Extended Forecast Discussion NWS Weather Prediction Center College Park MD 416 PM EDT Tue Jul 02 2019 Valid 12Z Fri Jul 05 2019 - 12Z Tue Jul 09 2019 1600 UTC Update... Minor changes were needed for the 16Z update across the CONUS with a typical summertime pattern expected to continue through the medium range period. Relatively progressive flow is expected for the northern tier of the U.S. with a shortwave crossing across the Northern Plains through Friday, then eventually through the Eastern US this weekend. By early next week, shortwave energy bottled up around Vancouver Island is likely to eject into the Northern Rockies with ridging building back through the central U.S. A multi-model deterministic blend was used for Days 3-4 followed by lesser weight of the 00Z CMC/UKMET for Day 5, with more emphasis on the ECMWF/ECENS and GFS/GEFS toward the latter portion of the forecast period. Model differences were mainly centered on how progressive the flow becomes by Day 6-7 (Mon-Tues). The GFS was much more progressive while the ECMWF/ECENS a bit slower with shortwave energy coming across the northern tier. A blended approach was favored for now and maintains a similar approach from continuity. Taylor/Campbell ...Overview and Guidance/Predictability Assessment... Typical summer pattern 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 near Hudson Bay. A shortwave crossing from the Northern Plains Fri to the Great Lakes/Northeast Sat-Sun is expected to push a cold front southward through the central U.S. and eastward across the Midwest/Great Lakes, Ohio Valley, and eventually off the Eastern Seaboard. The trailing end of this frontal boundary is expected to stall from the Southern Plains to the Southeast, lingering into early next week. Meanwhile, shortwave energy diving south on the eastern periphery of the Alaska ridge is expected to reach the Pacific northwest by the weekend, moving east across the northern Rockies through early next week. A blend of the ECMWF/GFS/CMC was used as a forecast starting point during days 3-4 (Fri-Sat). The CMC dropped starting on day 5 (Sun) as it became slower than consensus with the shortwave energy moving into the Pacific Northwest. Otherwise, these solutions handled the shortwave and cold front crossing the central/eastern U.S. during that time frame similarly. During days 6-7 (Sun-Mon) weighting of ECENS/GEFS ensemble means was gradually increased to comprise a majority of the forecast weighting. Model differences persist as to how quickly shortwave energy in the Pacific Northwest/northern Rockies will move eastward into the Plains, with the ECMWF consistently a bit faster than the GFS. Additional shortwaves moving south across central Canada on the western side of the mean upper low, and potential interactions with the shortwave energy across the northern CONUS further reduce predictability, lending to the greater use of ensembles by that time period. ...Weather Highlights/Threats... Numerous showers and thunderstorms with potentially heavy rainfall are possible from the northern High Plains to the Upper Midwest from Fri into the weekend as the surface front sags southward across the central U.S. and several waves of low pressure traverse the boundary. Showers and storms may also increase in coverage across the central/southern Plains by Sat-Sun as the surface front arrives in the vicinity. Scattered to numerous showers and thunderstorms are also expected across much of the eastern U.S., increasing in coverage by the weekend as the cold front moves east into the region. A cooler air mass is expected to move into the north central U.S. in the wake of the cold front, with highs forecast to be 5 to 15 deg F below average from Fri into the weekend, before temperatures begin to moderate. Meanwhile, the Northeast will initially see high temperatures 5 to 10 deg above average Fri-Sat before decreasing to near normal values behind the cold front by Sun. 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 Hazards: - Heavy rain across portions of the northern and central Plains, Fri-Sat, Jul 5-Jul 6. - Heavy rain across portions of the central Plains, and the coastal area of Georgia, Tue, Jul 9. - Heavy rain across portions of coastal North Carolina, Sun-Mon, Jul 7-Jul 8. - Flooding occurring or imminent across portions of the central Plains, the middle Mississippi Valley, the lower Mississippi Valley, the southern Plains, and the Ohio Valley. - Flooding likely across portions of the upper Mississippi Valley and the northern Plains. 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