Extended Forecast Discussion NWS Weather Prediction Center College Park MD 955 AM EDT Wed Jul 11 2018 Valid 12Z Sat Jul 14 2018 - 12Z Wed Jul 18 2018 ...Overview... Amplified flow should remain confined to the international border with Canada throughout the period. Broad long wave troughing will track from Alberta to the vicinity of Hudson Bay by Tuesday with the southern extension of height falls slipping across the Great Lakes and Ohio River valley. In its wake, renewed amplification is likely across western Canada into the Pacific Northwest by Day 6/7, July 17/18. Elsewhere, while an elongated upper trough lingers over the western Atlantic, an expansive 5940 meter 500 hPa ridge is likely to unfold over the Desert Southwest while a weaker anticyclone is expected across the southeastern U.S. ...Guidance evaluation and preferences... The guidance shows some issues with less predictable shortwave details, but is otherwise in good agreement. For pressures, 500 hPa heights, and winds, a compromise of the 00z Canadian, 00z UKMET, 00z ECMWF, and 06z GFS was used from Saturday into Monday, before increasing amounts of the 00z ECMWF ensemble mean and 00z NAEFS mean is used to replace the UKMET in the blend. The remainder of the grids are an even blend of deterministic and ensemble mean guidance to account for uncertainty. A compromise of the 12z National Blend of Models, 00z Canadian, 06z GFS, and 00z ECMWF is planned for the days 4-7 QPF. If the 12z GFS proves useful, it could be included into the QPF mix. ...Weather highlights/Threats... While many locations across the nation can expect temperatures near to above climatology, there are a few areas which will be on the cooler side. An active southwestern U.S. monsoonal circulation should keep these locations wet given daily thunderstorm chances with accompanying enhanced cloud cover. In a general sense, expected highs should be around 5 degrees below climatology. Meanwhile, Sunday should prove to be a rather cool day for July standards over the Northern Rockies into the adjacent Northern Plains as well as portions of Nebraska. 10 to 15 degree departures will keep highs in the 70s given cold advection in the wake of the trough passage. Regarding rainfall threats, as mentioned, the Four Corners region will remain wet due to monsoon considerations with activity gradually reaching the Central Great Basin and circulating into the Central Rockies. Farther east, widespread convection is anticipated along the slow moving front which will migrate from the middle of the nation toward the Eastern Seaboard from Day 4-7, July 15-18. There is also a signal for enhanced thunderstorm chances across the southeastern U.S. as high precipitable water air pools across the region. Roth/Rubin-Oster WPC medium range 500 mb heights, surface systems, weather grids, quantitative precipitation, winter weather outlook probabilities and heat indexes are found at: http://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif http://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif http://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html http://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml http://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 http://www.wpc.ncep.noaa.gov/heat_index.shtml