Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1047 PM EDT Fri Aug 17 2018 Valid 12Z Tue Aug 21 2018 - 12Z Sat Aug 25 2018 ...Pattern Overview... An amplified pattern -- with notable positive 500 hPa anomalies near western Canada and across Atlantic Canada -- is expected into August 22. Thereafter, the western Canadian anomaly moves westward across the Northeast Pacific which should cause some retrogression to the upper trough from the Great Lakes towards mid-continent by next Saturday. The guidance agrees on these ideas. The bigger issues involve the surface low near the mid-continent trough next Saturday, where the 18z GFS was more westward and the 12z Canadian more eastward. Overall though, this issue is rather minimal for a day 7-8 forecast. ...Model Preferences... Since the guidance was general agreeable, used a compromise of the 12z Canadian, 12z ECMWF, 12z UKMET, and 18z GFS through the period for 500 hPa heights/pressures/fronts/wind grids, with slowly increasing percentages of the 12z NAEFS mean and 12z ECMWF from Thursday onward which eventually make up 45% of the blend. For the other grids (weather, clouds, precipitation chances, dew points), a more even blend of the above deterministic and ensemble guidance was used. The days 4-7 QPF were an even blend of the 12z ECMWF, 18z GFS, 12z Canadian, 00z National Blend of Models, and 18z in-house bias corrected ensemble QPF. ...Weather Highlights and Hazards... The developing system tracking from the Plains through Great Lakes into eastern Canada as well as the trailing cold front will likely produce a broad area of showers/thunderstorms--some with heavy rainfall--from the eastern Plains/MS Valley into the eastern U.S. Tuesday into Wednesday. Rainfall/convection may also focus at times along a leading front that initially extends from the western Atlantic back to the surface low. Passage of the cold front will bring a drying trend to many areas in the eastern half of the country but areas from the Mid-Atlantic to the extreme Southeast and along the Gulf Coast may see rainfall persist or return late in the week as the front stalls. Meanwhile the trough/upper low initially near the West Coast may produce some scattered rainfall over northern parts of the West during the first half of the week. Then some showers/storms may emerge over the northern Plains/MS Valley later in the week as the shortwave energy ejects eastward. Most precipitation with the upper trough arriving from the Pacific late in the week should remain north of the Canadian border. Meanwhile the Four Corners states should continue to see periods of diurnally enhanced convection. The most extreme temperature anomalies during the period will likely be of the negative variety, with cool temperatures over the northern-central Plains during the first half of the week, though no records on the cool side are expected. Some locations may see one or two days with highs around 10-15F below normal. This cool air will modify as it progresses eastward, though refreshingly low dew points -- the lowest in 3-4 weeks -- are expected to infiltrate the Mid-Atlantic States. The West will tend to see above normal min temperatures (near record warm values) most of next week but with above normal highs tending to be early in the week. The overall evolution toward less amplified flow aloft over much of the lower 48 by late in the week should lead to many areas seeing temperatures within a few degrees of normal by next Friday and Saturday. Roth/Rausch 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