Extended Forecast Discussion NWS Weather Prediction Center College Park MD 201 PM EST Tue Nov 28 2023 Valid 12Z Fri Dec 01 2023 - 12Z Tue Dec 05 2023 ...Wet pattern for the Northwest to include heavy snow threats inland from the Cascades to the north-central Intermountain West/Rockies... ...Overview and Guidance Evaluation/Predictability Assessment... An active and tricky weather pattern is upcoming through medium range time scales, with larger scale flow now set to transition from overall split flow to one with more northern stream dominated positive PNA pattern in about a week. Most guidance is actually quite agreeable with respect to eventual positive PNA development to be mainly comprised of an amplified upper ridge over western North America and an amplified upper trough over east-central U.S.. However, guidance get there in different ways, showing significant run to run and model to model differnences from the weekend into early next week with flow embedded systems aloft and especially the often complex surface system reflection. Accordingly, there is quite a bit of uncertainty with sensible weather local and even regional foci. In particular, there is more than normal differences with even mid-larger scale areas of precipitation over the east-central U.S., but also in the wet pattern to/inland from the Northwest. Seemingly heavy NBM QPF amounts were overall lessened given uncertainties and as machine learning tools shed further doubt on mid-later forecast period QPF. Accordingly, the WPC medium range product suite was primarily derived from a composite of best clustered guidance of the 06 UTC GFS/GEFS mean, 00 UTC ECMWF/UKMET/ECMWF ensemble mean, The 13 UTC National Blend of Models (NBM) and WPC continuity for Friday into early Saturday to smooth out the embedded feature variances consistent with a pattern with seemingly less than average predictability. Opted to blend the GEFS/ECMWF ensemble means onward through the weekend into early next week along with input from the NBM. ...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 may linger into Sunday as well, with rainfall also lifting northward up the East Coast into Monday, albeit all with high uncertainty at these longer time frames. Even so, there does seem to be several uncertain opportunities for heavy rain over the Southeast to monitor along with moderate swaths of northern periphery snow into the Northeast and viciity. Stronger cyclogenesis with expected northern steam amplifcation into next week with aforementioned positive PNA pattern development may then also lead to widespread/larger scale precipitation to emerge. Out West, precipitation will increase Thursday through the weekend as a couple dynamic shortwaves move through the region, with some precipitation reaching southward into California by 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 to heavy snowfall looks to periodically impact parts of the north-central Intermountain West/Rockies as well this weekend into early next week. Santorelli/Schichtel 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