Extended Forecast Discussion NWS Weather Prediction Center College Park MD 117 AM EDT Sun Jul 15 2018 Valid 12Z Wed Jul 18 2018 - 12Z Sun Jul 22 2018 ...Overview... 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. The intensifying upper ridge farther south will help ensure that any such height falls in the Northwest are short-live, however. ...Guidance Evaluation and Preferences... Model spread was sufficiently low to start the forecast on day 3 (Wed) with a multi-model deterministic blend (including the GFS/ECMWF/CMC/UKMET). The trend in the guidance noted in recent days toward more amplification of features seems to have continued. Model consensus is now good with respect to the upper trough initially across the Great Lakes on Wed, which lifts out across New England on Thu. The aforementioned trend also applies to shortwave energy crossing the northern plains/Upper Midwest Wed-Thu and then amplifying further across the Great Lakes/Ohio Valley on Fri (day 5). Models continue to vary with respect to the amplitude of this feature, but given the trends, cannot rule out the more amplified solutions (such as the CMC and UKMET). Thus, the multi-model deterministic blend continued to serve as a basis for the forecast through day 5. During days 6-7 (Sat-Sun), the primary questions become how much amplification occurs with the eastern U.S. trough, as well as to what degree and for how long heights fall across the Pacific Northwest with the shortwave passing across western Canada. Given the increasingly amplified flow across the CONUS, opted to lean a bit toward greater amplification of the trough across the Great Lakes/Ohio Valley. The GFS has been among the least amplified solutions, but the FV3 is much more amplified, similar to the ECMWF and CMC. The GFS (prior to the 00Z run) has also been among the less amplified solutions with the western Canada energy. Thus, the forecast during days 6-7 was based on a blend of deterministic/ensemble mean solutions, including the ECMWF/CMC along with the ECENS/NAEFS means. ...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 midweek into next weekend. Areas of heavy rain will be possible, with guidance continuing to show a relatively strong signal for heavy rain across portions of the Southeast, from the coastal Carolinas to northern Florida. Farther north, the shortwave crossing the northern plains/Midwest Wed-Thu will generate convection with locally heavy rain. This feature should eventually generate a new wave along the aforementioned surface front, which is forecast to cross the Midwest/Great Lakes Fri-Sat, producing additional convection and locally heavy rains. The amplifying upper trough across the Great Lakes/Ohio Valley by next weekend could also result in the potential for latitudinal transport of deep tropical moisture along the Eastern Seaboard. A couple deterministic model solutions indicate the potential for widespread convection and areas of heavy rain across portions of the Mid-Atlantic and Northeast by late in the week into the weekend. Ensemble probabilities suggest that confidence in this scenario is relatively low at this time, however. 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. Convective activity should gradually become more sparse in nature through the week as the upper ridge strengthens overhead. 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 well past the 100 degree mark across much of Texas. 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