Extended Forecast Discussion NWS Weather Prediction Center College Park MD 250 AM EDT Mon Jun 25 2018 Valid 12Z Thu Jun 28 2018 - 12Z Mon Jul 02 2018 ...Overview... Once the amplified western trough/eastern ridge pattern becomes established late this week, guidance has displayed increased divergence over the past day or so regarding specifics of how/when western trough energy ejects downstream. Thus confidence has declined for the corresponding surface evolution over the central U.S. by the Sat-Mon time frame. Meanwhile guidance is diverse and variable with what type of surface reflection could evolve near/offshore the East Coast in connection with energy that originates from an upstream convective complex in the short range. Once the core of eastern ridging settles over the Ohio Valley/Mid Atlantic region, additional energy aloft and corresponding surface feature could evolve to the south near the Southeast Coast/Florida. ...Guidance evaluation/preferences... One source of the guidance spread for ejection of western trough energy lies with the timing of an upper low initially near the eastern Aleutians/Alaska Peninsula. The 12Z ECMWF/ECMWF mean were the fastest to bring this feature around the strong eastern Pacific ridge, likely aiding in faster ejection downstream and resulting in a broader/more shallow mean trough across the northern U.S. by the latter half of the period. CMC runs are slower with the upstream upper low but use other sheared energy flowing around the ridge to kick out leading western trough energy. ECMWF-based guidance has actually trended toward faster progression of height falls into the upper MS Valley and Great Lakes late in the period--in contrast to the GEFS mean whose past day of runs have trended stronger with the upper ridge covering the northeast quadrant of the contiguous U.S. GFS runs have been fairly persistent in depicting the most amplified trough-ridge pattern. Continuity leaned about 2/3 in the GFS/GEFS/NAEFS direction and see no reason to alter the philosophy at this time given the further increase in guidance spread since 12-24 hours ago and faster than consensus timing of the eastern Aleutians/AK Peninsula upper low in the ECMWF/ECMWF mean. In addition the 12Z CMC mean was closer to the GFS/GEFS scenario than the CMC/ECMWF idea. However note that teleconnections relative to the core of positive height anomalies over the eastern Pacific do offer eventual potential for the eastern ridge to be somewhat less pronounced than forecast by the GFS/GEFS. One last note, looking at guidance just beyond day 7 Mon suggests that early next week may feature a retrogression of the eastern Pacific/western U.S. pattern. This transition may lead to a period of increased uncertainty as well. Spread and recent variability among models/ensembles do not inspire confidence for the forecast near the East Coast. The past two ECMWF runs were wildly divergent for Atlantic low pressure originating from a Plains convective complex. CMC runs are tracking low pressure off the northeastern coast while GFS runs are quite diffuse. Additional energy may gravitate toward the Southeast Coast/Florida as the core of the eastern ridge aloft reaches the Ohio Valley/northern Mid Atlantic. Manual progs maintain yesterday's WPC/NHC-coordinated depiction of a surface trough/weak low near the Southeast Coast/Florida. The first half of the forecast incorporated portions of the 12Z-18Z GFS/GEFS and 12Z ECMWF/ECMWF mean/UKMET, with less than typical weight of the ECMWF due to its low confidence feature off the East Coast. The CMC was excluded due to northeastern troughing that become fairly extreme compared to the ensemble envelope. The new 00Z CMC appears more reasonable with a trend toward consensus. The remainder of the forecast started with 2/3 total GFS/GEFS/NAEFS and 1/3 ECMWF mean. ...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