Extended Forecast Discussion NWS Weather Prediction Center College Park MD 258 AM EDT Wed Aug 21 2019 Valid 12Z Sat Aug 24 2019 - 12Z Wed Aug 28 2019 ...Overview and Guidance/Predictability Assessment... Models and ensemble means continue to show the establishment of a mean trough aloft over the northern CONUS, between an eastern Pacific ridge that amplifies into the northeastern Pacific and another ridge that drifts across eastern Canada. Within this mean trough guidance becomes increasingly divergent for individual shortwave specifics by the latter half of the period. Farther east models/ensembles are still having some difficulty resolving the details of energy within an initial eastern Canada/northeastern U.S. trough which should progress eastward according to the majority cluster. At more southern latitudes ridging aloft should prevail over the southern half of the West while a developing upper weakness over the Lower Mississippi Valley and western Gulf Coast will likely split (the southern part retrograding and the northern part lifting out to the east/northeast). From the weekend into the start of next week guidance agrees decently for the gradually amplifying northern tier mean trough aloft, with Pacific shortwave energy nearing the Pacific Northwest/British Columbia coast as of early Sat leading to a sharpening trough over and just east of the northern Plains by day 5 Mon. As this occurs the models and means develop strengthening low pressure along/north of the U.S.-Canadian border with cold front extending south/southwest. As this trough lifts northeastward after Mon, there is considerable spread/decreasing confidence for specifics of the northeastern Pacific ridge and downstream energy that drops into the northern tier mean trough. A key piece of the puzzle involves a shortwave forecast to round the ridge Mon or later. The 12Z ECMWF looked particularly suspect as its shortwave dampened the ridge more than other guidance around Tue--not quite an outlier relative to the full ensemble spread but very close. The ensemble means were similar to each other and provided the most reasonable starting point given operational model differences and run-to-run variability, suggesting a shortwave reaching the northwestern U.S. by day 6 Tue and continuing eastward thereafter. There is still a minority cluster consisting of the 12Z ECMWF/latest UKMET runs which show energy with the Northeast upper trough closing off a slow-moving low. ECMWF runs have been inconsistent with the prior cycle having no upper low while the two runs before that had the low. Not enough ensemble members from ECMWF/GFS/CMC show an upper low for the means to have any more than an elongated trough, with the latter depiction likely the best option for a single deterministic forecast at this time. Based on the above considerations the updated forecast started with an operational model blend on day 3 Sat and then gradually decreased ECMWF input and increased ensemble mean weight. By days 6-7 Tue-Wed the primary emphasis was on the 18Z GEFS/12Z ECMWF means. The blend retained a minority component of the 18Z GFS due to reasonable comparison with the means, though it did trend a little fast with the northern tier trough by late in the period. ...Weather Highlights/Threats... There is a continued threat for heavy rainfall over areas from the northern half of the Plains into the Great Lakes as upper troughing becomes established over the northern tier states. From the weekend into early next week the primary surface focus will be with a cold front that pushes into the northern Plains as well as a leading warm front. As upstream energy feeds into the mean trough aloft, the Plains to Great Lakes part of the front may stall for a period of time by next Tue-Wed and provide an opportunity for training convection or repeat activity. Some convection may be strong to severe during the period so check Storm Prediction Center outlooks as the details become more clear in future days. The area encompassing the central Gulf Coast through the Southeast and southern Appalachians/southern Mid-Atlantic will also see the potential for areas of heavy rainfall with a combination of ample moisture, one or more upper level impulses, and the wavy surface front settling over the Southeast. Diurnal convection over the Florida Peninsula may produce locally significant totals as well. While some of the specifics of flow aloft by the middle of next week have yet to be resolved, leading up to that point confidence is increasing for a significant cooling trend over the northern Plains where some locations may see multiple days with highs of 10-15F or so below normal. On the other hand areas from southern Oregon/northern-central California into the southern High Plains should see above normal readings for most of the period with pockets of plus 10-15F anomalies and possibly a few daily records. Cooler air may reach the southern Rockies/High Plains by next Wed. Near to below normal temperatures over the eastern half of the country during the weekend should trend toward near or slightly above normal readings by next Tue-Wed. Rausch 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