Extended Forecast Discussion NWS Weather Prediction Center College Park MD 308 AM EDT Wed Oct 18 2023 Valid 12Z Sat Oct 21 2023 - 12Z Wed Oct 25 2023 ...Overview... Latest guidance continues to advertise a significant pattern change over the lower 48 during the period. A vigorous eastern U.S. upper trough will support strengthening low pressure that may bring locally heavy rain and gusty winds to the Northeast this weekend. Some degree of upper troughing will linger near the East Coast into early next week before a ridge builds into the East. On the other hand, expect mean troughing to replace the initial ridge over the West. A leading shortwave with possible embedded low should move into the region around Sunday while stronger upstream energy should feed into the developing mean trough by Tuesday-Wednesday, but with increasing uncertainty for details. This pattern transition would bring cooler and wetter weather to the West, with the Plains trending wetter as well by midweek, while the East will trend drier after the weekend with cool temperatures eventually moderating. ...Guidance Evaluation/Predictability Assessment... For the system affecting the Northeast this weekend, latest operational models are still showing finer-scale detail differences that have only medium to lower predictability 3-4 days out in time and will be important for resolving precipitation specifics in particular. Over the past day an average of the GFS/ECMWF runs and their ensemble means has held up the best for the overall evolution, with the developing coastal wave over or a little south of southern New England as of early Saturday and reaching over or a little beyond northern/eastern Maine by early Sunday. Some recent GFS runs have been on the south side of the spread but are trending closer to the majority. UKMET/CMC runs still differ with some details, as the new 00Z runs stray faster with the supporting upper low which other guidance says will still be closer to Maine as of early Sunday. Latest GFS runs are somewhat slower than the majority for the weak trailing system reaching the Midwest early Saturday. Forecast details have become more uncertain over the western and eventually central U.S. After most guidance had been showing a fairly diffuse leading upper trough moving into the West Sunday onward over recent runs, the past day has trended toward a better defined embedded low that may reach the Southwest by early next week. Beyond this trend, individual models and ensembles are suggesting that pattern complexities near and south of Alaska from the weekend into next week are leading to high sensitivity for the ultimate details of northern stream energy expected to drop into the western U.S. by Tuesday-Wednesday. Differences over that upstream area of sensitivity lead to the recent CMC runs being the most aggressive with digging energy by midweek versus the new 00Z GFS being at least a day later to bring its energy into the Northwest. Ensemble members are expectedly diverse but the means average out fairly close to the intermediate 12Z ECMWF. Corresponding to these upstream issues, a wide spread develops for how quickly the leading trough/low may eject northeastward--with minimal confidence in a specific solution (though trends lean away from fast timing like the 12Z CMC). Finally, the guidance still signals that the overall trough may incorporate moisture from Tropical Storm Norma which National Hurricane Center advisories expect to strengthen to hurricane status as it tracks northward offshore the western Mexico coast. Preferences based on the 12Z/18Z array of guidance led to starting the early part of the forecast with a blend of the 12Z/18Z GFS and 12Z ECMWF and then slowing incorporating more 18Z GEFS/12Z ECens means so that the means had half weight by late in the period. ...Weather/Hazards Highlights... During the Day 4 Excessive Rainfall Outlook that covers Saturday-Saturday, guidance continues to show the potential for some areas of significant rainfall over parts of the Northeast as northward moving low pressure deepens. Specifics are still uncertain and depend on fine-scale details and exact timing that would affect how long the system could bring in moist inflow from the Atlantic to enhance rainfall amounts. The new Day 4 ERO stays unchanged from the prior Day 5 issuance, depicting a Marginal Risk area from central/northern parts of eastern New York state through central and northern New England. Latest guidance trends have become a little more mixed in the far southwestern part of this area but are not emphatic enough to favor any change yet. Surface low intensification will also support the potential for a period of brisk to strong winds at least through Sunday. Depending on system strength/track, highest elevations could see a little snow. Next week will bring a drier trend over the East as this system departs. The West will see expanding coverage of precipitation from the weekend into next week. A leading system should bring light to moderate totals to northern and central latitudes after daytime Saturday, with this moisture shifting across the Intermountain West on Monday. Additional trough energy should bring more precipitation into the Northwest next week but details are becoming more uncertain. Leading moisture should spread across the Rockies and reach the central U.S. by Tuesday-Wednesday. Guidance continues to suggest that rainfall over the Plains could be enhanced by moisture originating from Norma in the East Pacific. Some of the precipitation over the West 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 from the Interior West into the Plains on Saturday. With time this warmth will moderate and shrink in coverage as it reaches portions of the Plains and Mississippi Valley by Tuesday. Above normal highs may expand into the Ohio Valley/Great Lakes by Wednesday but most anomalies as of midweek should be within 10 degrees of average. Potential for some record warm lows may extend into Saturday especially over California, while a few record highs may be possible from Arizona into Texas during the weekend. Still expect the West to see below normal highs expand in coverage from west to east, though latest trends have slightly delayed the arrival of colder air (Wednesday instead of Tuesday) that would bring highs at some locations down to 10-20F below normal. The Northeast should see above normal temperatures (especially for morning lows) into the weekend. Otherwise the amplified upper trough crossing the East will support highs up to 5-10F below normal from Saturday into early next week. The trough's departure will promote a rebound closer to or slightly above normal by midweek. 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