Extended Forecast Discussion NWS Weather Prediction Center College Park MD 251 PM EDT Tue Sep 13 2022 Valid 12Z Fri Sep 16 2022 - 12Z Tue Sep 20 2022 ...Overview... The medium range period begins Friday with broad upper troughing with embedded shortwaves across much of the western to north-central U.S., while warm ridging is in place over the east-central CONUS. The troughing over the West is forecast to deepen/sharpen by the weekend into early next week, though with some lingering model differences in the placement/timing. This will lead to cooler temperatures and some precipitation coming into the West. Farther 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. Temperatures should be above normal underneath ridging in the central U.S. and eventually into the East. ...Guidance Evaluation/Predictability Assessment... The general trend for the past few model cycles keeps the ECWMF faster/more progressive with less potential for a rounded and persistent closed low, the GFS remains a little slower but couples well with the EC/GEFS means while the CMC and UKMET fall between the GFS and EC solutions. Some model differences persist with timing of the trough's movement eastward and its potential for a closed low. Also, some of the solutions continue to depict Typhoon Merbok merging/absorbing as it recurves into the advancing Gulf of Alaska trough just prior to the extended period, which adds to model differences on the timing and placement of features downstream, particularly where ridging sets up dependent on how far east the trough pushes inland. The preferred WPC blend utilized the deterministic 00Z ECMWF, CMC, and UKMET and 06/00Z GFS early on, reducing the faster EC and slower GFS while increasing the weighting of the GEFS and EC ensemble means. This approach helped maintain continuity from the overnight forecast. ...Weather/Hazards Highlights... Light to moderate precipitation may spread across portions of the Northwest, northern California, Interior West, Great Basin and into the Northern Rockies into early next week thanks to approaching upper-level troughing and frontal systems trekking across the West. Temperatures aloft will cool in the coming days with this troughing, so some of the highest peaks across the Northwest and the Rockies may change over to snow; additionally, reaching the Sierra Nevada range by Sunday. There is also an increased signal for showers and thunderstorms to fire up across the Four Corners region Monday and Tuesday. This is in response to tropical moisture advecting into the Southwest from a system that may become tropical, as it tracks neat Baja California. Across the Gulf and Southeast a quasi-stationary front will remain draped for several days, providing a focus for multiple rounds of convective development. The heaviest rain continues to be forecast offshore of the Georgia/Carolina coasts and the Gulf Coast, but some rainfall could sneak onshore from southern Texas to the North Carolina Outer Banks. There may be periods of moderate to heavy rainfall across Florida. For the northern tier of the CONUS, a pair of fronts along with impulses of shortwave energy may generate rounds of rainfall through the extended forecast that may reach the Northeast by early next week. There is some signal for intermittent durations of higher intensity/heavy rainfall, however the confidence for excessive rainfall or localized flash flooding is not that high. With troughing over the West, cooler temperatures can be expected with some locations' daily maximums dipping to below average. Highs of 10-15F below average may be common in the West Coast states into the Intermountain West, with even cooler readings currently forecast centered over northern California on Sunday. Likewise, with ridging over the central/eastern U.S. temperatures will remain near seasonal up to 10-15F warmer than average (with actual temperatures well into the 90s) centered in the Central Plains/Middle Mississippi Valley for this weekend into early next week, and warmer than normal conditions could stretch into the Midwest/Great Lakes to eastern U.S. with time. Campbell 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