Extended Forecast Discussion NWS Weather Prediction Center College Park MD 231 PM EDT Sun Oct 15 2023 Valid 12Z Wed Oct 18 2023 - 12Z Sun Oct 22 2023 ...Overview... The forecast has been consistent with the general theme of a strong mid-late week upper ridge over the western U.S. supporting downstream trough amplification into the East by the end of the week. However guidance has varied for important trough details within an overall multi-day trend toward a more progressive and open feature with variable precipitation chances from the Plains to the Appalachians with potential but still uncertain East Coast surface low development by next weekend now providing the main possibility for heavier totals over the Northeast. The western ridge should weaken noticeably and drift a little eastward next weekend as a Pacific shortwave and leading front reach the West Coast with some rainfall for the Pacific Northwest. The forecast pattern will favor above normal temperatures over the West into the Plains mid-late week, with a moderating trend near the West Coast next weekend as the trough/front arrive. With the exception of above normal readings over New England, most of the East should see near to below average highs during the period. ...Guidance Evaluation/Predictability Assessment... The guidance is in decent agreement on the broader details through most of the medium-range period, which looks to continue an amplified pattern featuring mean long-wave troughing over the central-eastern U.S. and a ridge to the west. As one shortwave departs the East Coast, more upstream shortwave energy dropping southward over the Plains in the mean flow will reinforce the troughing in the east. The GFS has been consistent run-to-run with some of this energy dropping further southward over Texas while the 00Z CMC is less aggressive, keeping this energy further north, with the 00Z ECMWF in between the other solutions. Another trend across the guidance has been for a bit less amplification as additional upstream energies do not deepen the long-wave trough as far south into the Gulf mid- to late period compared to earlier runs. In this case, the 00Z ECMWF is still on the more aggressive side of the envelope with a larger degree of run-to-run variability in the GFS, which has trended less amplified in the 00Z/06Z runs than the ECMWF after showing a deeper, closed low developing over the Southeast in previous runs. This will have implications on the potential for subsequent coastal low development as the trough axis shifts eastward along the East Coast, as the 06Z GFS for example has trended faster and further northward by the end of the period, favoring surface low development further northwestward and inland in tandem with an upper-level closed low developing in the vicinity of the Lower Great Lakes/Interior Northeast/southeastern Canada. The ECMWF has been more consistent in taking a surface low northward along the coast. The varied solutions have an impact on precipitation chances through the period, with drier conditions with southwest extent over the Plains/Mississippi Valley in the 00Z CMC compared to the ECMWF and the 00/06Z GFS. THE ECMWF also favors higher rainfall chances along the coast of the Northeast with the coastal low as compared to the Interior Northeast in the GFS. Otherwise, although there are some timing differences, the guidance is in agreement that an upstream trough approaching the West Coast should begin to break down the ridge over the West by next weekend. The updated WPC forecast uses a composite blend of the deterministic guidance (00Z ECMWF/CMC/UKMET and 06Z GFS) early in the period, favoring the 00Z ECMWF which falls in the middle of the solutions with respect to the evolution of the troughing over the central-eastern U.S. A contribution from the 00Z ECens/GEFS means is added towards the end of the period as run-to-run variability among all the guidance, particularly the GFS, increases for the eastern trough as well as the system upstream along the West Coast. The contribution for the GFS is split between the 00Z and 06Z runs late to better capture the uncertainty in the evolution of the pattern as this variability increases. ...Weather/Hazards Highlights... An initial system progressing eastward across the Plains should spread mostly light rainfall across parts of the northern Plains through the Midwest. As upstream shortwave energy amplifies and sharpens the upper trough, rain should expand southward over the eastern U.S. with somewhat higher (but likely not excessive) localized totals possible, particularly for the Tennessee Valley/Southern Appalachians. Coverage and intensity of rainfall, as well as strength of winds, from the Mid-Atlantic into the Northeast next weekend will depend on specifics of the deepening surface low that may affect the region. A composite blend of the current guidance would favor rainfall and winds over at least the Northeast, but variable differences in the development and track of the surface low(s) could change the precipitation forecast significantly. Given the guidance changes over the past few days for evolution of the central-eastern U.S. upper trough, it will take some stabilization of the guidance over coming runs to gain much confidence in next weekend's details. Over the Northwest, the Olympic Peninsula may see some rain linger into Wednesday with heaviest totals staying focused over Vancouver Island. The upper trough/surface front reaching the West Coast during the weekend should expand coverage of light to perhaps locally moderate rainfall over the Pacific Northwest and into the Northern Rockies at that time. Strong upper ridging will support expansion of well above normal temperatures from the western U.S. into the Plains during Wednesday-Friday. By next weekend one or more fronts dropping southeast over the central U.S. may dent the eastern extent of the very warm temperatures while the upper trough/front pushing into the West Coast should bring highs down to near or slightly below normal levels over that region. Greatest anomalies should be in the plus 15-20F range for highs over the northern Rockies into portions of the High Plains Thursday-Saturday. Expect the best potential for daily records to be over the West during Thursday into early Saturday, with record warm lows tending to be more numerous than record highs. During next weekend the highs over parts of Texas/New Mexico could challenge record highs. Farther east, the Gulf Coast and Southeast will see below normal temperatures linger through midweek and then upper troughing amplifying into the East should expand the coverage of moderately below normal highs over the East by Friday-Sunday. New England should be somewhat above normal from late week into Saturday though. Putnam/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