Extended Forecast Discussion NWS Weather Prediction Center College Park MD 244 AM EDT Sat Aug 17 2019 Valid 12Z Tue Aug 20 2019 - 12Z Sat Aug 24 2019 ...Overview and Guidance/Predictability Assessment... Guidance agrees that some amplification will likely occur downstream from a system forecast to be over the northeastern Pacific as of early day 3 Tue. The upper trough that ultimately crosses eastern Canada and northeast quadrant of the lower 48 should push a cold front to the East Coast by the latter half of the week, with the front stalling as it reaches the South. The trailing part of the front will likely extend back to the High Plains and then move east/northeast as a warm front. Progressive Pacific flow extending into the northern CONUS/southern Canada should bring a front into the Northwest and then the extreme northern Plains. To the south of the main band of westerlies, expect initially strong Rockies/Plains ridging to weaken gradually after midweek. Within the agreeable pattern evolution there is some lingering spread for specifics of the upper trough amplifying into/crossing the northeastern U.S., timing of leading height falls associated with incoming Pacific flow, and then the character of energy reaching the Northwest/northern Plains late in the period. Recent trends toward a more amplified Northeast trough seem to favor leaning away from the flatter 12Z GFS/CMC for that part of the forecast while the 12-18Z GFS/12Z CMC become faster than most other solutions for upstream energy crossing southern Canada and northern tier U.S. The 00Z GFS has backed off from previous runs' fast timing of southern Canada flow, a trend supported by other 00Z guidance. Ensemble spread by late in the period and the low amplitude of expected mean flow yield low confidence in shortwave details across the Northwest/northern Plains by Fri-Sat. In differing ways the 12Z and new 00Z GFS runs show more troughing over the Northwest versus the majority of guidance so it may be reasonable to downplay their exact solutions. They are still within the full ensemble envelope though. Based on forecast considerations the updated forecast started with an operational model blend early in the period (18Z run for the GFS component) and then increased 18Z GEFS/00Z ECMWF mean input mid-late period so that ensembles comprised 70 percent of the blend by day 7 Sat. ...Weather Highlights/Threats... There may be pockets of locally heavy rainfall with the cold front moving from the Plains through the East but frontal progression should keep totals from being too extreme. However the central Plains may have a little more potential for areas of enhanced rainfall due to the trailing end of the front stalling over the High Plains and then returning as a warm front. Eventual deceleration of the front over southern latitudes of the East may lead to more rainfall over parts of the southern Mid-Atlantic/Southeast as well. Expect diurnal convection with generally localized heavy amounts over the central/eastern Gulf Coast region. The front reaching the Pacific Northwest Wed-Thu may bring a brief period of rainfall to northern areas. The cold front pushing through the central/eastern states will bring a noticeable cooling trend, with some highs up to 10F or so below normal over the northern Plains into midweek. The hot weather ahead of the front should persist over central U.S. into Tue and over the East into Wed. A majority of the central-eastern U.S. should see temperatures within a few degrees of normal by Fri-Sat. Expect above normal readings to prevail over most of the western U.S. through the period. The greatest anomalies (with some record highs possible over the Southwest) should be on 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