesday-Wednesday. Tropical moisture from a disturbance in the eastern Pacific that is likely to become a tropical cyclone (per the National Hurricane Center) is forecast to stream into the Four Corners states as early as Monday, but especially by Tuesday-Wednesday, increasing chances for showers and thunderstorms there while providing additional moisture for the precipitation activity in the Northwest. A frontal system and a shortwave trough rounding the northern side of the upper ridge should cause a round of rainfall tracking east from the Upper Great Lakes region spreading into the Northeast Sunday and continuing across the interior Northeast Monday. Some rainfall could be moderate to heavy, and increasing rain amounts in the forecast especially in the Northeast make isolated flash flooding a concern, though the details are uncertain as they will depend on the frontal position. A stronger front pushing across the north-central U.S. by midweek could lead to another round of rain in that area. Meanwhile, a stalled front is forecast to linger across central to southern Florida and could trigger showers and thunderstorms there. The western end of this front pushing south and dissipating across the Gulf should lead to drier conditions for the Gulf Coast after the weekend. The West Coast will be rather cool particularly in terms of high temperatures for the early part of the week, as highs 10-20F below average are expected. These cooler temperatures are forecast to spread into northern parts of the Rockies/High Plains and eventually into the Upper Midwest by next Thursday behind a cold front. Meanwhile warmer than average temperatures by 10-20F are forecast for the Central Plains and Middle Mississippi Valley through Wednesday before getting suppressed by the front, with actual temperatures well into the 90s. Warmer than normal temperatures will extend eastward all the way to the East Coast for much of next week, though with slightly lesser anomalies. 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