Extended Forecast Discussion NWS Weather Prediction Center College Park MD 203 AM EDT Fri Jun 28 2019 Valid 12Z Mon Jul 01 2019 - 12Z Fri Jul 05 2019 ...Overview and Guidance/Predictability Assessment... Relatively progressive northern stream flow will be squeezed in between a building ridge across the southern half of the CONUS, and more strongly blocked flow at higher latitudes, from the North Pacific across Canada. This will keep an active upper jet in place from the Pacific Northwest to the Midwest, Great Lakes, and Northeast through the medium range. A surface front will persist from the northern/central Plains to the Great Lakes through the forecast period along the northern periphery of the upper ridge, reinforced by another polar front by the middle of next week. A broad weakness in the upper ridge is expected to initially be in place across the southern Plains, with little in the way of movement until perhaps a stronger shortwave traverses the northern Plains/Midwest by late next week. Model consensus was sufficient through much of the forecast period to employ a multi-model blend of deterministic guidance (ECMWF/GFS/UKMET/CMC) as a basis for the WPC forecast. The largest differences among the guidance occurred by days 6-7 (Thu-Fri) with respect to a rather complex series of shortwaves diving south through western Canada and into the Pacific Northwest before ejecting east toward the northern Plains. Models showed significant differences on both timing and amplitude of these features, as well as with any potential interactions with the broader upper low near Hudson Bay and the aforementioned upper weakness across the central U.S. Given increased spread, the forecast by days 6-7 was shifted heavily toward a blend of ECENS/GEFS ensemble means, given the overall agreement of the means at larger scales. ...Weather Highlights/Threats... The persistent surface front extending from the north central U.S. to the Great Lakes will serve to focus areas of showers and thunderstorms across multiple days, with locally heavy rainfall possible. Showers and storms could spread east across portions of the Appalachians and Mid-Atlantic regions by the middle to end of next week. Farther south, the upper weakness will also focus scattered showers and thunderstorms across portions the southern/central Plains, especially by the middle to latter portion of next week as moisture inflow from the Gulf of Mexico increases. Meanwhile, high temperatures of 5 to 10 deg F above average across the Southeast and southern Mid-Atlantic through much of the forecast period will combine with a relatively humid air mass to produce potentially dangerous heat index values over 100 degrees. 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