Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Thu Oct 04 2018 Valid 12Z Sun Oct 07 2018 - 12Z Thu Oct 11 2018 ...Stormy Pattern for the West/Rockies and a Central U.S. Excessive Rainfall Threat... ...Pattern Overview and Guidance/Predictability Assessment... Guidance is overall well clustered with larger scale development of a highly amplified weather pattern for the nation this weekend well into next week. This pattern offers height anomalies of 2+ standard deviations above/below normal with a reinforcing western U.S. trough sandwiched between ridges over the eastern Pacific and the East Coast/Atlantic Ocean. A deepening/closed low sets up over the Great Basin region and multiple waves lift from the Great Basin/Southern and Central Rockies into the Plains where a deep layered southerly funnel of Gulf of Mexico moisture pools to 2+ standard deviations from normal into/over a slow moving baroclinic zone. However, deterministic models show significant run to run variance with the timing and emphasis of smaller scale systems embedded within the synoptic scale flow aloft and varied spin-ups of surface frontal lows. Accordingly, The WPC medium range product suite was primarily derived by blending the 18 UTC GEFS mean and 12 UTC ECMWF ensemble mean that show a strong and compatable pattern development signal. This maintains good WPC continuity. ...Weather Highlights/Threats... Highly unsettled/wet weather develops as a persistent and highly amplified trough aloft sets up and is reinforced over the West from the Great Basin/Southwest to the Rockies. Terrain/upslope flow snows spread down to focus over the northern Great Basin/Rockies/Plains, enhanced by a post-frontal cold high pressure surges. With the strong warming/blocking ridge over the East, waves along/over central U.S. fronts show little eastward progression until late period. This sets the stage for a prolonged period favoring multiple rounds of heavy rain/convection and ample training in a pronounced axis offers an extended threat of excessive rainfall. Warm sector temperatures south of the front will be summer-like, so a main frontal zone will remain quite defined. Regarding temperatures, well below average numbers are likely from the Upper Midwest back to the West Coast. The coldest air will reside across the Northern/Central Rockies and Plains with some departures from climatology upwards to 20 to 30 degrees. Highest elevations will struggle to get out of the 30s for highs while overnight lows plunge into the 20s. In stark contrast, it will feel like Summer in the vast warm sector with highs around 10 to 15 degrees above average, especially near the Great Lakes/Ohio Valley. Some daily records may be broken given the vast overnight warmth. Elsewhere, a deep layered easterly fetch off the Atlantic fortifies and persists into next week arcoss the Southeast, spreading showers across Florida and the Gulf Coast/ Gulf of Mexico underneath a northward shifting eastern U.S. closed ridge aloft. A Carribean based tropical disturbance may also work into the Gulf of Mexico, but models show a wide envelope of potential developments that is being monitored by WPC/NHC. Schichtel WPC medium range 500 mb heights, surface systems, weather grids, quantitative precipitation, winter weather outlook probabilities and heat indexes are found 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/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml