Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1207 PM EDT Sun Aug 18 2019 Valid 12Z Wed Aug 21 2019 - 12Z Sun Aug 25 2019 ...Overview... Deep low pressure pushing into southeast AK Tuesday night amplifies a ridge up the Canadian Prairies which in turn shunts a deep low pressure system on the southern shore of Hudson Bay on Wednesday across Quebec through Friday. Shortwave energy rounding this eastern Canadian low brings a trough across the Great Lakes Wednesday night into Thursday and across the northeast/New England Thursday night. A cold front ahead of this trough shifts east from MI Wednesday and off the New England coast Thursday. The shortwave energy associated with the Pacific system will move into western Canada and the Pacific Northwest Wednesday and serve as the leading edge of a fairly zonal flow regime extending from the North Pacific (south of mean troughing over the Gulf of Alaska through the period) across the northern tier states and provincial Canada that should cross the Great Lakes Saturday. Weak troughing persists over the Northeast U.S. into the weekend. Broad Ridging over the southwest states persists through the period. Meanwhile there is a moderate clustering of guidance that shows upper level energy drifting north through the western Gulf of Mexico which is on pace to reach the Gulf Coast Friday night. ...Guidance/Predictability Assessment... The guidance blend for Days 3-5 emphasizes operational guidance (00Z ECMWF/06Z/00Z GFS/00Z UKMET/00Z 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 maintains uncertainty. 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 blend for days 6 and 7 increases 06Z GEFS/00Z ECMWF mean weight (60 percent total by day 7/Sunday) as models/ensemble members continue to differ for specifics within the main band of westerlies. Relatively small scale keeps confidence low for the western Gulf energy aloft while the forecast maintains continuity for the possible surface reflection. ...Weather Highlights/Threats... A focus for precipitation will be the cold front that pushes into the East and Great Plains late week and settles over the Southeast, with the trailing part of the front extending back into the central Plains and returning northeast as a warm front. The best signals for highest rain totals along the front are currently over southern half of the Mid-Atlantic and the central Plains centered on KS/MO. 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 directs a weakening atmospheric river into northern parts of the Pacific Northwest though uncertainty remains with the magnitude of precipitation. One or more frontal systems looks to produce scattered convection over the northern plains. 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 some potential for reaching daily records any of the days. The cold front pushing into the East/South will provide a cooling trend after Wednesday. The northern/central Plains will see a couple days of below normal highs before warming over the weekend. Jackson 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