Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1221 PM EDT Wed Jun 27 2018 Valid 12Z Sat Jun 30 2018 - 12Z Wed Jul 04 2018 ...Overview and guidance evaluation/preferences... Model guidance this morning shows excellent agreement with the overall evolution of the synoptic pattern across the U.S. through next Wednesday. A warm subtropical high will likely become firmly established from the central U.S. into the East Coast this weekend, signaling the beginning of a heat wave. Meanwhile, a broad upper-level trough slowly lifting out of the northern Plains will be followed by the approach of a closed low forecast to drop southeastward from the Gulf of Alaska toward the Pacific Northwest by the middle of next week. The GFS solutions have trended faster toward the ECMWF regarding this closed low, and they both models agree that a positively tilted trough pattern with a vorticity center will reach the Pacific Northwest coast by Day 7 Wednesday. Farther east, as the subtropical high becomes firmly established early next week, some westward-moving disturbances are forecast to move across Florida into the Gulf coast. Over the western Atlantic, models have been suggesting the development of a cyclonic circulation south of the subtropical high. The diffuse nature of this feature lends itself to low predictability for specifics. However, the latest guidance shows a tendency for this feature to form farther south near Bermuda Sunday into Monday before turning westward by midweek next week. With the above considerations, the WPC morning grid package was a consensus between the 00Z ECMWF and the 06Z GFS, trending toward 70 percent of their ensemble means for days 6 and 7. Kong ...Weather threats/highlights... Expect potential for some heavy/strong convection between the upper Great Lakes and central Plains early in the period, in association with a cold front initially near this area. Progression of the front will eventually bring showers/thunderstorms into the East but with any heavy activity tending to be more localized as the best dynamics lift into Canada after the weekend. The trailing part of the front will stall over the Plains and lift back as a warm front so there will be potential for additional rainfall over portions of the central U.S. after the more concentrated activity during the weekend. The other area of focus for heavy rainfall potential will be over the Gulf Coast/Florida, with the combination of upper level energy and surface troughing contributing to activity. In terms of highest temperature anomalies, the core of the heat wave affecting the eastern half of the U.S. should extend from the Midwest/Great Lakes into the Northeast. The greatest coverage of plus 10-20F above normal readings will likely exist Sat-Mon, followed by gradual moderation as the Plains front approaches. Best potential for record highs will be from the lower Great Lakes/upper Ohio Valley into the northern Mid Atlantic/Northeast Sat-Sun while broader coverage of record warm lows is possible Sat-Mon. The northern Rockies/High Plains will see below normal highs during the weekend followed by a trend toward near normal temperatures. The evolving pattern by late in the period will tend to support moderately below normal temperatures along the West Coast and over the Northwest toward midweek. 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