Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1131 AM EDT Sat Jun 15 2019 Valid 12Z Tue Jun 18 2019 - 12Z Sat Jun 22 2019 1530 UTC Update... A blend of the 00Z ECMWF/UKMET/GFS was used as a starting point for this forecast update during days 3-4 (Tue-Wed), with a trend toward heavier weighting of ECENS/GEFS ensemble means starting on day 5 (Thu) and continuing through day 7 (Sat). Models show relatively small differences with waves of low pressure moving along a quasi-stationary surface front from the Southern/Central Plains to the Ohio Valley and Mid-Atlantic during days 3-5. Ensemble members were generally well-clustered around the 00Z GFS though much of this period, and that solution was weighted slightly more than the other deterministic guidance during that time frame. By days 6-7, models/ensemble show general agreement that a broad upper trough amplifying across the Northwest/northern Rockies earlier in the period should move east into the northern High Plains, with a cold front and stronger height falls pressing into the north central U.S. by that time period. Models differ on the details on this system and the potential transfer of energy to a new surface low across the Upper Midwest by Sat, but despite that they show relatively similar timing for the front into the central U.S. Meanwhile, phasing of energy from northern/southern streams may result in development of a surface low off the New England coastline by Fri, which then should move quickly into the Canadian Maritimes by the weekend. Solutions vary with respect to this system and an ensemble-based forecast was considered best at this time. Ryan Previous Discussion (issued at 0526 UTC)... ...Overview and Guidance/Predictability Assessment... Latest guidance indicates a persistent southeast Canada trough with confluent flow south in the the MS Valley and Oh Valley to the northeast and mid Atlantic; allowing a frontal system to progress across these areas. Consensus shows robust strengthening of the Pacific ridge and embedded next week. Teleconnections from an anomalously strong high in the northeast Pacific lead to a downstream trough over the northern Plains with an embedded closed low near the US Canadian border through the middle of next week before retreating into southern Canada next weekend. Prefer a blended approach giving more weight to the ensemble means of the 12z ECMWF and 18z GEFS Ensemble Means, with less weighting to the operational 12z/18z GFS and 12z ECMWF, which was more amplified than the ECMWF ensemble members with the sf-700 mb wave crossing the mid Atlantic and off the coast Fri 21 Jun. The 12-00z GFS are on the amplified side of the guidance for the wave crossing from the mid MS Valley Wed 19 Jun across the Ohio Valley and then Great Lakes Thu and northeast Fri. The 00z GFS run strengthened the low and moved it faster off the New England coast 00z Sat. Given the large run to run swings and teleconnection signal of a low amplitude Great Lakes trough, using the mean was preferred. The models indicate a trough with an embedded closed low drifting east across the northern Plains/adjacent southern Canada next Wed 19 Jun-Fri 21 Jun, drifting northeast into Manitoba Sat 22 Jun. The 18-00z GFS were located further south with the closed low and indicated lower heights/temperatures across the northern Plains as a result. The 18z GEFS Mean and 12z ECMWF Ensemble Means continue to show better than average agreement over the northern Rockies and northern Plains, so more weighting was given to the Ensemble Means of the 18z GEFS and 12z ECMWF. ...Sensible Weather Highlights/Threats... The expected progression of waves of low pressure along a front continues to be favorable for multiple episodes of convection from the southern half of the Plains across the lower-mid MS Valley across the Ohio Valley/Tn Valley/ Appalachians/Mid Atlantic/Northeast. The Florida peninsula should see diurnal convection through the period. During Tue-Thu expect showers/storms to spread into the northern Plains/Upper Mississippi Valley with the system forecast to affect those regions, with another round of showers/storms near the low level front across eastern OK/AR/TN/northern MS/AL/GA. Next Fri-Sat 22 Jun the showers/storms may extend from OK across the mid-upper MS Valley into the Great Lakes and Ohio Valley, depending on the evolution of the available moisture and instability near the next frontal wave. Over the West expect broad coverage of above normal temperatures Tue in CA/OR/interior WA/ID with some pockets of greater than plus 10F anomalies centered on interior northern CA. WA and ID/northern OR cool with height falls in the middle of next week with interior California/Nevada as the primary area of warmth midweek. Areas across the northern Plains will tend to see below normal highs by as much as 10 degrees Wed 19 Jun through Sat 22 Jun. Petersen 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