Extended Forecast Discussion NWS Weather Prediction Center College Park MD 254 AM EDT Thu Jun 28 2018 Valid 12Z Sun Jul 01 2018 - 12Z Thu Jul 05 2018 ...Overview and guidance evaluation/preferences... Today's guidance maintains good continuity/agreement for the expected longwave pattern across a majority of the forecast domain. An upper low dropping southeast from the Gulf of Alaska into southern BC and/or the Pacific Northwest will begin the process of trough amplification near the West Coast. With time this trough will further amplify and by next Wed-Thu likely drift offshore as the upstream Pacific ridge retrogrades. Farther east the guidance is also fairly consistent in depicting an upper ridge rebuilding over much of the central U.S. and southern Rockies after progressive shortwave energy passes through the Northern Tier states. As for one notable change in the models/means since yesterday, there is now a tendency to stall upper ridging over the northern Mid Atlantic/southern New England through early next week instead of having the ridge continue eastward and/or weaken. Latest consensus has this ridge eventually returning westward to become part of the very large overall ridge expected to be centered over the Plains by the end of the period. Around the periphery of the eastern ridge, most solutions still a broad upper level weakness to remain off the East Coast while some amount of upper level energy may brush the Gulf Coast. Based on guidance clustering or divergence as the case may be, a solution starting with mostly operational guidance (18Z GFS and 12Z ECMWF/CMC/UKMET) early followed by a transition toward a little more than half weight of the 18Z GEFS/12Z ECMWF means seemed to provide the best starting point for the forecast. Within the developing West Coast trough there are still some lingering differences for timing of the leading Gulf of Alaska low, in particular as the new 00Z GFS has reverted to a slower than consensus solution. By mid-late period the question is what proportion of the initial upper low energy ejects into Canada and when this occurs. Differences in models/ensembles suggest greater than average uncertainty in specifics across southern Canada and extreme Northern Tier areas especially by Wed-Thu. At that late-period time frame guidance is generally within the bounds of typical guidance error for the trough as it drifts offshore, fairly consistent with the forecast position of the upstream ridge/positive height anomalies in latest D+8 multi-day mean charts. Consensus changes in the upper pattern over the East have led to a weakening of the front that reaches the Northeast/northern Mid Atlantic by Tue and then a farther northward position by Thu. Guidance trends toward higher heights aloft since yesterday have been fairly unanimous but it will be worth watching to see if this adjustment persists. Teleconnections relative to the expected Pacific/West Coast pattern still signal the possibility of a little more mean troughing over the Canadian Maritimes and Northeast Coast as per earlier runs. It remains to be seen whether the upper feature near the Gulf Coast becomes as concentrated as in recent ECMWF runs, but teleconnections relative to the extent of eastern U.S. ridging do support the general tendency for above average rainfall probabilities over that region. Farther east, the UKMET runs bring the western Atlantic upper low/inverted trough closer to the coast than other solutions. ...Weather threats/highlights... The upper Great Lakes to Plains front as of early Sun will bring areas of showers and thunderstorms into the East. Most heavy activity should be fairly localized as upper dynamics lift into Canada. The trailing part of the front over the Plains will lift back north as a warm front during the period and help to focus periods of convection, with best potential for heavier activity over the northern Plains/upper Midwest regions. Locations along and/or inland from the Gulf Coast may also see one or more days of significant rainfall enhanced by upper level energy and a weak surface reflection. Confidence is low in the specifics of these features but the overall pattern is favorable for rainfall. Between this activity and the eastern front, expect some periodic convection of varying intensity in response to daytime heating. Portions of the Northwest may see scattered and mostly light-moderate rainfall in response to the upper trough developing near the West Coast. Parts of the southern half of the Rockies and vicinity may see a little scattered rainfall as well. Guidance changes over the past day suggest potential for greater persistence of hot/humid weather over the Northeast, though expect this weekend and Mon to be the focus for the most extreme heat index values. Record warm lows are possible within a fairly large area over the East. Record highs are likely to be less numerous and tend to be between the eastern Great Lakes and northern Mid Atlantic/Northeast. In that time frame some readings could locally reach 15-20F above normal. Anomalies may still reach or exceed plus 10F over some areas in the Great Lakes/Northeast into midweek. Meanwhile the strengthening central U.S. ridge aloft later in the period will bring a return of well above normal temperatures (up to plus 10-15F anomalies) to the northern Plains/Midwest by Wed-Thu. The most persistently cool temperatures in association with the West Coast upper trough will be over the Pacific Northwest/northern Rockies with Mon-Tue likely to have the coolest highs, approaching 10F below normal. California should trend cooler after the weekend as the upper trough amplifies. 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