Extended Forecast Discussion NWS Weather Prediction Center College Park MD 154 AM EDT Sun Jun 30 2019 Valid 12Z Wed Jul 03 2019 - 12Z Sun Jul 07 2019 ...Overview and Guidance/Predictability Assessment... Typical pattern for summer expected across the CONUS during the medium range. Downstream of a Rex block across the north Pacific, moderately progressive flow will pass between a building upper ridge across the CONUS southern tier, and a broad upper low in the vicinity of Hudson Bay. A wavy surface front will persist from the northern/central Plains to the Great lakes through much of the forecast period as a number of relatively small-scale shortwaves traverse the upper jet from the northern Rockies to the Great Lakes/Northeast. The front should be reinforced by the arrival of another polar front by midweek. Due to the establishment of the persistent Rex block and the resultant increase in stability of the large scale pattern, model solutions showed above average consensus at the synoptic scale through much of the medium range forecast period. A multi-model deterministic blend including the ECMWF/GFS/UKMET/CMC was used as a forecast starting point for days 3-6 (Wed-Sat). These solutions showed relatively modest timing and amplitude differences with shortwave energy moving from the Pacific Northwest eastward across the northern Rockies to the northern Plains as it the energy begins to shear and interact with additional energy rotating around the Hudson Bay upper low. By day 7 (Sun) the primary question remains how the upper flow will evolve across the northwestern U.S. downstream of the Rex Block. The GFS has consistently shown an upper trough persisting while the ECMWF develops a ridge axis. Teleconnections associated with the anomalous Alaska upper ridge support either flat flow or some degree of troughing across the Northwest, and this scenario is generally supported by the ensemble means. Thus trended to majority ECENS/GEFS ensemble means for day 7 to account for the increased uncertainty. ...Weather Highlights/Threats... The wavy surface front extending from the northern Rockies to the Great Lakes, and a second cold front arriving by the middle to latter portion of the week will focus areas of showers and thunderstorms across multiple days, with areas of heavy rainfall possible, especially from the northern/central Plains to the Upper Midwest. Scattered showers and thunderstorms will be possible farther east Wed-Thu, from the Southeast to the Ohio Valley and Mid-Atlantic, as a moist and unstable air mass remains in place. Storms may become more widespread across these areas by Fri-Sat as a shortwave crosses the Great Lakes pushing the surface cold front eastward. High temperatures are expected to be 5 to 10 deg F above average along the Eastern Seaboard through much of the period, with heat index values over 100 deg possible across much of the Southeast and southern Mid-Atlantic. High temperatures are expected to be 5 to 15 deg below average from the northern Rockies to the northern/central High Plains Wed-Sat due to widespread precipitation and the cooler air mass north of the polar front which should be moving southward across the region. Ryan 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