Extended Forecast Discussion NWS Weather Prediction Center College Park MD 129 AM EDT Fri Aug 17 2018 Valid 12Z Mon Aug 20 2018 - 12Z Fri Aug 24 2018 ...Pattern Overview and Preferences... Guidance is consistent in showing an evolution toward an amplified eastern US trough while mean ridging persists over the southwest and southern Great Basin to the southern high plains. Gradual deamplification of the pattern over the northeastern Pacific should ultimately lead to shearing and ejection of a trough from the weekend into early next week, with the arrival of a succeeding trough in the Pacific northwest centered on early Fri 24 Aug. Low pressure developing in the Plains and moving towards the Great Lakes is quite strong for August. Operational ECMWF/UKMET runs have tended to be on the deeper side of the envelope with the surface low. The depth in some of those runs would be sufficient to equal or exceed lowest sea level pressure records for the month of August. The operational GFS runs have generally been on the weaker and slower side of the spread. Overall favor an operational 12z ECMWF/12z Canadian model/12z ECMWF ensemble mean/12z NAEFS Mean compromise, but not as deep as the 12z ECMWF/12-00z UKMET given the possibility that feedback from convection is deepening the lows too much. This consensus also addressed remaining north-south spread in the location/timing of the low track. There has been fairly good agreement/continuity regarding the trailing front reaching the East Coast/Gulf Coast by midweek. Over the West Coast/Northwest the models and ensemble means are coming into better agreement over the evolution of the upper trough that arrives around the start of the period, possibly develops a closed low by Mon, and eventually ejects/shears out with the approach of upstream trough energy. Thus expect ejecting West Coast energy to be fairly weak by Wed-Thu. The models/ensembles are converging on the arrival o the next eastern Pacific trough into the Pacific northwest/northwest CA centered on early Fri 24 Aug, with better than average agreement. A blend among the 12Z ECMWF/ECMWF Ensemble Mean/12z CMC/18z GEFS Mean/12z NAEFS Mean was used for the height and adjusted sea level pressure forecasts, as well as gridded temperature, dewpoint, etc. forecasts. Greater weighting towards the means was used on day 7 Fri as the operational GFS and Canadian were on opposite ends of the timing spectrum from the northern Rockies across the northern Plains Fri 24 Aug with better agreement among the NAEFS/ECMWF/18z GEFS ensemble means. ...Weather Highlights and Hazards... The developing system tracking from the Plains through Great Lakes into eastern Canada will likely produce a broad area of showers/storms from Sun to Tue from the Plains into the Great Lakes/Ohio Valley. Expect the front trailing from the surface low to generate some showers/thunderstorms into the lower MS Valley and TN Valley across the southern Appalachians to the southeast coast as well. The possibility of a wavy front stalling off the mid Atlantic coast could lead to a pronged shower threat next Thu 23 Aug. The southwest will likely see diurnally favored convection. Portions of the northern/central Plains should be within the core of coolest daytime temperatures versus normal (minus 10-15F anomalies) from Sun-Tue. Departure of Great Lakes low pressure after Tue and southward/eastward progression of the trailing cold front will bring a cooling trend into the eastern states but with smaller anomalies. Expect warmest highs versus normal to be over parts of the Pacific Northwest with some readings locally 10-15F above normal Mon-Tue. Once the next Pacific trough comes onshore next Thu 23 Aug-Fri 24 Aug the northwest cools while downstream warm advection produces a surge of warmth into the northern Plains, resulting in an area of temperatures several degrees above normal. Petersen 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