Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1150 AM EDT Sun Jul 08 2018 Valid 12Z Wed Jul 11 2018 - 12Z Sun Jul 15 2018 ...Pattern overview/Guidance evaluation/Preferences... Latest models and ensembles continue to show a broad area of mean ridging across the central and western U.S., with the possibility of an embedded weakness for a time in the vicinity of the central Rockies/High Plains. To the east of the ridge expect an upper trough to support a fairly far southward extent of a cold front that should eventually stall and dissipate, while over the Atlantic the upper trough should encourage northeastward progression of Tropical Storm Chris. There has been a stray solution or two (such as the 00Z UKMET) that show greater effects on the East Coast but thus far such a scenario has very low probability. Consult NHC products for latest information on T.S. Chris. As the eastern upper trough lifts away, guidance is hinting that some weak energy could be left behind near the East Coast along with a modest lingering surface reflection consisting of a trough with one or more embedded waves. Meanwhile the consensus forecast led by the ensemble means has maintained good continuity in principle for two vigorous Canadian systems whose fronts will extend into the northern third to half of the contiguous U.S. Compared to yesterday guidance clustering has improved for North Pacific/Alaska flow compared to yesterday so latest operational runs are generally closer to the means from the northeastern Pacific into western North America in the late week/weekend time frame. By day 7 Sun the only items of note are that the 00Z GFS leans toward the amplified side of the envelope for northern U.S. troughing aloft while the 00Z ECMWF/CMC bring more shortwave energy into the West than most other solutions. Given the better than average agreement in today's guidance, the latest forecast blend utilized a majority weight of operational models (00Z-06Z GFS and 00Z ECMWF/CMC) versus 00Z GEFS/ECMWF means through day 5 Fri and then trended toward an even model/ensemble blend among GFS/ECMWF runs and their means. The 00Z GEFS was selected over the 06Z run due to better definition of the second Canadian system by day 7 Sun but otherwise the 06Z run provided a comparable forecast. ...Weather threats/highlights... Expect three general areas of focus for significant rainfall during the period. Over and just west of the Four Corners states an active monsoonal flow (with precipitable water values well above climatological averages) will promote periods of showers and thunderstorms with locally heavy amounts possible. The area extending from the northern Plains into the Midwest/and upper Great Lakes will first see episodes of convection along a leading cold front crossing the Northern Tier. This front may continue to serve as a focus for rainfall as it stalls over the north-central Plains/MS Valley, with more activity to follow as the front lifts back to the north and a second wave/cold front approach by next weekend. Meanwhile locations over the South will see diurnally enhanced convection with some enhancement provided by localized boundaries and the eastern U.S. front that should dissipate by the latter half of the period. Clouds and rainfall should keep high temperatures below normal over the Southwest/Four Corners region through the period while the second northern U.S. front will bring a push of cool air into the northern Plains by Sat-Sun. Otherwise the ridge aloft over the central/western U.S. will tend to keep readings above to well above average. The interior Pacific Northwest and parts of California/Nevada are likely to see highest anomalies exceeding plus 10F for min and/or max readings on multiple days. Surface high pressure behind the eastern U.S. front will bring a couple days or so of near or slightly below normal temperatures. Expect eastern U.S. temperatures to trend somewhat above normal by next weekend. Rausch 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