Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 AM EST Wed Dec 06 2023 Valid 12Z Sat Dec 09 2023 - 12Z Wed Dec 13 2023 ...Pattern Overview... Broad cyclonic mean flow is forecast to persist over the eastern two-thirds of the lower 48 through most of the medium range period. Initially, a deep upper-level shortwave will emerge from the West late this week, spawning a surface low over the southern Plains on Friday that will track to the northeast this weekend. Significant precipitation is forecast from the Gulf Coast to the Great Lakes as this system progresses, which could cause flooding concerns. Meanwhile, the Pacific Northwest will be dealing with another surge of Pacific moisture on Saturday that may result in scattered instances of flooding. Early next week, an amplifying Pacific ridge will help to reinforce the mean trough over the east, but with a positive tilt that will lead to more benign weather. ...Guidance Evaluation/Predictability Assessment... The most impactful model differences continue to concern the low pressure system that will track from the southern Plains to the Northeast this weekend. Models are in fairly good agreement on the timing of the low on Saturday, but notable differences arise as it approaches the Northeast on Sunday and Sunday night. The UKMET and the GFS are the most progressive with the system while the ECMWF is on the slower side, and the CMC falls in between. The ensemble means favor an in between solution, so a general deterministic model blend was used for days 3-4. Model guidance spread is as expected through the rest of the period. There is a general agreement on the mean trough becoming positively tilted with some differences in timing of shortwaves circulating through. To smooth out differences, the GEFS and EC ensemble means were added to the blend for days 5-7. ...Weather/Hazards Highlights... Another surge of Pacific moisture is forecast to impact the Pacific Northwest on Saturday, and the region will be particularly sensitive to any additional rainfall after receiving significant rainfall earlier in the week. The Day 4 Excessive Rainfall Outlook continues to show a Marginal Risk of Excessive Rainfall for western Washington and Oregon into far northwestern California with an embedded Slight Risk area along the coast. In addition to heavy rainfall, widespread moderate to locally heavy snow will be possible in the higher elevations of the Cascades and northern Rockies as moisture pushes inland. Precipitation will lighten up significantly across the region on Sunday. The surface low that will soak the eastern U.S. this weekend will likely produce significant rainfall from the Gulf Coast to the Great Lakes and interior Northeast. A Marginal risk area was maintained on the Day 4 Excessive Rainfall outlook from the Gulf Coast to parts of the Mid-Mississippi and lower Ohio Valleys, and another Marginal risk area was introduced in the Day 5 Excessive Rainfall Outlook from the Florida panhandle through the Mid-Atlantic and into the interior Northeast. The progressive nature of this system will help to limit rainfall totals; however, anomalous moisture and sufficient instability should still support rain rates high enough to produce some flooding concerns. In addition to potential flooding, thunderstorms will likely threaten areas along the Gulf Coast and the Lower Mississippi Valley on Saturday. The Storm Prediction Center has highlighted an area centered on East Texas and the Ark-La-Miss where severe thunderstorms are expected. Some snow will be possible in the northwestern side of the precipitation shield from the Central Plains/Midwest to the Great Lakes, but details are currently uncertain due to lack of guidance agreement for surface low track. Some snow may also fall after system passes, mainly over the Great Lakes into Appalachians. Finally, brisk to strong winds are forecast to develop over portions of the Rockies and Plains as the pressure gradient tightens around the low on Saturday. Overall, the lower 48 will trend drier through the rest of the period, but there are some lower precipitation chances (20-40%) that will accompany a couple of relatively weaker frontal systems moving across the nation. In terms of temperatures, much of the West will experience below normal temperatures in the wake of the upper-level shortwave on Saturday, and well above normal temperatures are expected in the warm sector ahead of the developing Plains-Great Lakes system. Highs should be 10-25 degrees above normal from the Lower Mississippi Valley to the Northeast this weekend. Temperatures will return to near normal after the system passes, and near normal temperatures (or temperatures within a few degrees of normal) will persist across most of the nation through early next week. 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