Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1154 AM EDT Fri Jun 28 2019 Valid 12Z Mon Jul 01 2019 - 12Z Fri Jul 05 2019 ...Overview and Guidance/Predictability Assessment... Today's guidance continues to show a fairly typical summer pattern over the lower 48. The northern tier states will see progressive and low amplitude flow to the south of a central Canada upper low while ridging will broaden over the southern U.S. by late in the week after a southern into central Plains weakness fills/shears out. Energy within an upper trough near the Pacific Northwest coast should eject inland as a mid-late week amplifying Alaska ridge encourages shortwave energy to settle over/just offshore British Columbia. The combination of an active upper jet over the northern states and wavy fronts over the northern half of the country will promote multiple episodes of convection and locally heavy rainfall from the northern Plains through the Midwest and into parts of the Great Lakes/Ohio Valley. The most prominent model discrepancy still involves the upper trough initially near the Pacific Northwest coast. 00Z-06Z GFS runs are pretty much on their own in weakening the trough much more quickly than consensus that includes multiple GEFS mean runs. Established consensus brings the trough only gradually inland and suggests that pronounced ejection/weakening will occur only after early Thu. The majority cluster would keep heights across the northern Plains into the Great Lakes higher than GFS runs, thus keeping greatest convective potential farther north than the GFS. However the tendency for some convection to stray south of where modeled by global guidance does keep the window open for significant activity to extend south of the preferred non-GFS mass field cluster. Northern U.S. shortwave details, as well as specifics of exactly how Pacific Northwest energy ejects during the latter half of the week, have low predictability due to the features' small scale. The updated forecast uses input from the 00Z ECMWF/UKMET/CMC and 06Z GEFS mean Mon into early Wed and then transitions to a blend of the 00Z ECMWF/CMC and 06Z GEFS/00Z ECMWF mean. ...Weather Highlights/Threats... During most of next week a broad area from the northern Plains through parts of the Great Lakes and Ohio Valley should see multiple episodes of showers/thunderstorms with some activity possibly strong to severe and capable of producing locally heavy rainfall. Currently the best signal for highest rainfall totals over the five-day period exists over the Upper Midwest. Some of the rainfall from this activity may extend toward the East Coast. Shortwaves within progressive northern U.S. flow aloft and two surface fronts--one settling across the north-central U.S. and a second that drops from Canada into the northern Plains--will provide ample focus for these anticipated convective systems. Meanwhile areas from the western Gulf Coast into the southern-central Plains may see showers and storms aided by an upper level weakness and some low level inflow of Gulf moisture. There may be somewhat more focus for rainfall along and just inland from the Gulf Coast but otherwise confidence is fairly low for pinpointing greatest coverage/intensity. Expect diurnally favored convection over Florida and parts of the Southeast. High temperatures over the Southeast of 5-10F above normal may not challenge too many daily record highs but the air mass may be humid enough to produce some potentially dangerous heat index values. Max heat index values could reach at least 105F at some locations. Elsewhere areas from the northern High Plains into the southern Plains will tend to see slightly below normal highs during the period while temperatures over the northern half of the West Coast states should trend warmer late in the week as the Pacific Northwest upper trough weakens/departs. Much of the lower 48 should see near to above normal morning lows most days next week. Rausch 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