Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1159 AM EDT Tue Oct 02 2018 Valid 12Z Fri Oct 05 2018 - 12Z Tue Oct 09 2018 ...Stormy Pattern for the West/Rockies and a Central U.S. Excessive Rainfall Threat... ...Pattern Overview and Guidance/Predictability Assessment... The initial configuration of the flow will feature a shortwave sweeping across the north-central U.S. while being forced east-northeastward due to the presence of a strong downstream ridge. By the weekend, models show the synoptic-scale pattern becoming much more meridional in nature. Strong height falls diving through the Central Great Basin and eventually the Four Corners region will help bolster the eastern U.S. positive height anomaly. By Sunday evening, the 00Z GFS depicts 500-mb height anomalies around 3 to 3.5 sigma below climatology over the southwestern U.S. with opposite phased departures around 2 sigma over the mid-Atlantic. This becomes a rather stagnant set up into next week as the upper trough likely reloads with additional shortwave energy diving down from western Canada. For the most part, the available guidance is in reasonable agreement to start the period. By Saturday, the 00Z CMC becomes a slow, deep outlier with the surface cyclone traveling toward the lower Great Lakes into Ontario/Quebec. Toward the end of the weekend, some variance exists with the placement of the southwestern U.S. trough. Most notably, the 00Z UKMET is ahead of the pack which sits east of the stronger ensemble consensus. The biggest differences emerge early next week across the Pacific Northwest into British Columbia/Alberta as a system from the polar latitudes attempts to drop southward toward the continental U.S. The 00Z ECMWF generally keeps this disturbance confined to the Yukon Territory which has implications on the amount of modified Arctic air workings its way toward the international border with Canada. A majority of the guidance favor a tight pressure gradient setting up across the Pacific Northwest into the Intermountain West. The 00Z ECMWF backed off on this scenario which remains in stark contrast from the consensus. Otherwise, the pattern over the eastern U.S. seems fairly straightforward with a large positive height anomaly sitting over the Eastern Seaboard. Through Day 4/October 6, utilized a combination of the 06Z GFS/00Z GFS-parallel in conjunction with the 00Z ECMWF/UKMET. Thereafter, took away the 00Z UKMET given the progressive nature of the southwestern U.S. trough with a more balanced combination of operational and ensemble guidance. Toward the Day 6/7, October 8/9 timeframe, leaned a bit more toward the ensemble means with a slight preference toward the 06Z GEFS/00Z NAEFS ensemble means. Confidence sits above average with the synoptic-scale flow but the uncertainty sits more with the higher latitude flow interactions. ...Weather Highlights/Threats... Given the amplified set up in place, there will be a heavy rainfall threat across the middle of the country while a discernible temperature gradient separates the western and eastern U.S. With meridional flow evolving, tremendous poleward moisture fluxes are noted with 850-700 mb standardized anomalies around 4 to 5 sigma at times during Days 5-7, October 7-9. The multi-day nature of the threat could prove to create flash flooding issues in some portions of the central U.S. Too difficult to pinpoint exact locations but anywhere from the Southern/Central Plains up into the Middle Mississippi Valley/Upper Midwest will be in the threat area. Farther west, colder temperatures across the higher terrain should spread areas of snow over the region, particularly over the Northern/Central Rockies potentially into the adjacent High Plains. Regarding temperatures, while a majority of the western U.S. can expect below average temperatures, the largest departures should sit over the Northern/Central Rockies into the Northern Plains with numbers in the 20 to 30 degree range. By early next week, highs could struggle to get above freezing over areas of Montana and Wyoming. Meanwhile, from the Mississippi River eastward, high temperatures should be around 10 to 15 degrees above climatology with some chance for daily records for warm overnight minima. Rubin-Oster 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