Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1200 PM EDT Mon Jul 30 2018 Valid 12Z Thu Aug 02 2018 - 12Z Mon Aug 06 2018 ...Pattern Overview and Guidance/Uncertainty Assessment... Model guidance this morning continues to advertise a slow-moving positively-tilted trough to weaken gradually in place along the Mississippi Valley as a very strong Bermuda high attempts to extend westward into the Southeast through the medium range period. Meanwhile, a cool airmass moving across the northern Plains into the Great Lakes will quickly be modified by the next shortwave trough moving from west to east just north of the U.S.-Canadian border. This will be followed by the approach of a more amplified closed low forecast to drop southeastward from Alaska to a position off the Pacific Northwest coast by next Monday. Model solutions are in good agreement with the synoptic pattern evolution over the U.S. with the Mississippi Valley upper trough slowly eroding with time as the Bermuda high extends its influence westward toward the warm high aloft over the southern Rockies. The main uncertainty lies mainly from the Northern Tier into Canada where models continue to show run-to-run variability on handling the eastward-moving shortwave trough just north of the border. By this weekend into Monday of next week, the GFS has been indicating a more amplified southward-plunging closed low across the Northeast Pacific compared with the ECMWF. The Canadian model shows a decent deep low positioned in between the GFS and the ECMWF. The morning WPC grid fields were derived using a 50-50 blend of the 06Z GFS with the 00Z ECMWF, mixing in with more of their ensemble means toward Day 7. This yields grid fields that are close to continuity with the previous WPC package. Kong ...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