Extended Forecast Discussion NWS Weather Prediction Center College Park MD 247 PM EDT Tue Oct 17 2023 Valid 12Z Fri Oct 20 2023 - 12Z Tue Oct 24 2023 ...Overview... A deep upper level trough will be positioned over the eastern U.S. late this week into this weekend, which will support precipitation chances east of the Mississippi Friday through Sunday. A strong coastal low is forecast to develop off the Mid-Atlantic on Friday and track north along the East Coast over the weekend, which may cause locally heavy rainfall and gusty winds for parts of the Northeast. The upper trough and surface low pressure system will push offshore early next week, and upper level troughing will move over the West. The weather pattern will trend cooler and wetter for the West next week. Precipitation chances will increase across the Pacific Northwest and along the West Coast on Sunday, then expand east into the Plains and Upper Midwest through Tuesday. ...Guidance Evaluation/Predictability Assessment... Model guidance is in generally good agreement on the timing, strength, and position of the eastern U.S. trough through the weekend, but there are some minor differences in how the deterministic models handle shortwaves moving around the base of the trough. The latest runs of the GFS/ECMWF/CMC/UKMET all show a wave developing off the Mid-Atlantic coast on Friday and moving north up the East Coast on Saturday accompanied by a strong coastal low at the surface. There are slight differences in the timing/speed of the coastal low, which will affect the forecast rainfall. A mainly deterministic model blend was used for the WPC forecast for Friday through Sunday with small amounts of the GEFS mean and EC mean. Slightly less weight was given to the CMC, which continues to differ from the majority this afternoon with how it handles the waves of energy in the eastern trough. There is quite a bit more uncertainty surrounding the troughing over the West next week. Two successive troughs will likely move into the West through mid-next week, but the CMC/ECMWF/GFS disagree on the speed of the second trough. The ECMWF strengthens the second trough significantly more than the other models and brings it into the West the slowest, keeping both troughs separate. The GFS is on the other end of the spectrum, keeping the second feature weaker and bringing it into the West faster, allowing the two troughs to combine quickly. The CMC seems to fall between the GFS and ECMWF. Additional uncertainty is introduced from the potential tropical system in the eastern Pacific, which may affect the evolution of the upper level pattern as the upper level energy moves into the mid-latitudes. For the forecast blend, ensemble means were assigned the heaviest weight with smaller amounts given to the deterministic models to produce a middle of the road solution. ...Weather/Hazards Highlights... The front crossing the eastern U.S. on Friday will produce 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 coast will likely produce another relative focus for rainfall in the coastal Mid-Atlantic. 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. The potential for significant rainfall seems to be slightly higher on Day 5 (Saturday-Saturday night) as the coastal low moves up the Northeast coast. There is still uncertainty in the location of the heaviest rainfall amounts, which will depend on the track, intensity, and speed of the coastal low. Models do indicate that there will be at least some moist inflow from the Atlantic ahead of the system, which may enhance rainfall 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 on 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 and Upper Midwest by 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 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 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. The Northeast should see above normal temperatures (especially for morning lows) late this 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 the start of next week, followed by a return toward normal. 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 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