Extended Forecast Discussion NWS Weather Prediction Center College Park MD 332 PM EDT Wed Oct 26 2022 Valid 12Z Sat Oct 29 2022 - 12Z Wed Nov 02 2022 ...Heavy rain/snow threat for the Northwest/Rockies Sunday through early-mid next week... ...Weekend closed low heavy rain threat from the Lower Mississippi Valley through the Southern Appalachians... ...Guidance and Predictability Assessment... The global model guidance was in reasonable agreement regarding the evolution of the synoptic pattern throughout the medium range. A slightly more progressive trend in the pattern was realized since the overnight forecast. This translated into notably less qpf for parts of the Northwest and Southeast/Southern Appalachians. A general model blend consisting of the 00z EC/UK/CMC and 06 GFS was applied through day 4 since they all captured the weakening closed low's progression through the Lower Mississippi and Tennessee Valleys fairly well. The 00z ECE and 06z GEFS means were introduced to the blend on day 5 to account for uncertainty revolving around the evolution of a piece of shortwave energy moving toward southern California/Baja peninsula. The ensemble means make up a majority of the blend by days 6 and 7 due to uncertainty surrounding an upper trough which will approach the Pacific Northwest. The ensembles themselves have quite a bit of spread with respect to this trough. ...Pattern Overview and Weather/Hazard Highlights... The details with a potent series of Pacific systems to work into the West/Northwest this weekend and through early-mid next week remains more run to run varied than normal in recent model guidance, but the compatible GEFS/ECMWF ensemble means offer a increasingly strong signal for a pattern with long fetch moisture to fuel periodic moderate to locally enhanced precipitation from northern California and the Pacific Northwest inland across the north-central Great Basin/Rockies. This will offer heaviest rainfall potential for the coastal ranges/Olympics and Cascades of the Pacific Northwest along with a signifciant threat for terrain enhanced elevation snow set to also spread early-mid next week into the mountains of northern California inland across the north-central Great Basin/Rockies as depicted in the WPC Winter Weather Outlook probabilities in NDFD. Meanwhile, an intially closed/separated southern stream system is slated to lift from the southern Plains through the Ohio Valley this weekend. This will act to focus across the South enhanced rain/convection given upper and surface system support to pool moisture/instability to fuel activity. There is some guidance signal for locally moderate to heavy rainfall given the risk of repeat/training cells. While locally heavy rainfall is possible this weekend, not rates are not expected to be substantial enough to overcome high flash flood guidance. Nevertheless, also expect a swath of moderately heavy rainfall to lift across the Ohio Valley/Appalachains and across the Eastern Seaboard through early next week with the lead system and with rejuvinating backside upper trough energy feed that may spawn modest coastal/frontal genesis and moisture from the Atlantic in addition to lingering Gulf moisture. Kebede/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 Hazards: - Heavy precipitation across portions of the Pacific Northwest and the Northern Great Basin, Sun-Mon, Oct 30-Oct 31. - Heavy rain across portions of the Southeast, the Lower Mississippi Valley, and the Tennessee Valley, Sat, Oct 29. - Flooding occurring or imminent across portions of the Southeast. WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, experimental excessive rainfall outlook, winter weather outlook probabilities and heat indices 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