Extended Forecast Discussion NWS Weather Prediction Center College Park MD 105 AM EDT Mon Jul 16 2018 Valid 12Z Thu Jul 19 2018 - 12Z Mon Jul 23 2018 ...Overview... Confidence is now fairly high that the flow pattern across the contiguous U.S. will become more amplified during the medium range, with an upper-level ridge expanding from the Great Basin to the southern plains, and a series of upper waves gradually carving out a trough across the Great Lakes/Ohio Valley. A shortwave originating in the Gulf of Alaska will skirt the Pacific Northwest by late in the week, briefly lowering heights across that region, before moving east toward the northern plains/Upper Midwest by Sun-Mon. A look at hemispheric teleconnections shows support for this pattern evolution across North America, further increasing confidence in the impending pattern change. An anomalous and persistent Rex block across eastern Asia favors upper ridging across the southwestern and south central U.S., with a relatively strong vortex across the Arctic, and an equatorward extension in the form of favored troughing across Hudson Bay and the Great Lakes. Given the persistent nature of the aforementioned Rex block, would expect that the upcoming pattern shift across North America will not be a transient one. ...Guidance Evaluation and Preferences... Model spread was sufficiently low to justify use of a majority deterministic multi-model blend (including the ECMWF/GFS/CMC/UKMET) as a basis for the forecast starting on day 3 (Thu) and continuing through day 5 (Sat). During days 6-7 (Sun-Mon) weighting of ensemble means (ECENS/NAEFS) was gradually increased to account for an increase in spread during that time frame. Models continue to show some variability with the amplitude of shortwave energy crossing the Midwest Thu-Sat, although the trend seen in recent days toward more amplification of the feature seems to have slowed. In general, continue to favor the somewhat deeper ECMWF/CMC as a closed upper low develops by Fri-Sat, and this is reflected in the forecast blend. As the feature amplifies further across the Great Lakes/Ohio Valley Sun-Mon, the ECMWF/CMC continue to be the preferred solutions over the less amplified GFS. The past couple runs of the FV3 also showed a more amplified low over the Ohio Valley similar to the ECMWF/CMC, lending further credence to that scenario. Farther west, timing and amplitude differences remain with respect to shortwave energy crossing British Columbia and skirting the U.S. Northwest Fri-Sat. No clear outliers were evident, but in general a solution with timing resembling the ECENS mean was favored, and the described multi-model blend yielded such a forecast. By days 6-7 as the system crosses the northern plains, the ECMWF was perhaps a bit fast with the eastward progression of the system (the 12Z CMC may actually be the closest solution to consensus throughout the progression of this system across the northern tier). The heavier ensemble mean weighting by that time frame kept a forecast that is in line with the consensus for this system. ...Weather Highlights/Threats... A cold front expected to become stationary from the Southeast to the southern/central plains will focus scattered to numerous showers and thunderstorms from late in the week into next weekend, with areas of heavy rain possible. Farther north, the shortwave, and a developing associated surface low pressure system crossing from the northern plains/Midwest to the Ohio Valley will generate convection with locally heavy rain. The amplifying upper trough across the Great Lakes/Ohio Valley by next weekend could also result in the potential for transport of deeper moisture northward across the eastern U.S., with widespread convection and areas of heavy rain possible across portions of the Mid-Atlantic and Northeast by the weekend. Finally, monsoonal moisture will result in scattered afternoon/evening convection from the southern Great Basin to the southern/central Rockies through much of the week and into the weekend. The expanding ridge from the Southwest to the southern plains will result in rising temperatures by later in the week. High temperatures across the southern plains are forecast to be 5 to 15 deg F above average from Thu onward, with temperatures expected to soar past the 100 degree mark for many areas. A number of record high temperatures across the southern plains could be in jeopardy. 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