Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Tue Oct 17 2023 Valid 12Z Fri Oct 20 2023 - 12Z Tue Oct 24 2023 ...Overview... From late this week into the weekend, expect initially strong western U.S. upper ridging to weaken somewhat and drift eastward while a vigorous eastern U.S. trough with potential embedded low likely supports strengthening low pressure along the East Coast and possibly into the Canadian Maritimes. There are still important details to be resolved for this system but there is a fair clustering of guidance indicating some potential for significant rainfall and a period of brisk to strong winds over the Northeast. Then as this system and supporting upper trough lift away from the eastern states, the West should see a significant pattern change toward more troughing. A Pacific trough of moderate strength arriving around Sunday will start this evolution, followed by deeper energy expected to drop down from northwestern North America and the far northeastern Pacific. The overall mean trough will support expanding precipitation coverage and a pronounced cooler trend across the western half of the country. ...Guidance Evaluation/Predictability Assessment... For the eastern U.S. upper trough and surface system, the forecast depends upon some still lower predictability details for shortwave energy digging into the East as of Friday and upstream energy likely to feed into the overall trough. Among latest/recent cycles, there has been some loose clustering among recent GFS runs, GEFS/EC means, and 00Z/16 ECMWF run that would support a developing coastal wave near the northern Mid-Atlantic by Saturday and reaching the vicinity of Maine by Sunday (the means being faster, GFS slower, 00Z/16 ECMWF in the middle). This occurs while guidance generally hints at an embedded upper low tracking somewhere over the Great Lakes/northern Mid-Atlantic/New England/southern Canada. CMC runs have differed from the majority lately, while the 12Z/16 ECMWF showed more digging of energy in the base of the eastern trough than other guidance. The new 00Z ECMWF has returned to a solution somewhat closer to the majority. Preference was for a compromise among the means/GFS/old 00Z ECMWF which also maintained reasonable continuity. Across the West, the leading trough arriving by Sunday continues to be fairly well behaved via model/ensemble blend though the new 00Z CMC has returned to a somewhat more amplified depiction as had been its tendency before the 12Z/16 runs. To the south of this trough, a significant wild card will be a tropical system expected to develop over the eastern Pacific and lift northward offshore the western Mexico coast. The combination of faster northward progression and more amplified trough in the CMC has led that model to be on the fast side of incorporating the system and its moisture. There is more spread for upstream troughing that may reach the Northwest by next Tuesday, along with the corresponding surface pattern. The ensemble means provide the most consistent starting point here, with some aspects of GFS/CMC runs and 00Z/16 ECMWF also valid contributors to a blend. The deeper/offshore depiction of the 12Z ECMWF (along with a strong surface system) appear less probable. Preferences across the above regions led to starting the forecast with a 12Z/18Z model blend early in the period, but with the 00Z/16 ECMWF providing the ECMWF component instead of the 12Z run. The latter half of the forecast incorporated some 18Z GEFS/12Z ECens means, reaching a total of half weight by the end of the period. ...Weather/Hazards Highlights... The front crossing the eastern U.S. on Friday may produce some rainfall of varying intensity from the eastern Great Lakes southward into the Southeast, with some lingering uncertainty over best emphasis within this broad area. Low pressure developing near the East Coast may produce another relative focus for rainfall. In both cases the ingredients still do not yet appear sufficiently favorable for depicting any risk areas in the current Day 4 Excessive Rainfall Outlook that covers Friday-Friday night. Depending on the track and timing of strengthening low pressure during the Day 5 period (Saturday-Saturday night), there is somewhat more potential for significant rainfall over parts of the Northeast as some duration of moist Atlantic inflow may help to enhance amounts. The Day 5 outlook depicts a Marginal Risk area from central/northern parts of eastern New York state through central and northern New England, which represents the best overlap of heaviest rainfall potential (between the slow GFS and faster ensemble means). Surface low intensification will also support the potential for a period of brisk to strong winds at least through Sunday. Depending on system evolution/track, highest elevations could see a little snow. Next week will bring a drier trend over the East as this system departs. In contrast, much of the West start out dry late this week and then see expanding coverage of precipitation from the weekend into next week. A leading upper trough/surface front should bring mostly light rainfall to the Pacific Northwest around Saturday night, with this moisture shifting across the Intermountain West into Monday. Additional trough energy should bring more precipitation into the Northwest early next week while leading moisture spreads across the Rockies and possibly reaching the Plains by next Tuesday. At that time it will be worth monitoring how much East Pacific tropical moisture may get incorporated into the mean flow. Some of the precipitation should fall as snow in the higher northern Rockies, with snow levels gradually declining with time. A broad area of temperatures 10-20F above normal will extend through the West and into the Plains on Friday. With time this warmth will moderate and shrink in coverage as it reaches portions of the Plains/Mississippi Valley by next Tuesday, at which time the high temperature anomalies should be only in the plus 5-12F range. Late this week a scattering of daily record highs will be possible across the West, with record warm lows tending to be a little more numerous and concentrated more over California. During the weekend the potential for record highs should shift into the southern Rockies and Texas. With the West transitioning to more mean troughing aloft from Sunday onward, expect the region to see a pronounced cooling trend, with highs likely dropping to 5-15F below normal by early next week. Over the East, the Northeast should see above normal temperatures (especially for morning lows) late week into the weekend. Otherwise the amplified upper trough crossing areas east of the Mississippi River will support multiple days with highs moderately below normal from late this week into start of next week, followed by a return toward normal. 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