Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1158 AM EDT Sat Aug 03 2019 Valid 12Z Tue Aug 06 2019 - 12Z Sat Aug 10 2019 ...Overview and Guidance/Predictability Assessment... Retrogression of an upper high initially to the south of mainland Alaska should allow cyclonic mean flow to develop over the Gulf of Alaska/extreme northeastern Pacific. By next Sat this evolution will likely support the ejection of an upper trough/embedded closed low forecast to persist just off the West Coast for most of next week. Downstream the Rockies/Four Corners ridge aloft should drift into the Plains while eastern North America troughing should likewise shift a bit to the east with time. Guidance consensus shows the eastern Canada/Northeast U.S. part of the upper trough deepening by late next week/weekend. Overall the latest and recent guidance agrees well for the general pattern evolution with some typical spread and run-run variability for medium/smaller-scale details. Thus confidence is fairly high for the mean pattern but lower for some regional specifics. Primary guidance notes involve the 00Z GFS and some 00Z GEFS ensembles possibly being quick to eject the eastern Pacific trough/upper low into western North America by the latter half of the period and the 00Z GFS being somewhat deeper and/or sharper than most other solutions within portions of the eastern trough aloft. The latter issue shows up early in the period as well as with upstream energy that digs into the mean trough Fri-Sat. The 06Z GFS/GEFS mean compare more favorably to remaining consensus for both western and eastern features. Also the 00Z CMC strays a bit south of the guidance majority for the upper low reaching the Northwest by Sat. In light of these considerations, the updated forecast started with an operational model blend of 06Z GFS and 00Z ECMWF/UKMET/CMC days 3-5 Tue-Thu and then transitioned toward an even model/mean weight of the 06Z GFS/00Z ECMWF and their means. ...Weather Highlights/Threats... Monsoonal moisture should enhance scattered showers and storms with locally heavy rainfall over/near the Four Corners states through the period. Interior West locations could see one or more episodes of scattered convection in response to shortwaves embedded in southwesterly flow between the eastern Pacific upper trough/low and Rockies/Plains ridge. Confidence is quite low in details though. Ejection of the Pacific trough aloft may also generate rainfall at some locations over the Northwest by Fri or Sat. To the east of the Rockies two fronts--one weakening as it progresses from the Great Lakes/central Plains to the East Coast and a second eventually stronger one reaching the northern tier by midweek and the East Coast by Fri-Sat--will provide the main large-scale focus for convection. Some locations within the northern two-thirds of the Plains/Mississippi Valley may see higher heavy rainfall potential at times as the fronts will tend to settle over the central/south-central Plains/MS Valley. Locally heavy rainfall will also be possible farther east but frontal progression may temper highest accumulations. The Southeast will see scattered diurnal convection. The Pacific Northwest/northern Rockies will see above normal temperatures (locally plus 10F or so anomalies) into midweek. Eventual ejection of the initial eastern Pacific upper trough/low should promote a gradual cooling trend over the West later in the week. Modestly below normal temperatures (by up to 5F or so) will become established over the northern Plains after passage of front dropping out of southern Canada Tue-Wed. By late week/weekend near to slightly below normal readings should extend into the Great Lakes toward the Northeast. The southern High Plains will likely see plus 5-10F anomalies for most of the period. Rausch Additional 3-7 Day Hazards information can be found on the WPC medium range hazards chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, winter weather outlook probabilities and heat indices are 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