Extended Forecast Discussion NWS Weather Prediction Center College Park MD 303 PM EDT Wed Sep 14 2022 Valid 12Z Sat Sep 17 2022 - 12Z Wed Sep 21 2022 ...Overview... The upper level flow pattern during the medium range period will be characterized by troughing deepening in the West 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 by Tuesday-Wednesday of next week. Meanwhile warmer than average temperatures will be the norm across the central and eastern U.S. into the first half of next week, though a cool down may reach the north-central U.S. by around Wednesday as the trough and cold front edge east. 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 and the Gulf Coast. ...Guidance Evaluation/Predictability Assessment... The latest run of guidance shows an improvement with the overall clustering of the upper-level pattern through the most of the extended forecast period, as they have converged with the notion that an upper-level trough/closed low will develop by the beginning of the week and reside near the eastern Pacific Ocean/West Coast. The CMC continues to be more of an outlier since it quickly looses the closed low feature and advances the trough across the West while the rest of the pack maintains the low and is slow to progress eastward. The CMC has the northern/southern streams phased thus the faster progression. As previously noted, the timing differences increase during Tuesday-Wednesday with the trough's slow eastward motion, depending in part on how long the closed low holds on and how phased the southern/northern streams may be. Both the GFS and ECWMF keep the southern stream closed low cut off and lingering in the Great Basin while the northern stream flow rushed a low amplitude trough eastward into the Great Lakes next Wednesday. The ECWMF was on the western edge of the guidance like the previous run which keeps the precipitation more near the coast offshore than the bullish GFS moving much further inland across California/Great Basin. The ensemble means favor the slower evolution of their parent guidance but also have some spread as well. Downstream, ridging will build over the central and eastern states in response to the troughing over the West. Similar to the forecast challenge of where the trough/low setups, there is some uncertainty of where the western edge of the ridge will be. Additionally, some shortwave energy may pass through the northern periphery of the ridge which may suppress it and allow for showers to spread across the Plains, Upper Midwest, Great Lakes and into New England. The WPC preferred model blend used the ECWMF, GFS, GEFS mean, EC ensemble means through all periods and very light amounts of the CMC and UKMET early on. ...Weather/Hazards Highlights... Upper troughing across the eastern Pacific and western U.S. will promote a cool season type precipitation pattern this weekend into early next week, with rain and even some snow in the highest elevations across California into the Northwest. The best potential for snow is the Northern Rockies but may reach as far south as the Sierra Nevada. Tropical moisture from a disturbance in the eastern Pacific that is likely to become tropical (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. A frontal system or two and a shortwave trough rounding the northern side of the ridge should cause a round of rainfall tracking east from the Upper Midwest and Upper Great Lakes region this weekend into the Northeast Monday. There may be instances of moderate, possibly higher intensity rains, however the confidence of flash flooding conditions arising and where is low at this time. A stalled front is expected to linger over Florida and the Gulf of Mexico which may trigger showers and thunderstorms for Florida and parts of the Gulf Coast. Much of the West will have cooler temperatures while locations east of the Rockies will remain toasty. Highs in particular are forecast to be 10-20F below normal in the West this weekend, with below average highs gradually shifting eastward into the Northern Rockies and northern High Plains as next week progresses, while lows remain closer to normal. Meanwhile the Central Plains into the Middle Mississippi Valley can expect high temperatures 10-20F above average through Tuesday, with actual temperatures well into the 90s. Warmer than normal temperatures will extend eastward all the way to the East Coast for the first half of next week, though with slightly lesser anomalies. Campbell/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