Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1159 AM EDT Sat Jul 14 2018 Valid 12Z Tue Jul 17 2018 - 12Z Sat Jul 21 2018 ...Overview... The flow pattern across the contiguous U.S. will become a bit more amplified during the medium range period. Expect two separate upper ridges, centered near California and the southern Plains as of early Tue, to merge with the more dominant western center tracking across the Four Corners and then settling over the southern Rockies/Plains (while also strengthening/expanding) by Fri-Sat. Meanwhile one or more upper waves originating from the eastern Pacific will be rounding the upper ridge at the start of the period and should gradually carve out a trough across the eastern U.S. A leading upper trough will cross eastern North America early-mid period. Heights aloft will fall across the Pacific Northwest and eventually eastward across the Northern Tier from midweek onward as a northeastern Pacific upper trough with embedded low digs southeastward and then continues eastward. ...Guidance Evaluation and Preferences... Most of the latest model/ensemble guidance agrees well with the overall evolution aloft during the period. Canadian ensembles are a bit of an exception as they are somewhat slower with the northeastern Pacific into Canada upper trough/low--counter to the general trend of the GEFS/ECMWF means toward slightly faster progression. Operational runs have varied on timing though, and the full envelope of GEFS/ECMWF members would suggest a significant degree of uncertainty on specifics across the western half of the northern U.S./southern Canada. There is also a reasonably high level of uncertainty in exactly how the Thu-Sat upper trough over the East will evolve with corresponding effects at the surface. Both the GFS and ECMWF easily fit into the expected mean pattern but the deep/closed 00Z ECMWF aloft generates a rather strong Great Lakes surface low by day 7 Sat while the 00Z-06Z GFS weak/open wave depiction yields a very diffuse surface pattern. Ultimately prefer an intermediate solution between these two extremes, which provides reasonable continuity while awaiting better guidance agreement. Overall a blend among the 00Z/06Z operational models represented consensus well for days 3-5 Tue-Thu. With the increasing detail uncertainty later in the period the forecast transitioned toward a blend of operational guidance with the 06Z GEFS/00Z ECMWF means and WPC continuity. ...Weather Highlights/Threats... Showers and storms with locally heavy rain will accompany a cold front from the Northeast/Mid-Atlantic/Appalachians to the Southeast Tue-Wed. The trailing end of the frontal boundary will likely stall from the Southeast into the southern/central Plains, which could provide an ongoing focus for locally heavy showers and thunderstorms through later next week. Monsoonal moisture will also produce scattered afternoon/evening thunderstorms from the southern Great Basin/Southwest to the southern/central Rockies through next week. Expect the best potential for locally heavy rain to be early in the week as the building upper ridge should gradually reduce the coverage/intensity of convection by late week/weekend. There is decent agreement in the guidance that shortwave energy emerging into the Plains should encourage some organized convection/rainfall over the northern half of the Plains Tue-Wed. However after that time the uncertainty in details at the surface and aloft holds confidence lower than desired for the rainfall forecast from the eastern Plains into the Great Lakes/Appalachians. The general evolution toward upper troughing should at least promote some increase in shower/thunderstorm activity. The combination of relatively cooler air behind the initial eastern U.S. cold front and clouds/rainfall ahead of the potential feature emerging over the Plains/east-central U.S. should lead to an area of below normal temperatures from the northern-central Plains into the East. Some locations may see one or more days with minus 5-10F anomalies. Hot temperatures across the Pacific Northwest (some highs greater than 10F above average over interior areas) should moderate by later next week as a Pacific cold front moves inland. Evolution aloft will keep warm mins over a large area covering the West and into the northern Plains, while the strengthening southern Rockies/Plains ridge aloft by Fri-Sat will lead to somewhat more focus on the southern Plains and vicinity for highest max temperature anomalies. Rausch/Ryan 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