Extended Forecast Discussion NWS Weather Prediction Center College Park MD 333 PM EDT Wed Jun 02 2021 Valid 12Z Sat Jun 05 2021 - 12Z Wed Jun 09 2021 ...Record Heat possible from parts of the Northern Plains into the Northeast.... ...Multiple Day Heavy Rainfall Event Likely For Parts of the Southern Plains and Lower Mississippi Valley... ...Overview and Guidance Evaluation/Preferences... A large scale upper trough entering the Northwest U.S. at the start of the period (Saturday) should finally suppress the strong Western U.S. ridge that is currently anchored over the region. Amplification and persistence of this trough near the West Coast into the first half of next week will promote mean ridging over most of the East and shift much of the above normal temperatures/possible record heat into areas across the northern tier. Between these features, from the weekend into early next week expect a slow-moving upper low/trough to generate unsettled weather and potentially heavy rainfall over the Southern Plains and Lower Mississippi Valley regions that are already overly saturated. The upper system should eventually weaken enough for rainfall to become lighter and less organized by midweek. Shortwave energy passing over the East Coast this weekend may form a modest upper low off the Mid-Atlantic coast but with low confidence for details. Compared to continuity, the 00Z-06Z model/ensemble runs provided similar aspects of large-scale agreement as well as spread later in the period. This maintained the preference for an operational model composite this weekend followed by a trend toward more GEFS/ECMWF ensemble input, reaching 70 percent total by day 7 Wednesday. Developing issues with the West Coast trough originate from widely differing solutions for aspects of central Pacific flow early next week, ultimately affecting northeastern Pacific flow and in particular the track and timing of an upper low that may drop south/southeast from the western coast of Mainland Alaska. 00Z ECMWF/06Z GFS runs suggested this upper low could hold far enough west to weaken the West Coast trough by midweek, but in contrast very different upstream flow in the 00Z CMC led to an amplified West Coast trough at that time. New 12Z runs of both the GFS and CMC have trended toward more moderate ideas per the more consistent means. A significant change in ECMWF specifics from the 00Z to new 12Z runs further highlights the uncertainty over this area. Meanwhile the GFS/ECMWF and their means offer a good middle ground for the Plains upper low/trough. 00Z/12Z UKMET runs are on the southern side of the envelope and CMC runs are on the eastern side. There is a reasonable consensus that the feature will gradually weaken/open up by later in the period. Specifics of associated potential heavy rainfall will be very sensitive to exact details of the upper feature as well as mesoscale boundaries which all have low predictability multiple days out in time. The possible weak upper low near/offshore the East Coast also has low predictability given its small scale and being embedded within a larger-scale ridge. ...Weather Highlights/Hazards... A multi-day heavy rainfall event is looking more likely across portions of Texas, Louisiana, and into the ArkLaTex region with a persistent deep-layer flow of Gulf moisture interacting with a Southern Plains upper level trough/low and western Gulf Coast surface front. Models continue to suggest the potential for several inches or more of rain within a region which has seen well above normal precipitation over recent weeks. In less extreme form, some of this rainfall may extend northward over the east-central U.S. toward the middle of next week as the upper feature opens up/elongates. Elsewhere, much of the East should remain fairly dry although diurnally-favored showers may linger over parts of the Southeast and flow around Bermuda high pressure may eventually extend some moisture northward through the Appalachians. Specifics of a possible but low-confidence upper low near/offshore the East Coast will have to be watched as well. Expect a gradual increase in moisture over the Pacific Northwest as mean troughing aloft settles near the West Coast while ejecting shortwaves/frontal systems should extend some activity into the northern Rockies and eventually the northern Plains and Upper Mississippi Valley. The most extreme temperature anomalies are likely to shift from the Great Basin through northern Plains/Upper Midwest at the start of the period Saturday into the Upper Great Lakes and Northeast Sunday onward. Highs and/or morning lows may reach up to 20-25F above normal over parts of the northern Plains and vicinity early in the weekend and northern New England early next week. While the heat will moderate over the Great Basin with time, areas from the north-central Rockies through the northern tier should continue to see areas of plus 10-20F anomalies into midweek. On most days expect various portions of the northern tier to see temperatures approach or exceed daily records for highs/warm lows. Meanwhile Texas and Louisiana are likely to see below normal highs (up to minus 10-12F anomalies on Saturday) at least from the weekend into early next week due to persistent clouds and rainfall. West Coast highs will trend moderately below normal as amplified upper troughing becomes established over the region. Rausch/Santorelli Hazards: - Flooding possible across portions of the Central/Southern Plains. - Flooding occurring or imminent across portions of the Southern Plains. - Flooding likely across portions of the Northern Rockies and the Northern Great Basin. - Heavy rain across portions of the Lower Mississippi Valley and the Southern Plains, Sat-Tue, Jun 5-Jun 8. - Much above normal temperatures across portions of California and the Northern/Central Great Basin, Sat-Sun, Jun 5-Jun 6. - Much above normal temperatures across portions of the Northern/Central Rockies, the Northern/Central Plains, the Upper Mississippi Valley, and the Upper Great Lakes, Sat-Wed, Jun 5-Jun 9. - Much above normal temperatures across portions of the Northeast and the Central Appalachians, Sun-Wed, Jun 6-Jun 9. Additional 3-7 Day Hazard information can be found on the WPC medium range hazards outlook 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