Extended Forecast Discussion NWS Weather Prediction Center College Park MD 249 AM EDT Sat Jun 29 2019 Valid 12Z Tue Jul 02 2019 - 12Z Sat Jul 06 2019 ...Overview and Guidance/Predictability Assessment... Relatively progressive northern stream flow will be squeezed between a building upper ridge across the southern half of the CONUS, and increasingly blocked flow at higher latitudes, from the North Pacific and Alaska east across Canada. This will keep an active upper-level jet in place from the Pacific Northwest to the Midwest, Great Lakes, and Northeast through the medium range. A wavy surface front will persist from the northern/central Plains to the Great Lakes through much of the forecast period along the northern periphery of the upper ridge, and will be reinforced by the arrival of another polar front by the middle of next week. A broad weakness in the upper ridge is also expected to initially be in place across the southern Plains, with little in the way of movement until a somewhat more significant shortwave traverses the northern Plains/Midwest by late next week. Model consensus was sufficient initially to use a multi-model deterministic blend (ECMWF/GFS/UKMET/CMC) during days 3-5 (Tue-Thu). Models showed timing differences with shortwave energy digging into the Pacific Northwest during this time frame, and a consensus approach was preferred. By days 6-7 (Fri-Sat), model differences continue to gradually increase as the aforementioned shortwave moves east across the northern Plains/Midwest, potentially interacting with additional shortwave energy diving south around the broad upper low near Hudson Bay. Additionally, as a blocking upper ridge strengthens across Alaska by late next week, this should favor some degree of troughing/negative height anomalies downstream of the ridge across western Canada and perhaps the Pacific Northwest, but models are highly variable in their solutions. Given the spread, leaned heavily toward ECENS/GEFS ensemble means by days 6-7, with a bit more weight toward the GEFS by day 7 as it showed an upper pattern more consistent with what would be expected given teleconnections from the Alaska ridge. ...Weather Highlights/Threats... The wavy surface front extending from the northern Rockies to the Great Lakes, and a second cold front arriving later in the week, will focus areas of showers and thunderstorms across multiple days, with heavy rainfall possible. Showers and thunderstorms should spread east across portions of the Appalachians and Mid-Atlantic regions by later next week. Meanwhile, high temperatures of 5 to 10 deg F above average across the Southeast and southern Mid-Atlantic will combine with a fairly humid air mass to produce potentially hazardous heat index values over 100 degrees through much of next week. 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