Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1154 AM EDT Mon Aug 19 2019 Valid 12Z Thu Aug 22 2019 - 12Z Mon Aug 26 2019 ...Overview/Guidance/Predictability Assessment ... The days 3-7 fronts/sea level pressures/500 mb heights were derived from a blend of the 06z GEFS Mean and 00z ECMWF Ensemble Mean (with the 00z NAEFS clustering well with these solutions), with less weighting on the operational 00-06z GFS and 00z ECMWF. ...Eastern US... From late this week into the weekend expect an upper level trough to cross the Great lakes and northeast, with the trough departing either late Sunday or early next week. The leading cold front reaching the East as of early Thu will likely stall over the Southeast while the trailing part of the front over the Plains returns as a warm front (though with slower multi-day trends). For the northeastern upper trough there is decent consensus in principle. However shortwave differences between models and in consecutive runs lead to some spread/variability in specifics of low pressure forecast to track just north of New England and the trailing wavy front that sinks through the Mid-Atlantic into the Southeast. The 00z ECMWF was an outlier in closing a 500 mb low and developing a stronger wave of low pressure along the front in the mid Atlantic. The vast majority of ECMWF ensemble members are more progressive in moving the trough off the coast and don't develop a 500 mb low, which is a rare occurrence in Aug. Consequently, the ECMWF Ensemble Mean is preferred over the operational 00z run. ...Western US... Guidance is starting to converge on the evolution of the 500 mb trough crossing the northern Rockies on Thu and northern Plains on Fri, deamplifying as it moves east towards the upper MS Valley Sat. The 00z Canadian global was a high amplitude outlier, with the good clustering of the GFS/ECMWF and their respective ensembles preferred. Low amplitude mean flow should prevail from the North Pacific into western-central parts of the northern CONUS/southern Canada, with a trend toward some troughing moving out of the northern Rockies this weekend and into the northern Plains early next week. The latest ECMWF/CMC ensemble means have been more in agreement with their depiction of the trough relative to the latest GEFS mean runs that are somewhat slower, so this aspect of the forecast remains to be resolved. Here the intermediate NAEFS forecasts would be helpful. The ECMWF/GFS/Canadian have waves at the base of the trough in the central Plains to the upper MS Valley that differ in timing and location early next week, so predictability of that aspect of the forecast is low. ...Weather Highlights/Threats... The front dropping through the Mid-Atlantic into the Southeast continues to serve as a likely focus for areas of showers and thunderstorms, with potential for highest totals along and just inland from the southern Mid-Atlantic/Southeast coast. A separate wave along the trailing front in the Plains will result in other clusters of showers/storms in parts of the central/south-central Plains into Mississippi Valley. Moisture return from the Gulf and possible low level trough should enhance showers/storms in the Gulf Coast this weekend and into the southeast early next week. Areas from the northern Plains east into the Mississippi Valley may see one or more episodes of convection with fronts reaching the region. Expect above normal temperatures to be most persistent from southern Oregon and the northern two-thirds of California into the southern High Plains with upper ridging in place during the period. Scattered locations within this area may see max and/or min temperatures at least 10F above normal. The central/northern Plains into Great Lakes seeing the best opportunity for below normal highs from Thu into Sat, moderating thereafter. Petersen Additional 3-7 Day Hazards information can be found on the WPC medium range hazards chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, winter weather outlook probabilities and heat indices are 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