Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM 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 that favors less precipitation to the west of the Appalachians, with potential but still uncertain East Coast surface 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 12Z/18Z models and ensemble means offered a reasonable composite overall, reflecting the aforementioned trend toward keeping the amplifying central into eastern U.S. upper trough open and more progressive through late week. Recently some GFS runs and GEFS members have represented the southwestern side of the spread for any potential elongation of the trough, but the 18Z GFS and now the new 00Z run represent the ongoing trend toward the open and progressive trough through at least Friday. An embedded compact upper low could track across the northern tier but it should be fairly transient. There is increasing detail uncertainty given the spread/lower predictability for how Pacific energy will round the western ridge and feed into the eastern trough late week into the weekend, but there is a fairly common theme in the guidance for low pressure to develop near the northern Mid-Atlantic/southern New England by around Friday-Saturday and deepen as it tracks northward into Sunday while the core of the upper trough (with possible low) reaches eastern Canada/New England. Latest CMC runs are somewhat on the northern side of the spread with this system by Sunday while the 12Z ECMWF was somewhat south. Low confidence for specifics of shortwaves feeding into the eastern trough extends to any corresponding waves/fronts dropping southeastward over the central U.S. Solutions for the shortwave reaching the West Coast by next Sunday show better than average clustering for forecasts that far out in time. A 12Z/18Z operational model blend early in the period and then a transition to a combination of 18Z GEFS/12Z ECens means and 12Z/18Z models provided a good intermediate solution through the period, including the desired model-mean compromise for strength/track of the storm system potentially affecting the Northeast next weekend. ...Weather/Hazards Highlights... The system initially progressing eastward from the Plains should spread mostly light rainfall across parts of the northern Plains through the Midwest. As the supporting upper trough amplifies and sharpens, rain should expand southward over the eastern U.S. with somewhat higher (but likely not excessive) localized totals possible. 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. The current majority guidance cluster would support the idea of some enhanced rainfall and winds over at least the Northeast, but a slight difference in low track 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 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. 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