Extended Forecast Discussion NWS Weather Prediction Center College Park MD 430 PM 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 upper trough in the eastern U.S. will support a strengthening low pressure system 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 side of the nation, 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... Model guidance still shows uncertainty in the finer scale details of the low pressure system forecast to impact the Northeast late this week into the weekend, which will have impacts on the precipitation forecast. The GFS/ECMWF runs and their ensemble means have held up the best for the overall evolution of the upper wave that will form the Northeast coastal low. The latest GFS still brings the low up the coast slower than the ensemble means and other deterministic guidance, which would result in heavier rainfall spreading further west as well. Since this solution seems the least likely, heavier weight was put on the other guidance for this forecast. Latest GFS runs are also 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. Ensemble members are expectedly diverse but the means average out to a solution falling between the to the 00Z ECMWF and 12Z GFS. Corresponding to these upstream issues, a wide spread develops for how quickly the leading trough/low may eject northeastward, leading to low forecast confidence for mid-next week. Finally, the guidance still signals that the overall trough may incorporate moisture from Tropical Cyclone Norma, which may contribute to potential heavy rainfall in the Southern Plains Tuesday into Wednesday. Preferences based on the latest guidance led to starting the early part of the forecast with a deterministic blend with heavier weights on the GFS/ECMWF than the CMC/UKMET. The GFS and EC ensemble means were slowly incorporated into the blend in increasing amounts through the second half of the forecast period. ...Weather/Hazards Highlights... During the Day 4 Excessive Rainfall Outlook that covers Saturday and Saturday night, 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 slow GFS solution remains the outlier with heavier rainfall spreading further west than the other deterministic guidance and ensemble means. For the afternoon forecast, confidence was not high enough to make any significant changes, and the Marginal Risk area remains in place. 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, but there is still significant uncertainty in the timing and location of any potential heavy rainfall. 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/Dolan 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 Hazards: - Heavy rain across portions of the Central/Southern Plains, Tue-Wed, Oct 24-Oct 25. - Heavy rain across portions of the Northeast, Sat, Oct 21. - Flooding occurring or imminent across portions of the Southeast. - Frost/freeze across portions of the Northeast, the Central/Southern Appalachians, the Tennessee Valley, the Mid-Atlantic, the Southern Appalachians, the Southeast, the Great Lakes, and the Ohio Valley, Mon-Tue, Oct 23-Oct 24. 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