Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1200 PM EDT Mon Jun 25 2018 Valid 12Z Thu Jun 28 2018 - 12Z Mon Jul 02 2018 ...Overview with guidance evaluation/preferences... Model guidance this morning generally agrees that a warm subtropical high will gradually become established in the eastern U.S. while a broad trough will remain the dominant feature over the western U.S. by this weekend. However, notable uncertainties are indicated over the past day or so regarding specifics of how/when western trough energy ejects downstream, leading to uncertainties with the evolution of surface features over the northern and central U.S. by the Sat-Mon time frame. A compromise between the 06Z GFS/GEFS and the 00Z EC/EC mean solutions were incorporated to handle this feature. Meanwhile, the ECMWF has been showing a wide variety of solutions regarding the possibility of low pressure wave acquiring more tropical characteristics off the Southeast U.S. coast later this week in connection with a dissipating front. The last couple of runs have trended northward with the low moving rapidly off the Mid-Atlantic coast late this week. However, the GFS has continued to indicate minimal development regarding this feature. Given the unstable solution displayed by the ECMWF and the marginal environment near and off the East Coast, a conservative approach is adopted with a low moving off the Mid-Atlantic coast but with little development shown on the WPC prognostic charts. Kong ...Weather threats/highlights... A system affecting the Northeast around the start of the period will be accompanied by areas of rainfall, some of which may be moderate to heavy. Behind this system the focus for significant rainfall will turn to an area encompassing the northern Rockies/High Plains to upper Great Lakes and possibly extending southward close to the central Plains. A persistent frontal boundary with one or more waves, along with shortwave energy ejecting from the western trough aloft, will likely promote multiple convective episodes which may produce heavy rainfall at some locations. Guidance spread for details aloft keeps confidence low for the details of timing/location within a broader pattern evolution that should support the heavy rainfall threat. Farther west the upper trough will bring scattered rainfall with lighter amounts. Meanwhile the Southeast/Florida should see areas of locally heavy convection during the period. Upper level energy and possible surface reflection to the south of the deep layer ridge over the Ohio Valley/Mid Atlantic may provide some enhancement of this activity. Expect a broad area of hot/humid weather across much of the central-eastern U.S. during the period. Highest temperature anomalies of plus 10-20F should extend from the central Plains/Midwest through the Northeast. Daily records are possible for both highs and warm lows, with warm low records likely to be more numerous. In addition humidity will be sufficient to yield very high heat index values over many areas. On the other side of the temperature spectrum, much of the West and northern High Plains will see multiple days with below normal highs. The northern Rockies and High Plains should see the most extreme anomalies with highs as much as 10-15F below normal Fri-Sun. Rausch WPC medium range forecasts of 500 mb heights, surface systems, weather grids, quantitative precipitation, and winter weather outlook probabilities 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