Extended Forecast Discussion NWS Weather Prediction Center College Park MD 419 PM EDT Thu Jun 03 2021 Valid 12Z Sun Jun 06 2021 - 12Z Thu Jun 10 2021 ...Record Heat Threat from the Northern Plains to the Northeast.... ...Multi-Day Heavy Rainfall Event For the Southern Plains and Lower Mississippi Valley... ...Overview and Guidance Evaluation/Preferences... Guidance highlights the amplification and persistence of a West Coast mean upper trough throughout the period which will promote mean ridging over most of the East. This will spread much above normal temperatures from the Great Basin/Northern Plains to the Great Lakes and Northeast next week. In between, expect a slow-moving upper low/trough to generate unsettled weather and potentially heavy rainfall over parts of the already over saturated Southern Plains and Lower Mississippi Valley. Shortwave energy passing over the East Coast this weekend may form a modest upper low embedded within the ridge off the Mid-Atlantic coast, but with low confidence for maritime details. Model solutions remain well clustered days 3-5 (Sun-Tue), with the 00 UTC GFS/ECMWF seemingly best supported by the latest GEFS/ECMWF ensemble means in a pattern with above normal predictability. The WPC medium range product suite was primarily derived from the 00 UTC GFS/ECMWF along with the 13 UTC National Blend of Models and WPC continuity in this time frame. The 00 UTC GFS/ECMWF maintain amplified West Coast upper troughing into Days 6/7 (Wed/next Thu) more in line with ensemble means and pattern continuity while the 06/12 UTC GFS runs become less amplified as more energy works inland. The 12 UTC ECMWF remains on the amplified side of the forecast envelope and the 12 UTC Canadian has trended more amplified than the 00 UTC run, bolstering confidence. Considering increasing forecast spread and lingering uncertainty, WPC guidance was mainly derived from the ensemble means in this longer time period. ...Weather Highlights/Hazards... A multi-day heavy rainfall event is likely across portions of Texas, Louisiana, and into the ArkLaTex region with a persistent deep-layer flow of Gulf moisture interacting with the Southern Plains upper level trough/low and a 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, leading to flooding concerns. As the trough opens up and elongates, rainfall should become lighter with time with moisture also extending northward over the east-central U.S. toward the middle of next week. Elsewhere, the Northeast should remain fairly dry with diurnally-favored showers lingering over parts of the Southeast and flow around Bermuda high pressure may eventually extend some moisture northward through the Appalachians. Out West, increasing moisture over the Pacific Northwest is expected this weekend, with ejecting shortwaves/frontal systems extending some activity into the northern Rockies and eventually the northern Plains and Upper Mississippi Valley. Extreme temperature anomalies are likely from parts of the Great Basin/northern Plains to the Upper Great Lakes and Northeast. Highs and/or morning lows may reach up to 15-20+F above normal. Especially across parts of the Northeast next week, these values may exceed daily records for highs and warm lows. Meanwhile Texas/Louisiana and vicinity are likely to see slightly below normal highs early next week due to persistent clouds and rainfall. West Coast highs will refresh to moderately below normal as amplified upper troughing becomes established over the region. Schichtel/Santorelli 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 Hazards: - Heavy rain across portions of the Lower Mississippi Valley and the Southern Plains, Sun-Tue, Jun 6-Jun 8. - Flooding possible across portions of the Southern Plains. - Flooding occurring or imminent across portions of the Southern Plains, the Northern Rockies, and the Northern Great Basin. - Much above normal temperatures across portions of the Great Lakes, the Great Basin, and the Upper Mississippi Valley, Basin, Sun-Tue, Jun 6-Jun 8. - Much above normal temperatures across portions of the Northern/Central Plains, the Central Great Basin, the Northern/Central Rockies, the Northern Great Basin, and the Upper Mississippi Valley, Sun-Thu, Jun 6-Jun 10. - Much above normal temperatures across portions of the Great Lakes, the Mid-Atlantic, the Northeast, and the Central Appalachians, Sun-Wed, Jun 6-Jun 9. 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