Extended Forecast Discussion NWS Weather Prediction Center College Park MD 206 AM EST Tue Nov 28 2023 Valid 12Z Fri Dec 01 2023 - 12Z Tue Dec 05 2023 ...Overview... An upper-level shortwave will lift from the Middle Mississippi Valley to the Northeast Friday into Saturday, while at the same time, a couple of short waves into the West act to amplify mean troughing which should translate eastward with time, reaching the East Coast by early next week. Upper ridging will build over the East this weekend and the West by early next week. This overall pattern would bring daily bouts of moderate to heavy rainfall to the Southeast as well as the Pacific Northwest. Widespread but lighter rain should spread up the East Coast as well next Monday into Tuesday. ...Guidance Evaluation/Predictability Assessment... The latest model guidance remains in very good agreement for the medium range time period, though still with plenty of uncertainty in the details. The greatest uncertainty continues with individual shortwaves into the West, acting to reinforce a building amplified trough over the region. There is some considerable run to run variability with timing/location of Western U.S. energies, but there is an overall consensus on a more amplified and active pattern. A possible atmospheric river looks to impact the West around Sunday-Monday time frame, and the ECMWF is by far the strongest, indicating a possible moderate to strong AR, according to the CW3E AR scale tool. The new 00z ECMWF came in just as strong (if not stronger) but about 12 hours faster. At this point, the ECMWF is a strong outlier, but the potential is there so this should be monitored. The guidance shows similar timing as the trough shifts into the East early next week, just with considerable differences in the details. For tonights WPC blend, a purely deterministic model blend was used for days 3-5, with increasing amounts of the ensemble means by days 6-7 (still only to about 50% of the overall blend) to account for the increased uncertainty late period. This maintains good continuity with the previous WPC forecast as well. ...Weather/Hazards Highlights... Upper-level shortwave energy will move over the Mid-South while a warm front lifts north out of the Gulf of Mexico late this week, and an amplified trough shifts out of the West this weekend providing a renewed surge of moisture. This will support daily widespread moderate to heavy rainfall for the Gulf Coast and Southeast. There is a Marginal Risk of Excessive Rainfall for this region valid Days 4 and 5 (Friday and Saturday) as anomalous moisture (PWs 2-3 standard deviations above normal) and instability will be present. It is possible an embedded slight risk will be needed at some point but there is still some uncertainty on exact amounts. The heavy rainfall threat across the Southeast will linger into Sunday as well, with less intense rainfall also lifting northward up the East Coast on Monday. 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 Friday-Saturday and possibility for accumulating snows in the higher elevations. The heaviest precipitation (coastal rain and mountain snow) will be across the Washington/Oregon coast and Cascades Sunday into Monday, where some models indicate a potential atmospheric river to impact the region. Modest snowfall looks to impact parts of the northern Rockies as well this weekend into early next Week. 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 with well above normal temperatures (on the order of +10-15F) returning to the North-Central Plains next Tuesday. Santorelli 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