Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM EDT Sun Aug 18 2019 Valid 12Z Wed Aug 21 2019 - 12Z Sun Aug 25 2019 ...Overview and Guidance/Predictability Assessment... Guidance continues to show a fairly vigorous upper trough crossing the northeast quadrant of the lower 48 after a short-range northeastern Pacific system helps to amplify the flow to its east. The shortwave energy associated with the Pacific system will move into North America after midweek and serve as the leading edge of a flow regime that has low amplitude mean flow extending from the North Pacific across the northern tier states and southern Canada. An initial Rockies-Plains upper ridge should weaken/retrograde a bit during the period. By next weekend there is some suggestion of at least flat ridging that builds off the West Coast--which teleconnections say should eventually lead to a mean trough over the eastern half of the country. Meanwhile there is a moderate clustering of guidance that shows upper level energy drifting north through the western Gulf of Mexico and reaching the Gulf Coast by the latter half of the period. During the first half of the period a primary emphasis on operational guidance (12Z ECMWF/18Z GFS/12Z UKMET/12Z CMC in order from most to least weight) provides a reasonable starting point for the forecast. Within the eastern upper trough there are still smaller scale detail differences that will affect progression of the leading cold front and which will take more time to be resolved. There are also meaningful differences for the leading energy that progresses into the northern U.S./southern Canada, with a blend approach providing the most stable solution while awaiting any confident adjustments. The rest of the period increases 18Z GEFS/12Z ECMWF mean weight (60 percent total by day 7 Sun) as models/ensemble members continue to differ for specifics within the main band of westerlies. The majority guidance cluster shows a trough reaching the northern Plains and vicinity by next Sun, a reasonable lead-in to the eastern trough that teleconnections say should ultimately develop in response to the positive height anomalies that recent D+8 multi-day mean charts show off the West Coast. Relatively small scale keeps confidence low for the western Gulf energy aloft while the forecast maintains continuity for the possible surface reflection--a trough per 17Z WPC-NHC coordination. ...Weather Highlights/Threats... One focus for convection will be the front that pushes into the East mid-late week and settles over the Southeast, with the trailing part of the front extending back into the Plains and returning northeast as a warm front. The best signals for highest totals along the front are currently over southern half of the Mid-Atlantic and the central Plains. The energy aloft/surface reflection expected to lift northward over the Gulf may bring areas of heavy rainfall to parts of the central-western Gulf Coast by the weekend. The rest of the Southeast/Gulf Coast region should see diurnal convection with greater influence from localized boundaries. A cold front approaching/reaching the Northwest Wed-Thu should bring a brief period to northern parts of the Pacific Northwest. One or more frontal systems may produce scattered convection over the far northern tier. Expect most of the West and southern High Plains to see above normal temperatures during the period. Anomalies may reach or exceed plus 10F at some locations over the West with the best potential for reaching daily records around midweek. The cold front pushing into the East/South will provide a cooling trend after Wed. The northern/central Plains will see a couple days of below normal highs early in the period followed by parts of the Great Lakes/Northeast. Locations to the south should be close to normal by Fri-Sat. The Great Lakes/Ohio Valley may see a warming trend by Sun though. 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