Extended Forecast Discussion...Amended NWS Weather Prediction Center College Park MD 647 PM EDT Mon Jul 01 2019 Valid 12Z Thu Jul 04 2019 - 12Z Mon Jul 08 2019 ...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 wavy surface front initially across the north central U.S. is forecast to weaken before another polar front overtakes it by later this week. An upper trough skirting the Great Lakes/Northeast should push the cold front eastward, eventually off the Eastern Seaboard by the weekend. Meanwhile, another shortwave should reach the Pacific Northwest by the weekend before moving east across the northern Rockies into early next week. A multi-model deterministic blend was used for the first half of the medium-range period then a near equal blend of the 00Z ECMWF/GFS and their respective ensemble means was used. The biggest model differences were seen by Day 6 and 7 with respect to energy coming out of a Northwest US trough. The 00Z ECMWF featured a more progressive flow pattern which resulted in a stronger surface high in its wake across the Northern Plains and Upper Midwest by Day 7. Conversely, the 00Z GFS/GEFS (and to some degree the UKMET) showed a slower shortwave progression with a surface low developing over the western Great Lakes. Using a multi-model blend of the ECMWF/GFS and their ensemble means resulted in a slight slowing trend for the northern US surface high. Meanwhile, a cold front was adjusted further south from the Mid-Atlantic region to the lower Ohio Valley on Day 7 (Monday). ...Weather Highlights/Threats... Numerous showers and thunderstorms with potentially heavy rainfall are possible from the northern High Plains to the Upper Midwest from Thu 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 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 Thu into the weekend, before temperatures begin to moderate. Meanwhile, the Northeast will initially see high temperatures 5 to 10 deg above average Thu-Sat before decreasing to near normal values behind the cold front. Taylor/Rausch/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 from across portions of the northern Plains into the upper Midwest, as well as over parts of North and South Carolina, Thu-Fri, Jul 4-Jul 5. - Flooding occurring or imminent across portions of the central and southern Plains, the lower and middle Mississippi Valley, the lower Ohio Valley, as well as a small part of the central Appalachians. - Flooding likely across portions of the central Rockies, the central Plains, and the upper Mississippi Valley. - Excessive heat across portions of the Southeast, Thu, Jul 4. - Enhanced wildfire risk across portions of the the Alaska Panhandle and mainland Alaska, Thu-Mon, Jul 4-Jul 8. - Excessive heat across portions of southern mainland Alaska, Thu-Mon, Jul 4-Jul 8. 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