Extended Forecast Discussion NWS Weather Prediction Center College Park MD 157 AM EST Tue Dec 05 2023 Valid 12Z Fri Dec 08 2023 - 12Z Tue Dec 12 2023 ...Pattern Overview... Mean upper-level troughing will gradually swing east across the continental U.S. through the weekend, accompanied by a deepening surface low that will track from the southern Plains to the eastern Great Lakes. This system is forecast to bring moderate to locally heavy rain to areas east of the Mississippi this weekend. Heavy rainfall will also be possible in the Pacific Northwest as another surge of Pacific moisture moves into the region on Saturday ahead of an approaching frontal system. The upper-level mean trough will flatten out over the east early next week while ridging builds over the West Coast, which will lead to relatively benign weather. ...Guidance Evaluation/Predictability Assessment... Model guidance continues to show good agreement on the pattern in the beginning of the period, so a fairly even blend of the available deterministic solutions was used for the days 3-4 forecast. There are some significant timing differences between the models with the upper level wave that will support the deepening surface low this weekend. The GFS and ECMWF form lows of similar strengths over the southern Plains, but then the GFS pulls the low to the northeast much faster than the ECMWF. To get a middle of the road solution, ensemble means were added into the blend for the days 5-7 forecast. ...Weather/Hazards Highlights... After the ongoing atmospheric river event ends in the Pacific Northwest, there will be a brief break in heavy precipitation on Friday before another surge of moisture reaches the coast on Saturday. The region will be sensitive to additional rainfall after the first event, so a Marginal Risk of Excessive Rainfall was introduced for western Washington and Oregon and 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. The surface low that will deepen over the southern Plains Friday into Saturday will likely cause widespread showers from the Gulf Coast up through the mid-Mississippi Valley and into the Ohio Valley and southern Great Lakes region. Conditions will be favorable for thunderstorm development on Saturday from the Gulf Coast to the mid-Mississippi Valley, and the Storm Prediction Center has highlighted this area for a severe thunderstorm threat. As the low pushes northeast on Sunday, precipitation will spread to the East Coast and up into New England and thunderstorm chances will decrease. Locally heavy rainfall will be possible from the Mid-South to the Ohio Valley on Saturday and for parts of the Mid-Atlantic and Northeast on Sunday. At this time, uncertainty in the timing and location is still too high to highlight specific areas that may have a risk of flash flooding. Precipitation will come to an end in the East on Monday as the system pushes offshore, and high pressure will build into the Central U.S. and Southeast. Temperatures will be mild from the southern Plains to the Upper Midwest and Great Lakes on Friday, reaching 10-20 degrees above average for this time of year. Warmer air will shift into the Mid-Atlantic and Northeast over the weekend, and then temperatures will return to near normal early next week. Though not abnormal for this time of year, colder temperatures will persist across the Intermountain West through the period. 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