Extended Forecast Discussion NWS Weather Prediction Center College Park MD 309 AM 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 cooldown 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 main area of model discrepancies on the larger scale continues to be with the trough in the eastern Pacific/West, though models have converged reasonably well on timing of the trough digging through the weekend. Recent model guidance including the ECMWF runs (which had been a holdout) are now also agreeable for closing off a low in the southern end of the trough through Monday. By Tuesday-Wednesday greater timing differences arise with the trough's slow movement eastward, depending in part on how long the closed low holds on and how phased the southern/northern streams may be. The 12Z and now 00Z CMC runs are on the more phased and thus faster side with the overall trough. With the GFS, the 18Z run in particular kept 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 12Z ECMWF seemed to be a middle ground approach and was somewhat similar to the ensemble means--though each ensemble mean showed similar characteristics to their deterministic runs so the means were not perfectly agreeable either. However, the 00Z ECMWF has a solution where it lingers the closed low centered offshore in the Pacific even into midweek--unlike any of its previous runs and farther west than other deterministic guidance too. Farther east, overall ridging is forecast east of the trough across the central and eastern U.S. with good agreement, but guidance varies with where the western side of the ridge will be (necessarily due to where the eastern side of the trough will be). Models also vary some with shortwave energy/troughing that suppresses the northern side of the ridge in the northern tier of the U.S. around Sunday-Tuesday, affecting the exact frontal position and placement of rainfall, but a multi-model blend seemed reasonable. Thus the WPC forecast blend was based on multi-model deterministic blend of the 12/18Z GFS and 12Z ECMWF, CMC, and UKMET early on, adding in and increasing the weighting of the GEFS and EC means to over half as the period progressed, while keeping in some of the 12Z ECMWF for added detail definition late in the period. This provided reasonable continuity from the previous forecast. ...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. Some snow is most likely in the Northern Rockies but may reach the Sierra Nevada as well. Then 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. While there could be some localized heavy rain with this activity, confidence in placement of heavy rain and the potential for any flooding/flash flooding impacts remains low. Additionally, a stalled front across the Florida Peninsula and Gulf of Mexico could continue to spark scattered thunderstorms across the Sunshine State, with some rain sneaking into the western/central Gulf Coast as well. Cool weather west of the Rockies and warm weather east of the Rockies will be the main weather story in terms of temperatures given the western trough/central-eastern ridge upper pattern. 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. 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