Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1154 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... For the medium range period (Sun - Thurs), the guidance remains overall well clustered with the larger scale development of a highly amplified pattern showing a reinforcing western U.S. trough (with 2+ standard deviation height anomalies) sandwiched between 2 ridges over the eastern Pacific and the East Coast/Atlantic Ocean. Within the western trough, a deepening/closed low sets up over the Great Basin with multiple waves lifting from the Great Basin/Rockies into the Plains. A deep southerly funnel of anomalous Gulf of Mexico moisture pools along a slow moving frontal boundary, leading to multiple days of heavy to excessive rainfall. After about days 4-5, the deterministic models continue to show fairly significant run-to-run variance with timing and amplitude of smaller scale systems entering and reinforcing the large scale trough, which leads to the spin-up of numerous surface frontal lows. Accordingly, this cycle of the WPC medium range surface progs were based on a mainly deterministic blend for days 3 and 4 (between the 06z GFS/00z ECMWF), transitioning to a majority 06Z GEFS/00Z ECENS blend by the end of the period in order to mitigate and smooth out the more unpredictable small scale features. This blend maintains good WPC continuity as well. ...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 post-frontal cold high pressure surges. With the strong warming/blocking ridge over the East, waves along/over the Central U.S. fronts show little eastward progression until late in the period. This sets the stage for a prolonged period favoring multiple rounds of heavy rain/convection, ample training, and an extended excessive rainfall threat. 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 across the Southeast, spreading showers across Florida and the Gulf Coast/ Gulf of Mexico underneath a northward shifting eastern U.S. closed ridge aloft. Models show a possible tropical disturbance in the Gulf of Mexico, with a wide envelope of potential developments that is being monitored by WPC/NHC. Santorelli/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