Extended Forecast Discussion NWS Weather Prediction Center College Park MD 142 PM EST Mon Nov 27 2023 Valid 12Z Thu Nov 30 2023 - 12Z Mon Dec 04 2023 ...Overview... An upper-level shortwave initially located over the Southwest on Thursday will travel across the Midwest and into the Northeast by this weekend. Periods of moderate to heavy rainfall will accompany this feature and are forecast to impact parts of the South, Southeast, and Mid-Mississippi Valley. Meanwhile, a couple quick-moving shortwaves will move into the West late this week, promoting amplified mean troughing that shifts from the West into the Central U.S. this weekend into early next week. As the pattern amplifies, upper ridging will build downstream over the East Coast. This setup will favor wet, unsettled weather in the West and bring another round of potentially heavy rainfall to the Gulf Coast and Southeast over the weekend. ...Guidance Evaluation/Predictability Assessment... Model guidance is in very good agreement on the upper-level pattern for the medium range time period. The highest uncertainty continues to surround activity moving into the West late in the period. There is a significant amount of run to run variability with the timing/location of the western shortwaves, but there is an overall consensus on a more amplified and active pattern. For the afternoon forecast package, a purely deterministic blend was used for days 3-5 with slightly more weight on the GFS and ECMWF than the CMC and UKMET. For days 6-7, the deterministic GFS/ECMWF were blended with their ensemble means to smooth out some model differences. ...Weather/Hazards Highlights... Upper-level shortwave energy will move over the Mid-South while a warm front will lift north out of the Gulf of Mexico late this week, which will support widespread moderate to heavy rainfall in the South and Lower Mississippi Valley. There is a Marginal Risk of Excessive Rainfall for this area on Thursday-Thursday night. Models have consistently shown the heaviest rainfall amounts in the vicinity of the eastern Texas Gulf Coast and western Louisiana Gulf Coast, which may be where the highest risk of flash flooding will be located. Heavy rain may linger across the Central Gulf Coast and into the Southeast on Friday, and there is a Marginal Risk of Excessive Rainfall for this area. Amplified troughing shifting out of the West this weekend will provide a renewed surge of moisture and support widespread moderate to heavy rainfall across the Southeast. Out West, precipitation will increase Thursday through the weekend as a couple shortwaves move through the region, with some precipitation reaching southward into California and the Southwest. The heaviest precipitation (coastal rain and mountain snow) will be across the Washington/Oregon coast and Cascades next weekend with generally lighter amounts farther inland. The interior West should stay cooler than normal underneath reinforced troughing through the weekend. Temperatures for the rest of the country should be closer to normal late this week, but trend warmer with time across the Northern Plains into the Midwest and eventually the East by the weekend. Daytime highs could be 5-10 degrees above average across this region. The Western U.S. may finally trend back towards normal, or above normal in some places, early next week as the trough moves east and ridging builds back in. Santorelli/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