Extended Forecast Discussion NWS Weather Prediction Center College Park MD 225 AM EDT Thu Jun 03 2021 Valid 12Z Sun Jun 06 2021 - 12Z Thu Jun 10 2021 ...Record Heat possible from parts of the Great Lakes into the Northeast.... ...Multiple Day Heavy Rainfall Event Likely For Parts of the Southern Plains and Lower Mississippi Valley... ...Overview and Guidance Evaluation/Preferences... The medium range will feature amplification and persistence of a mean trough over the West Coast throughout the period which will promote mean ridging over most of the East. This will shift much above normal temperatures this weekend over the Great Basin/Northern Plains, into parts of 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 also form a modest upper low embedded within the mean ridge off the Mid-Atlantic coast but with low confidence for details. The latest operational model runs show reasonable agreement to warrant a purely operational model blend for the WPC progs for days 3 and 4. After this time, developing issues with the West Coast trough originate from differences in the details of energy dropping into the mean trough and in particular the track and timing of an upper low that may drop south/southeast from Alaska. By late in the period, this affects both placement and strength of the trough over the West Coast. Especially out towards days 6 and 7, run to run continuity in much of the guidance is poor so uncertainty in the details is high. Therefore, a blend towards the better agreeable ensemble means is preferred beyond day 5. For the Plains upper low/trough, the GFS/ECMWF and their means offer a good middle ground for the solution. UKMET runs continue to be 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 however, 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. ...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. The most extreme temperature anomalies are likely from parts of the Great Basin/northern Plains into the Upper Great Lakes and Northeast. 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. Especially across parts of the Northeast next week, these values could come close to or exceed daily records for highs and warm lows. Meanwhile Texas and Louisiana are likely to see slightly below normal highs 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. Santorelli/Rausch 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