Extended Forecast Discussion NWS Weather Prediction Center College Park MD 435 PM EDT Thu Sep 15 2022 Valid 12Z Sun Sep 18 2022 - 12Z Thu Sep 22 2022 ...Overview... The overall large-scale pattern through the medium range period will feature a trough and closed low over the eastern Pacific/western U.S. that will slowly progress eastward, ridging that builds in over the central-eastern U.S. with shortwave energy tracking along the northern periphery over the ridge top. Cooler temperatures and some precipitation is in store for California and the Northwest beginning this weekend. Moisture increasing ahead of a possibly forming tropical system in the eastern Pacific could lead to showers and thunderstorms over the Four Corners region by early next week prior to northern stream trough edges a cold front east and considerably cools the north-central CONUS. Multiple rounds of rain along and ahead of this front will spread from the Plains to the Great Lakes and Northeast progressive shortwaves and frontal boundaries passing through. Further south, scattered thunderstorms will remain possible for Florida near a lingering quasi-stationary frontal boundary. ...Guidance Evaluation/Predictability Assessment... Another cycle of guidance that favors having a closed low centered near the California coastline by Sunday that moves slowly inland as the week progresses. Most of the models are depicting some separation from the trough/closed low within the southern stream from the northern stream; which moves more quickly toward the East. There is some differences on the strengthen and timing of the shortwave in the northern stream which reduces confidence of where QPF is expected to fall, the intensity and the amounts. The ECWMF has been persistently getting slower it seems that past few days and quickly lags while the cluster moves inland and east. The 06Z and 00Z GFS maintain separation of the streams and is the continues to be the closest "middle ground" solution for the extended forecast. As such, the WPC blend used a multi-model deterministic blend that favored the 06/00Z GFS but also utilized the UKMET, CMC, ECWMF sparingly early on and then eliminated ECWMF and CMC while increasing the weighting of the ensemble means. This combination maintained the 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... With multiple days with upper-level troughing in place, cooler temperatures that may lead to some mixed precipitation, or snow for the highest peaks across the Northwest and California. The daily temperatures are forecast to be 10 to 15 degrees F below seasonal average for mid September for the West with the cool airmass spread to the Rockies/High Plains early in the week and then to the Upper Mississippi Valley by the middle/end of the week as the cold front makes its way east. The onshore rain may be heavy at times across portions of northern California Sunday, 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. Some of this deep moisture will advect further north into the Rockies and adjacent High Plains. 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. The models suggest there could be instances where moderate to heavy rainfall will fall, particularly over New York and parts of New England where higher amounts may lead to isolated flash flooding. However at this time the exact location of where there concerns may be is uncertain. A second and strong cold front is forecast to advance across the northern tier which would bring another round of showers. 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. With the ridging building into the central U.S., 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. Campbell Hazards: - Heavy rain across portions of New England and northern California, Sun-Mon, Sep 18-Sep 19. - Heavy rain across portions of the Four-corners region, as well as the northern Rockies and nearby foothills, Tue-Wed, Sep 20-Sep 21. - Excessive heat across portions of the central Plains, Sun-Wed, Sep 18-Sep 21. 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