Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1200 PM EDT Wed Jul 25 2018 Valid 12Z Sat Jul 28 2018 - 12Z Wed Aug 01 2018 ...Heat wave lingers over the West... ...Heavy rainfall threat to spread from the Central Plains to Mid-South then the Southeast/Eastern U.S... ...Pattern Overview/Guidance Evaluation/Preferences... Some impressive mid-level height anomalies appear across North America and the surrounding oceanic basins during the period. All such features remain around 1.5 to 2 sigma above/below average, respectively. A pair of 594-dm ridges will inhabit the southwestern U.S. as well as the western Atlantic with the latter retrograding back toward the Gulf Stream. Sandwiched in between this pair of positive height anomalies will be another cut-off low meandering about the Middle Mississippi Valley into the mid-South early to mid next week. Into the higher latitudes, a rather deep upper low sinking southward from Nunavut could be primed to carry lowering heights to just south of the international border with Canada depending on the model. Otherwise, a weak longwave trough may position itself off the Pacific Northwest coast early Monday before gradually sliding inland the subsequent days. Generally speaking, the hemispheric pattern remains rather intact with ensemble spaghetti spread on the lower end. Over the weekend, most of the issues loom upstream across the northeastern Pacific as the 00Z ECMWF ensembles favor a much flatter ridge. While the pair of upper ridges inhabiting the southwestern U.S. and western Atlantic are well agreed upon, differences continue in between with spatial/temporal uncertainties shown in the guidance. Beginning with the upper trough drifting from the north-central states toward the mid-South, a slow evolution is supported although the 00Z UKMET is a western outlier. Eventually forward progression is stunted as downstream ridging continues to shift toward the west. The 06Z GFS diverges from the pack as well as its own ensemble mean tracking lower heights into the lower Great Lakes by Day 7/August 1. Farther to the north, likely the most contentious sector of the continent as discernible amplitude differences are noted by as early as Day 5/July 30. With substantial height rises depicted across far eastern Alaska into the Yukon Territory, a more amplified flow regime is to be expected downstream. In spite of similar ridge strengths across this region, the 00Z GEFS/ECMWF ensembles depict completely different downstream patterns. The 00Z GEFS mean, and to some degree the 00Z NAEFS mean, favor more significant amplification while over 90 percent of the 00Z ECMWF ensembles stay much flatter. This would have an impact on whether a cold front with associated precipitation threats could cross into the northern tier by the middle of next week. Weighted the forecast toward operational guidance through Day 4/Sunday with most emphasis on the 06Z GFS/00Z ECMWF. Thereafter, moved toward a 50/50 approach of ensembles vs. operational solutions with model spread rising. Given mentioned differences across the higher latitudes, leaned a bit more toward the 06Z GEFS/00Z NAEFS ensemble means. ...Weather Highlights/Threats... With cold advection continuing across the middle of the country, well below average temperatures are anticipated from Saturday into Monday from the Northern Rockies eastward into the Northern/Central Plains as well as the Middle Mississippi Valley. Over the weekend, expect highs to remain in the 70s which would be around 10 to possibly 15 degrees below average. Gradually this cool anomaly will migrate east of the Mississippi River by next week although numbers should be slightly closer to climatology. On the converse, forecast highs should be around 5 to 10 degrees above average over the Pacific Northwest through Monday before the longwave trough approaches from the offshore waters. While an upper ridge will remain fixed over the Desert Southwest, readings in the lower 110s would only be around 5 degrees or so above late July climatology. A band of moderate to heavy rainfall will accompany the slow moving upper trough with the main axis initially over the Central Plains into the Ozarks. Convection should gradually push eastward along the primary cold front with plenty of warm sector thunderstorms further downstream. By the early to middle part of next week, the squeeze play between the nearly stalled upper low across the mid-South and the blocking downstream ridge could bring more heavy rainfall across the eastern third of the U.S. 850-700 mb moisture flux anomalies begin to exceed 3 sigma by the end of the period. Farther west, monsoonal activity should not be terribly widespread with a majority of the action focusing over the Southern Rockies perhaps back into Arizona depending on the model. 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