Extended Forecast Discussion NWS Weather Prediction Center College Park MD 415 PM EDT Thu Sep 21 2023 Valid 12Z Sun Sep 24 2023 - 12Z Thu Sep 28 2023 ...Potential Tropical Cyclone Sixteen to Bring Heavy Rainfall to Mid-Atlantic and Northeast This Weekend... ...Overview... Strong upper ridging is forecast to meander in place in an omega/Rex block configuration between Hudson Bay and the Upper Midwest/Corn Belt/Mid-MS Valley. On its southeast side, a frontal complex will move into the MS Valley with heavy rain and severe weather chances. On the East Coast, an upper low and developing coastal low (potentially sub-tropical) will be lifting north/northeastward this weekend with a rainfall shield mainly just affecting parts of the Mid-Atlantic to New England by the start of the period on Sunday. Across the North Pacific, a strong upper jet arcing across the Aleutians will promote digging of troughing southeast of the Gulf of Alaska and west of the Pacific NW this weekend before translating eastward/inland early next week. ...Guidance Evaluation/Predictability Assessment... Forecast guidance has trended westward and generally slower with the newly formed Potential Tropical Cyclone Sixteen off the Southeast coast. This system is now forecast by the National Hurricane Center to track generally northward through eastern North Carolina into the Mid-Atlantic Saturday into Sunday. While guidance has clustered better overall since yesterday, there remains some timing and track differences late Sunday through Monday with how fast the system gets pulled away from the Northeast coast. Overall, a slower trend was preferred based on the latest NHC track and 12Z guidance. Elsewhere, there are some notable differences in the large scale pattern with the blocky pattern evolving that features a ridge south of Hudson Bay and the approaching shortwave energy from the PacNW and northern Rockies. Most of the 12Z guidance slows this feature down and forms a closed low at 500 mb over the Dakotas early next week with general troughing slowly moving east/southeast through days 7 while a slow moving front eventually washes out/stalls over the Mid-South by days 6-7. Meanwhile, the latest model guidance is in pretty good agreement for a strong trough to push onshore the Pacific Northwest that will start a rather unsettled and active period of precipitation for the region. The WPC forecast blend this cycle generally used a blend of the latest European and GFS for Days 3-5 followed by higher weights of the ECENS and GEFS means for Days 6-7 which trended well from the previous forecast and was largely consistent with the PTC 16 track from the NHC. ...Weather/Hazards Highlights... Potential Tropical Cyclone Sixteen is forecast to be along the Mid-Atlantic coast Sunday morning and the higher tropical moisture associated with it will likely bring rounds of moderate to heavy rainfall to portions of the Mid-Atlantic through the Northeast and southern New England. This heavy rainfall may bring instances of flash flooding Sunday through Monday morning as the storm system tracks along the Northeast coast. Continue to monitor the latest information on PTC 16 from the National Hurricane Center for all additional weather hazards. A slow moving frontal system across the central U.S. will act as the focus for areas of showers and thunderstorms late this weekend into early next week. Some locally heavy rainfall will be possible, particularly across portions of the ArkLaTex region and Southern Plains where interactions with greater instability and moisture may help fuel stronger storms. The Day 4 Excessive Rainfall Outlook highlights a Slight Risk across the ArkLaTex while a Marginal Risk remains for Monday further south into Texas and Louisiana. The associated frontal boundary will stall and wash out across the Southern U.S. into next week and may act as the focus for additional showers and thunderstorms though confidence on details and locations is low at this point. An early season storm system will bring unsettled and active weather to the Pacific Northwest beginning late this weekend and through early next week. A few rounds of heavier precipitation are likely, favored for the coastal regions and upslope areas of the Washington and Oregon Cascades into northern California where some localized flooding may be possible with repeating rounds of moderate rainfall. Below normal temperatures out West should moderate by Sunday as an initial upper low shifts east in the central CONUS. Elsewhere, temperatures could be moderately above normal across parts of the southern Plains Sunday into Monday. By Tuesday and beyond though, much of the country should be near normal or within a few degrees of normal. The one exception will likely be along the West Coast (esp NorCal northward into the northern Great Basin) where incoming rainfall may keep daytime highs at least 5-10 degrees below normal. Taylor 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 California, and the Pacific Northwest, Mon, Sep 25. - Heavy rain across portions of the Middle/Lower Mississippi Valley, and the Southern Plains, Sun-Mon, Sep 24-Sep 25. - Heavy rain across portions of the northern Mid-Atlantic, and the Northeast, Sun, Sep 24. - Hazardous heat across portions of the Southern Plains, Sun, Sep 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