Extended Forecast Discussion NWS Weather Prediction Center College Park MD 130 AM EDT Tue Oct 30 2018 Valid 12Z Fri Nov 02 2018 - 12Z Tue Nov 06 2018 ...Pattern Overview and Guidance/Predictability Assessment... A mean upper-level trough is expected to persist across the central U.S. through the extended period. The trough will be reinforced by a series of shortwaves originating in the north Pacific, potentially interacting with additional shortwave energy of polar/arctic origin as they cross the Canadian Rockies before amplifying as they encounter the area of broad and persistent cyclonic flow across the central U.S. A series of frontal/low pressure systems will accompany these shortwaves across the CONUS, generally 1-2 days apart, resulting in a relatively active period of weather. Models are still struggling with the wave of low pressure crossing the eastern U.S. on what is now day 3 (Fri), with the GFS continuing to show a somewhat weaker and faster low pressure system and the ECMWF the slowest/strongest solution (with a fairly significant degree of ensemble support). Deterministic solutions have still shown a fairly large degree of variability looking back several runs, so believe that a compromise approach between the two extremes is still best. Even a compromise, however, will result in a significant slowdown of the surface low compared to continuity. Additional shortwave energy and an associated surface low will cross the central plains/Mississippi Valley on day 3, with yet another entering the northern plains by the weekend, but model solutions for these systems are closer on timing and intensity. Given these considerations, a multi-model deterministic blend (including the ECMWF/GFS/UKMET) was used as a basis for the WPC forecast during days 3-5. By next Mon-Tue (days 6-7), model spread continues to quickly increase. Ensemble guidance suggests that the amplified and blocky flow prevalent across the north Pacific early in the period may begin to trend toward a more zonal pattern, which introduces more chaos into the forecast, particularly by the end of the extended period. Nonetheless, the central U.S. trough appears likely to persist for at least a bit longer, with the arrival of additional shortwave energy continuing to lower heights by Tue. Model solutions differ quite substantially, however, with respect to the timing and amplitude of this feature along with another wave nearing the Pacific Northwest by this time. As a result, substantial weight was placed toward the ECENS and GEFS ensemble means during the day 6-7 time period. ...Weather Highlights/Threats... Rain may be ongoing at the beginning of the extended period on Fri across much of the eastern U.S. in association with a low pressure system/cold front. The Mississippi Valley will see the potential for rain to become more widespread late in the weekend into early next week as a couple frontal systems traverse the region and tap into Gulf of Mexico moisture. Heavy rain will be possible across the Pacific Northwest through the weekend and into early next week as upper shortwaves/surface fronts traverse the northern periphery of the upper ridge. These features will also bring rounds of snow to the higher elevations of the northern and perhaps the central Rockies. Chilly conditions will be prevalent through the weekend across much of the central U.S. and portions of the eastern U.S. beneath the persistent upper trough, where highs will generally be 5 to 10 deg F below average. Meanwhile, above average temperatures will persist across portions of the West, from California into the Great Basin, where high temperatures may reach 5 to 15 deg F above average through the weekend and into early next week. Ryan 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