Extended Forecast Discussion NWS Weather Prediction Center College Park MD 250 AM EDT Mon Jul 30 2018 Valid 12Z Thu Aug 02 2018 - 12Z Mon Aug 06 2018 ...Pattern Overview and Guidance/Uncertainty Assessment... During the first half of the period there will be a well-advertised flattening of the mean pattern, as southwest Canada/Pacific Northwest shortwave energy progresses eastward while an initial MS River upper trough dissipates through a combination of weakening in place and ejection of energy around a strong Bermuda high. A very strong ridge south of the Aleutians will promote amplification of an eastern Pacific trough after early Fri. Consistent with teleconnections relative to the Pacific pattern, guidance consensus expects flow over North America to evolve toward a west-central ridge/eastern trough by late in the period and just beyond. Over the eastern Pacific the primary question over recent runs has been the longitude of the evolving trough. GFS/GEFS runs had been on the western side of the spread and not especially favored by teleconnections relative to the upstream ridge's corresponding height anomaly center. Latest versions have trended favorably eastward thus allowing for a more even weight in the forecast blend. On the other hand the 12Z ECMWF may actually be a little overdone with the extent of leading height falls reaching the West--thus favoring some inclusion of the 00Z/29 run to balance the operational ECMWF component of the forecast. For the shortwave energy progressing eastward from the Pacific Northwest/southwestern Canada the UKMET continues to lie on the fast side of the envelope. Best consensus of the GFS/ECMWF and their means has trended a little faster by Fri or Sat though. Aside from this trend there is reasonable continuity for the front reaching the northern Plains by Fri-Sat. Specifics become increasingly murky after Sat with varying ideas regarding how quickly the best defined shortwave aloft will progress, with detail uncertainties for upstream impulses and higher latitude flow possibly playing a role. By late in the period the 12Z ECMWF is a notable faster solution at the surface and aloft. Farther south/southeast there are only low-predictability detail differences for the MS Valley trough aloft and associated frontal system that will weaken during the first half of the period. Otherwise the Bermuda high will be the prevailing feature affecting the eastern U.S. pattern. Around the upper high center most solutions show a weakness tracking just off the East Coast by Fri onward but with significant timing differences. Early in the period a blend of the 18Z GFS/12Z ECMWF and their means--with most weight to the operational runs--best reflected the most common ideas of latest guidance. By mid-late period aforementioned question marks with the 12Z ECMWF supported adding some 00Z/29 ECMWF to the blend as well and ensemble mean input increased to half weight by day 7 Mon given typical detail uncertainty at that time frame. ...Weather Highlights/Threats... Within the favorable area for locally heavy rainfall over the Southeast/southern half of the Appalachians and possibly extending into parts of the Mid-Atlantic, the best organized activity should be late this week when flow between the MS Valley upper trough and Bermuda high will direct the most focused Gulf/Atlantic moisture into the region. A weakening cold/stationary front will provide a low-level focus for convection as well. By Sat-Mon expect showers/storms to become gradually lighter and more scattered as flow aloft becomes less defined and the surface ridge axis drifts southwestward. The northeastern Plains into the Great Lakes will be another region with enhanced rainfall potential during the period. Some of the rain will come with a wavy front settling into the region late this week. Then expect heavier/stronger convection with the system emerging into the northern Plains by Fri with the leading warm front providing an added focus. The cold front trailing from this system may decelerate with time, allowing for some persistence/training later in the period. Meanwhile diurnally favored convection of varying intensity will continue over portions of the Four Corners states. The amplifying upper trough nearing the West Coast later in the period may eventually bring some moisture to the Pacific Northwest. Expected pattern evolution will likely lead to an area of above normal min and/or max temperatures becoming oriented from the north-central/central Plains through the Great Lakes/Northeast. Anomalies exceeding 10F should be somewhat more common for morning lows. The southeastern third of the country will see coolest daytime highs versus normal late this week when clouds/rainfall should be most prevalent, followed by a trend toward normal values. Over the West, min temps will likely remain quite warm over and south of the Great Basin through the period while slightly below normal highs should prevail over the Northwest. 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