Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM EDT Sat Oct 14 2023 Valid 12Z Tue Oct 17 2023 - 12Z Sat Oct 21 2023 ...Overview... The models and ensemble means continue to show the upper pattern transitioning to an amplified western ridge/eastern trough configuration for the latter half of the coming week, aided by the extratropical reflection of Bolaven which should reach the east-central Pacific by the start of the forecast period early Tuesday. The amplifying central-eastern U.S. trough and leading front/low pressure should produce some areas of light to moderate rainfall, with some areas of heavier activity possible over the East by the end of the week depending on uncertain exact details of upper flow and surface evolution. Meanwhile an upper trough over the East Coast on Tuesday should quickly depart, and shortwave energy behind the building western U.S. upper ridge may bring a front and associated moisture close to the West Coast by next Saturday. The forecast pattern will promote above normal temperatures over the West and at times into the High Plains, along with near to below normal readings over the East. ...Guidance Evaluation/Predictability Assessment... The primary forecast challenge involves the details of flow within the overall central-eastern U.S. amplifying trough. Models show multiple pieces of shortwave energy, including the initial feature over and just north of the northern Rockies as of Tuesday, and at least one and possibly more upstream impulses that may feed into the trough. The scale of individual impulses is small enough to keep predictability for specifics fairly low when projecting several days in advance. Over the past couple days sporadic GFS runs and one CMC run have been trying to pull off upper low energy as far southwest as the southern High Plains while ECMWF runs have remained more open and progressive with the trough (aside from perhaps a transient low over the northern tier) until closing off a larger scale low over/near the Southeast by day 7 Saturday per the past two runs. While the ensemble spread has included the possibility of the slower/separated scenario, the majority of members and thus the means have sided with the open and progressive scenario. Latest GFS/CMC trends have aligned with this idea as well, waiting until the trough reaches the East before closing off a significant upper low if at all. Needless to say the upper flow detail uncertainty toward next Saturday keeps confidence low for surface specifics by then, though in a broad sense the guidance does show the potential for a meaningful system to develop somewhere near the East Coast. An operational model blend early in the period followed by a transition to half models/half ensemble means by day 7 Saturday provides a reasonable account for decreasing detail confidence with time and adds a little detail to the means late while not committing to any specific model solution. Elsewhere, recent guidance trends have held onto the idea of farther offshore Atlantic low pressure through midweek due to the departing eastern U.S. upper trough being sufficiently open, while guidance shows only modest spread relative to day 7 forecasts for the shortwave/surface front nearing the West Coast late in the period. The average of guidance provides a reasonable starting point for those aspects of the forecast. ...Weather/Hazards Highlights... Some locations over and near the northern Rockies may see mostly light precipitation on Tuesday as a shortwave and surface system cross the region. After that time, upper trough amplification should support somewhat greater coverage of rainfall spreading across the central and eastern U.S. along and ahead of the corresponding surface system(s). Confidence in specifics is still not great though, given the uncertainty in upper trough/surface details during the latter half of the week. Currently expect progression to be sufficient for most central U.S. rainfall to be in the light to moderate range, with somewhat heavier activity possible over the southeastern Plains/Lower Mississippi Valley. Moist easterly low level flow across Florida should increase rainfall over the state late in the week, and some of this moisture could ultimately get pulled farther north over the eastern U.S. by Friday-Saturday. Coverage and intensity of rainfall over the East will depend on how much the upper trough may decelerate/close off at that time. Elsewhere, northern portions of the Pacific Northwest could see a little rainfall during the period with a slight increase possible by the end of the week as a front approaches. Some scattered light rain may linger into Tuesday over the eastern Great Lakes/Appalachians into Northeast before a drier trend. Expect above normal temperatures over the West into High Plains and near to below normal readings over the East. Warmest anomalies will likely be over the High Plains on Tuesday and then with increasing coverage over the northern Rockies into northern-central High Plains late in the week, with highs 10-20F above normal. Coolest anomalies over the East, including Florida, should be on Tuesday with some highs at least 10F below normal. After some moderation with the departure of the Tuesday trough, the southern half of the East should cool down once again with the arrival of the upstream amplified trough by the end of the week. Rausch 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 forecast, excessive rainfall outlook, winter weather outlook probabilities, heat indices and Key Messages 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 https://www.wpc.ncep.noaa.gov/#page=ovw