Extended Forecast Discussion NWS Weather Prediction Center College Park MD 249 AM EDT Sun Jul 29 2018 Valid 12Z Wed Aug 01 2018 - 12Z Sun Aug 05 2018 ...Pattern Overview and Guidance/Uncertainty Assessment... A strong Bermuda high will extend its influence into the eastern U.S. through the period. This feature will hold in place a steadily weakening upper trough over or just east of the MS River while flow around the surface high will eventually overwhelm a front initially trailing from surface low pressure tracking northeast from the Great Lakes. Meanwhile flow across the northern half of the West will flatten with the eastward progression of energy associated with a modest upper low near northern Vancouver Island as of early Wed. The shortwave will bring an area of surface low pressure across southern Canada and the northern U.S.. Guidance continues to advertise a dramatic pattern change over the Pacific as a strengthening North Pacific/Aleutians ridge encourages troughing to develop over the northeastern Pacific. The leading edge of the trough may begin to influence the weather over the Pacific Northwest toward the end of the period. For depicting primary features of interest a blend consisting of mostly the 18Z GFS/12Z ECMWF with the rest composed of their ensemble means worked well for days 3-5 Wed-Fri. CMC runs through 12Z/28 had been too weak with the western Atlantic ridge aloft, allowing the east-central U.S. trough to extend too far eastward. The 00Z CMC has finally trended close to consensus that has also included the UKMET. On the other hand UKMET runs have tended to be on the fast side of the spread with the system tracking along the western/central U.S.-Canadian border. CMC tendency over the past couple runs has been toward slower than consensus timing for this system. Days 6-7 Sat-Sun transitioned the blend to a more even model/ensemble weight with a slight tilt in the ECMWF/ECMWF mean direction, plus using the last two ECMWF runs for the operational ECMWF component due to increasing detail differences. An overall consensus among GFS/ECMWF runs and their means has been fairly stable for the system near the Canadian border thus far (albeit with some variability/spread among individual solutions/runs), in spite of what have been some significant differences for Pacific evolution. Recent GFS/GEFS runs have been on the western side of the spread for the overall Pacific ridge-trough pattern. Through the 00Z cycle they have been nudging toward consensus for the ridge but continue to depict a deep/westward trough and embedded upper low. Teleconnections relative to the positive height anomaly center near the Aleutians in D+8 charts favor a northeastern Pacific trough closer to the ECMWF/ECMWF mean, though the amplified nature of the pattern allows for partial inclusion of the GFS/GEFS scenario. ...Weather Highlights/Threats... The Southeast continues to show the best potential for highest rainfall totals during the period. Flow around the Bermuda high will support a persistent feed of low level Atlantic/eastern Gulf moisture into the region. This moisture should interact with the slow moving front over the East during the first half of the period. After the front dissipates expect convection to be somewhat less organized with increasing focus along terrain and localized boundaries. Convection/rainfall through the period may be problematic over areas that have received significant rainfall over the past two weeks. Expect some moisture/rainfall to extend into the Mid-Atlantic/Northeast, with pockets of moderate-heavy activity but in general less extreme totals than over the Southeast. Periods of convection with varying intensity will continue over the Four Corners states with showers/storms possibly extending into the southern High Plains at times. Across the northern states, advancing shortwave energy and surface low pressure/leading warm front should encourage convective development within an area from the northern Plains into western Great Lakes from late week into the weekend. Some thunderstorms may produce heavy rainfall. Across the West, flatter flow aloft over northern areas will bring temperatures down to near or slightly below normal. Expect the Great Basin/Southwest to see above normal min temps through the period but highs will likely be more moderate. A wavy front pushing into the northern U.S. will bring an area of well below normal readings into the northern Plains/upper MS Valley Wed-Thu. Then flattening of the upper pattern will encourage a warming trend to above normal levels, progressing from the northern Plains into Great Lakes during Fri-Sun. Clouds/rainfall should keep the Southeast below normal for highs on most days. Flow around the Bermuda high will promote above normal lows over the Mid-Atlantic/Northeast. Rausch WPC medium range 500 mb heights, surface systems, weather grids, quantitative precipitation, winter weather outlook probabilities and heat indexes are found at: https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml